Dissertations / Theses on the topic 'Accretion disk'

We examine the nature of the boundary layer in α-viscous accretion disks. The boundary layer is the interface between the disk and the accreting central star or black hole. We develop two models for the boundary layer by expanding and generalizing the standard disk equations, and then solving our new set of equations numerically using a relaxation method. First, we use a model which includes a polytropic equation of state to examine the disk dynamics. This allows us to ignore the energetics and radiative transfer and simplifies the problem considerably. We find two types of boundary layer solutions with this model, depending on the rotation rate of the accreting star. One of these is a new type of solutions in which the angular momentum accretion rate can be small or negative. These solutions allow accretion to continue even after the star spins up to breakup speed. We apply a causally-limited viscosity prescription to our solutions, and find that it prevents the radial velocities from becoming supersonic in the boundary layer, thus preserving causality. We apply the same prescription to a model for disks around black holes, and find that it allows us to calculate solutions for reasonable values of α, where none existed before. We develop a more complete model, which includes the energetics and radiative transfer of the boundary layer, for comparison with observations. We apply this model to cataclysmic variables, and find that the nature of the boundary layer in these systems depends strongly on the optical depth, which in turn depends largely on the mass accretion rate and the rotation rate of the accreting star. The dependence of our results on the accretion rate agrees well with X-ray observations of these systems. We also apply the model to accretion disks in pre-main sequence stars, such as T Tauri and FU Orionis stars, and find that the temperatures and radial widths of the boundary layer in our solutions agree well with those inferred from observations.

Outflows are ubiquitous in active galactic nuclei (AGN). They can take the form of either dramatic radio jets, which extend vast distances into the inter-galactic medium, or of much smaller scale winds - whose existence can be inferred only indirectly via their influence on the observed spectra. There is good evidence to show that winds are likely to arise from the accretion disks thought to form the central engine of all AGN; they should therefore be observable in all such systems. The clearest observational signature comes in the form of the broad, blue-shifted absorption features seen in the spectra of broad absorption line quasars (BALQSOs) and the aim of this work is to investigate how the geometry and physical parameters of disk winds affect their absorption spectra. We first discuss the changes made to an existing Monte Carlo radiative transfer code, python, in order to extend its capabilities to include modelling of AGN. These changes include the implementation of an approximate ionization scheme which takes account of arbitrary illuminating spectral energy distributions (SEDs), and the inclusion of heating and cooling effects likely to be important in the presence of high energy photons. Next, we describe the second stage of the project which was to gain insight into the general properties of a wind exhibiting broad absorption line (BAL) features. We did this by performing radiative transfer and ionization calculations on a simple kinematic wind representation using python. We show that BAL features can be produced for plausible AGN parameters. Finally, we present calculations carried out on a more complex wind geometry, generated from a hydrodynamic simulation of a line driven disk wind. This calculation does not produce BAL features at all, and in fact the wind is too highly ionised to permit efficient line driving. This result is significant because it illustrates the importance of detailed radiative transfer in hydrodynamic modelling, and suggests future work.

Aims. We investigate the suitability of alpha-viscosity in self-similar models for self-gravitating disks with a focus on active galactic nuclei (AGN) disks. Methods. We use a self-similar approach to simplify the partial di ff erential equations arising from the evolution equation, which are then solved using numerical standard procedures. Results. We find a self-similar solution for the dynamical evolution of self-gravitating alpha-disks and derive the significant quantities. In the Keplerian part of the disk our model is consistent with standard stationary alpha-disk theory, and self-consistent throughout the self-gravitating regime. Positive accretion rates throughout the disk demand a high degree of self-gravitation. Combined with the temporal decline of the accretion rate and its low amount, the model prohibits the growth of large central masses. Conclusions. alpha-viscosity cannot account for the evolution of the whole mass spectrum of super-massive black holes (SMBH) in AGN. However, considering the involved scales it seems suitable for modelling protoplanetary disks.

The focus of this study is the change in accretion disk size in Dwarf Novae (DN), IP Peg. DN systems are a type of cataclysmic variable that experience periodic outbursts. These outbursts are caused by the release of gravitational potential energy from an increased rate of matter flow through the accretion disk. Throughout outburst, the radius of the accretion disk of the DN changes. Recent research done at Ball State University has suggested that the disk radius may not change as the disk instability model predicts. According to the disk instability model, the accretion disk should be at its largest radial size when the DN is at the peak of outburst. IP Peg in September and October of 2006 underwent outburst. It was found that during that particular outburst that the accretion disk was at its largest radial size on the decline from outburst and not peak. Further research into how the accretion disk changes with time is needed.
Department of Physics and Astronomy

Chondritic meteorites largely formed 4.6 billion years ago, and can range from being metamorphosed as a result of processing on their asteroid parent bodies to entirely unaltered since their formation in the protoplanetary disk. The magnetic grains within these meteorites can potentially record and retain the magnetic conditions on the parent body and the conditions in the protoplanetary disk during the formation of our planetary system. However, the complex history of these meteorites can make their magnetic records difficult to interpret, and their age prompts the question of whether a magnetic remanence can be retained for so long. In this thesis, to help identify the origin of the magnetic remanence, a new method for the palaeomagnetic conglomerate test that uses micro-CT scans to accurately mutually orient chondrules from chondrites was developed. When applied to Vigarano (CV3) and Bjurböle (L/LL4), a more in-depth understanding of parent body processing was achieved that provides evidence for magnetic dynamo activity on their parent bodies. To understand the magnetic record of CK chondrites, a palaeomagnetic analysis of Karoonda (CK4) was conducted, and found no evidence of a significant magnetic field recording, supporting the solar radiative heating model for the CV-CK chondrites. To determine whether magnetic remanence can be retained from the early Solar System, the high thermal stability of single and multi-vortex kamacite grains from Bishunpur (LL3.1) was demonstrated by performing in-situ temperature-dependent nanometric magnetic measurements using electron holography and numerical micromagnetic energy barrier calculations. This study found that the majority of kamacite grains in dusty olivines are capable of retaining magnetic field information from the early Solar System, a key finding in our quest to understand the formation of our Solar System.

This Thesis focuses on the X-ray study of the inner regions of Active Galactic Nuclei, in particular on the formation of high velocity winds by the accretion disk itself. Constraining AGN winds physical parameters is of paramount importance both for understanding the physics of the accretion/ejection flow onto supermassive black holes, and for quantifying the amount of feedback between the SMBH and its environment across the cosmic time. The sources selected for the present study are BAL, mini-BAL, and NAL QSOs, known to host high-velocity winds associated to the AGN nuclear regions. Observationally, a three-fold strategy has been adopted: - substantial samples of distant sources have been analyzed through spectral, photometric, and statistical techniques, to gain insights into their mean properties as a population; - a moderately sized sample of bright sources has been studied through detailed X-ray spectral analysis, to give a first flavor of the general spectral properties of these sources, also from a temporally resolved point of view; - the best nearby candidate has been thoroughly studied using the most sophisticated spectral analysis techniques applied to a large dataset with a high S/N ratio, to understand the details of the physics of its accretion/ejection flow. There are three main channels through which this Thesis has been developed: - [Archival Studies]: the XMM-Newton public archival data has been extensively used to analyze both a large sample of distant BAL QSOs, and several individual bright sources, either BAL, mini-BAL, or NAL QSOs. - [New Observational Campaign]: I proposed and was awarded with new X-ray pointings of the mini-BAL QSOs PG 1126-041 and PG 1351+640 during the XMM-Newton AO-7 and AO-8. These produced the biggest X-ray observational campaign ever made on a mini-BAL QSO (PG 1126-041), including the longest exposure so far. Thanks to the exceptional dataset, a whealth of informations have been obtained on both the intrinsic continuum and on the complex reprocessing media that happen to be in the inner regions of this AGN. Furthermore, the temporally resolved X-ray spectral analysis field has been finally opened for mini-BAL QSOs. - [Theoretical Studies]: some issues about the connection between theories and observations of AGN accretion disk winds have been investigated, through theoretical arguments and synthetic absorption line profiles studies.

This Thesis focuses on the X-ray study of the inner regions of Active Galactic Nuclei, in particular on the formation of high velocity winds by the accretion disk itself. Constraining AGN winds physical parameters is of paramount importance both for understanding the physics of the accretion/ejection flow onto supermassive black holes, and for quantifying the amount of feedback between the SMBH and its environment across the cosmic time. The sources selected for the present study are BAL, mini-BAL, and NAL QSOs, known to host high-velocity winds associated to the AGN nuclear regions. Observationally, a three-fold strategy has been adopted: - substantial samples of distant sources have been analyzed through spectral, photometric, and statistical techniques, to gain insights into their mean properties as a population; - a moderately sized sample of bright sources has been studied through detailed X-ray spectral analysis, to give a first flavor of the general spectral properties of these sources, also from a temporally resolved point of view; - the best nearby candidate has been thoroughly studied using the most sophisticated spectral analysis techniques applied to a large dataset with a high S/N ratio, to understand the details of the physics of its accretion/ejection flow. There are three main channels through which this Thesis has been developed: - [Archival Studies]: the XMM-Newton public archival data has been extensively used to analyze both a large sample of distant BAL QSOs, and several individual bright sources, either BAL, mini-BAL, or NAL QSOs. - [New Observational Campaign]: I proposed and was awarded with new X-ray pointings of the mini-BAL QSOs PG 1126-041 and PG 1351+640 during the XMM-Newton AO-7 and AO-8. These produced the biggest X-ray observational campaign ever made on a mini-BAL QSO (PG 1126-041), including the longest exposure so far. Thanks to the exceptional dataset, a whealth of informations have been obtained on both the intrinsic continuum and on the complex reprocessing media that happen to be in the inner regions of this AGN. Furthermore, the temporally resolved X-ray spectral analysis field has been finally opened for mini-BAL QSOs. - [Theoretical Studies]: some issues about the connection between theories and observations of AGN accretion disk winds have been investigated, through theoretical arguments and synthetic absorption line profiles studies.

We conduct a multiwavelength continuum variability study of the Seyfert 1 galaxy NGC 5548 to investigate the temperature structure of its accretion disk. The 19 overlapping continuum light curves (1158 angstrom to 9157 angstrom) combine simultaneous Hubble Space Telescope, Swift, and ground-based observations over a 180 day period from 2014 January to July. Light-curve variability is interpreted as the reverberation response of the accretion disk to irradiation by a central time-varying point source. Our model yields the disk inclination i = 36 degrees +/- 10 degrees, temperature T-1= (44 +/- 6) x 10(3) K at 1 light day from the black hole, and a temperature-radius slope (T proportional to r(-alpha)) of alpha = 0.99 +/- 0.03. We also infer the driving light curve and find that it correlates poorly with both the hard and soft X-ray light curves, suggesting that the X-rays alone may not drive the ultraviolet and optical variability over the observing period. We also decompose the light curves into bright, faint, and mean accretion-disk spectra. These spectra lie below that expected for a standard blackbody accretion disk accreting at L/L-Edd = 0.1.

We present new Atacama Large Millimeter/sub-millimeter Array (ALMA) 1.3 mm continuum observations of the SR 24S transition disk with an angular resolution less than or similar to 0'.18 (12 au radius). We perform a multi-wavelength investigation by combining new data with previous ALMA data at 0.45 mm. The visibilities and images of the continuum emission at the two wavelengths are well characterized by a ring-like emission. Visibility modeling finds that the ring-like emission is narrower at longer wavelengths, in good agreement with models of dust-trapping in pressure bumps, although there are complex residuals that suggest potentially asymmetric structures. The 0.45 mm emission has a shallower profile inside the central cavity than the 1.3 mm emission. In addition, we find that the (CO)-C-13 and (CO)-O-18 (J = 2-1) emission peaks at the center of the continuum cavity. We do not detect either continuum or gas emission from the northern companion to this system (SR 24N), which is itself a binary system. The upper limit for the dust disk mass of SR 24N is less than or similar to 0.12 M-circle plus, which gives a disk mass ratio in dust between the two components of M-dust,M-SR 24S/M-dust,M-SR 24N greater than or similar to 840. The current ALMA observations may imply that either planets have already formed in the SR 24N disk or that dust growth to millimeter sizes is inhibited there and that only warm gas, as seen by rovibrational CO emission inside the truncation radii of the binary, is present.

Theoretical models and spectroscopic observations of newborn stars suggest that protoplantary disks have an inner "wall" at a distance set by the disk interaction with the star. Around T Tauri stars, the size of this disk hole is expected to be on a 0.1 au scale that is unresolved by current adaptive optics imaging, though some model-dependent constraints have been obtained by near-infrared interferometry. Here we report the first measurement of the inner disk wall around a solar-mass young stellar object, YLW 16B in the rho Ophiuchi star-forming region, by detecting the light-travel time of the variable radiation from the stellar surface to the disk. Consistent time lags were detected on two nights, when the time series in H (1.6 mu m) and K (2.2 mu m) bands were synchronized while the 4.5 mu m emission lagged by 74.5 +/- 3.2 s. Considering the nearly edge-on geometry of the disk, the inner rim should be 0.084 au from the protostar on average, with an error of order 0.01 au. This size is likely larger than the range of magnetospheric truncations and consistent with an optically and geometrically thick disk front at the dust sublimation radius at similar to 1500 K. The widths of the cross-correlation functions between the data in different wavebands place possible new constraints on the geometry of the disk.

Le binarie a raggi-X sono tra le sorgenti più brillanti in banda X nella nostra Galassia, e furono le prime sorgenti cosmiche in banda X ad essere scoperte all'inizio degli anni 60. Le prime osservazioni pionieristiche di queste sorgenti venivano realizzate utilizzando dei razzi; tuttavia, nel breve tempo in cui questi razzi operavano era impossibile raccogliere le informazioni necessarie per comprendere la vera natura di queste sorgenti. Soltanto circa 10 anni più tardi, con l'avvento della prima generazione di satelliti per osservazioni in banda X fu possibile stabilire che queste sorgenti erano effettivamente dei sistemi di stelle binarie, contenenti una stella di neutroni o un buco nero in orbita intorno alla stella compagna. La gran parte dell'emissione in raggi X in queste sorgenti è dovuta all'accrescimento di materia della stella compagna sull'oggetto compatto. In questa tesi ci concentriamo sui sistemi binari contenenti stelle di neutroni. La presenza di stelle di neutroni nelle binarie a raggi X è in genere dedotta dalle pulsazioni osservate nel flusso in banda X proveniente da queste sorgenti. Queste pulsazioni sono dovute al cosiddetto "effetto-faro", che è il risultato dell'azione combinata della rotazione e del campo magnetico della stella di neutroni. Quest'ultimo incanala la materia in accrescimento verso i poli magnetici della stella di neutroni, e può avere un intensità pari a circa 1.000 bilioni di volte quella della Terra; è perciò il campo magnetico più intenso che si conosca nell'Universo. Non tutte le stelle di neutroni hanno però un campo magnetico così elevato, e in alcune di esse sembra che possa essere molto più debole. In questi casi, il flusso di materia in accrescimento non è sempre incanalato dal campo magnetico verso i poli della stella di neutroni e talvolta vengono osservate soltanto delle pulsazioni transienti. Molte di queste sorgenti con campi deboli mostrano anche delle rapide oscillazioni quasi-periodiche che sono il risultato dell'interazione tra la stella di neutroni ed il disco di accrescimento. In alcuni casi la materia in accrescimento, costituita per lo più da idrogeno ed elio, si accumula sulla superficie della stella di neutroni, e raggiunge una certa massa critica al di sopra della quale si innescano delle esplosioni termonucleari, dette burst in raggi-X. Questo ci mostra dunque che, in generale, le proprietà dell'emissione di una stella di neutroni dipendono dai parametri fisici della stella stessa (come, ad esempio, il suo periodo di spin e l'intensità del campo magnetico). Al contrario, la natura della stella compagna determina la modalità con cui ha luogo il trasferimento di materia tra le due stelle. Nelle binarie a raggi-X di piccola massa, le stelle compagne hanno tipicamente un età di ~10^8 anni, e una massa simile o inferiore a quella solare. In questi sistemi il trasferimento di massa avviene a mezzo del cosiddetto "Roche Lobe Overflow", che in genere porta alla formazione di un disco di accrescimento intorno alla stella di neutroni. Nel caso in cui la stella compagna sia molto più giovane (~10^6 anni) e più massiva (>>1 massa solare), allora il sistema prende il nome di sistema binario di grande massa, e il trasferimento di materia tra le due stelle avviene a mezzo della cattura del vento stellare. Infatti, tipicamente le compagne delle binarie di grande massa sono stelle O o B il cui intenso vento stellare può facilmente esser catturato dalla stella di neutroni per emettere in raggi X. Nel corso degli ultimi anni, il lancio di molti satelliti per osservazioni in banda X, come RXTE, XMM-Newton, Chandra, Swift, e INTEGRAL, ha aperto nuove possibilità per lo studio delle binarie a raggi X, fornendo sempre maggiori dettagli e mostrando comportamenti che ancora necessitano di uno studio approfondito e di una corretta interpretazione. In questa tesi ci occupiamo sia della binarie di piccola massa che di quelle di grande massa, e analizziamo diversi aspetti di queste sorgenti sia da un punto di vista osservativo che teorico. In particolare, utilizzando i dati raccolti grazie ai telescopi a bordo dei satelliti X della generazione attuale, studiamo in dettaglio i processi di accrescimento da disco e da vento che hanno luogo nei sistemi binari. Confrontiamo poi i risultati ricavati dall'analisi dei dati con quelli aspettati dai modelli teorici, e discutiamo anche gli avanzamenti nella teoria dell'accrescimento da disco e da vento che abbiamo sviluppato nel corso di questa tesi. Nel capitolo 1 presentiamo un'introduzione generale sulle binarie a raggi X, mentre in tutti gli altri capitoli che seguono descriviamo i risultati originali ottenuti durante lo svolgimento di questa tesi. Suddividiamo questi capitoli in due parti. Nella prima parte (Capitoli 2, 3, 4, 5, e 6) ci concentriamo sulle binarie X di piccola massa. In particolare, nel capitolo 2 riassumiamo il "magnetic threaded disk model", ovvero il modello ormai largamente accettato che spiega l'interazione tra una stella di neutroni magnetizzata e il suo disco di accrescimento. Utilizzando le oscillazioni quasi-periodiche, presentiamo un nuovo metodo sviluppato per testare questo modello rispetto alle osservazioni delle QPO da binarie di grande e piccola massa (Bozzo, E., Stella, L., Vietri, M., et al. 2008, A&A, in press [astro-ph/0811.0049]). Discutiamo anche alcuni miglioramenti che intendiamo apportare al magnetic threaded disk model e che svilupperemo in una successiva pubblicazione. Nel capitolo 3 proponiamo invece il modello della "recycling magnetosphere" per spiegare il comportamento di spin-up e spin-down di alcune sorgenti che non può essere interpretato nell'ambito del magnetic threaded disk model (Perna, R., Bozzo, E., Stella, L. 2006, ApJ, 639, 363). Il modello della recycling magnetosphere prevede una trattazione dettagliata del cosiddetto "effetto-propeller", che viene discusso anche in maggior dettaglio nel capitolo 4 (Falanga, M., Bozzo, E., Stella, L., et al. 2007, A&A, 464, 807). Nei capitoli 5 e 6, ci concentriamo invece sulle osservazioni di due binarie X di piccola massa, 4U 2129+47 e XTEJ1701-407. Della prima sorgente riportiamo i risultati dell'analisi di due osservazioni XMM-Newton, che ci ha permesso di misurare un ritardo di circa 190 s tra due eclissi distanti 22 giorni. Nella discussione del capitolo 6 mostriamo come questo ritardo possa esser naturalmente spiegato come l'effetto del moto della binaria intorno al centro di massa con una terza stella; questa misura costituisce dunque la prima prova diretta in raggi X dell'appartenenza di un sistema binario a raggi X ad un sistema triplo di stelle (Bozzo, E., Stella, L., Papitto, A., et al. 2007, A&A, 476, 301). Nel caso della sorgente XTEJ1701-407 ci occupiamo invece di analizzare i dati relativi al primo burst di tipo I in raggi X osservato da questa sorgente e discutiamo i risultati di questa analisi in merito ai più recenti modelli di bruciamento termonucleare sulla superficie di una stella di neutroni (Falanga, M., Cumming, A., Bozzo, E., et al. 2008, A&A, in press [astro-ph/0901.0314]). Nella seconda parte di questa tesi (capitoli 7 e 8) ci concentriamo sulle binarie a raggi X di grande massa. Nel capitolo 7 analizziamo in dettaglio il processo di accrescimento da vento stellare, e applichiamo questo scenario al caso delle "supergiant fast X-ray transients" (SFXT), una nuova sottoclasse di binarie X di grande massa scoperte recentemente con INTEGRAL. In particolare, noi suggeriamo che queste binarie possano contenere delle "magnetar", ovvero delle stelle di neutroni con un campo magnetico eccezionalmente elevato (~10^14-10^15 G). In questo caso, le sorgenti SFXT sarebbero il primo esempio di sistemi binari contenti magnetar, e fornirebbero dunque una preziosa occasione per poter osservare e studiare questi oggetti peculiari (Bozzo, E., Falanga, M., Stella, L. 2008, ApJ, 683, 1031). Nel capitolo 8 riportiamo invece i risultati di un osservazione XMM-Newton della sorgente IGR J16479-4514. L'analisi temporale, spettrale e spaziale di questa osservazione ha rivelato una fenomenologia molto complessa che è stata interpretata in termini di un eclissi della sorgente X da parte della stella compagna. Inoltre, il flusso X residuodurante l'eclissi si è potuto spiegare come dovuto all'effetto di scattering della radiazione X per mezzo sia di un gas parzialmente ionizzato posto vicino alla sorgente, che di un alone di polvere interstellare situato lungo la linea di vista tra noi e la sorgente IGR J16479-4514 (Bozzo, E., Stella, L., Israel, G., et al. 2008, MNRAS, 391, L108). Infine descriviamo brevemente i possibili sviluppi futuri dei campi di ricerca trattati in questa tesi.
X-ray binaries are among the brightest X-ray sources in our Galaxy, and were the first extra-solar X-ray sources discovered in the early '60s. The first pioneristic observations of these sources were made by using rocket flights; however, in the brief interval in which rockets operated it was hard to get sufficient information to understand the nature of these sources. Only about ten years later the first generation of X-ray satellites established that most bright X-ray sources in the Galaxy are in fact binary systems containing either a neutron star (NS) or a black hole orbiting a companion star. Most of the X-ray emission of these sources is due to the accretion of matter onto the compact object. This thesis focuses on X-ray binaries hosting neutron stars. The presence of NSs in X-ray binaries is often inferred from pulsations in their X-ray flux. These occur because of the so called "lighthouse" effect, that is due to the combination of the NS rotation and misaligned intense magnetic field, which funnels the accreting matter onto the star's magnetic poles. In these sources the inferred NS magnetic fields can be as high as about 1,000 billion times that on the earth, and are thus the strongest magnetic fields known in the universe. Not all NS systems have such intense magnetic fields: in many NS X-ray binaries the field is much weaker. In these cases the flow of material onto the NS is not always channeled towards the magnetic poles and sometimes only transient pulsations are observed. Many of these low magnetic field systems also show fast quasi-periodic oscillations in their X-ray flux that originate from the interaction of the NS with the surrounding accretion disk. In some cases the accreted material (mostly hydrogen and helium) accumulating on the NS surface reaches a critical mass, at which a thermonuclear explosion takes place and the source undergoes an X-ray burst. The emission properties that characterize an accreting NS in a binary system thus depend mainly on the NS physical parameters (e.g. spin period and magnetic field strength). On the contrary, the mode in which mass transfer takes place, as well as the geometry of the accretion flow, depends on the nature of the companion star. In low mass X-ray binaries the companion star has a typical age of ~10^8 yr, and its mass is similar to or less than that of our sun. In these systems the transfer of mass takes place through the so called Roche Lobe Overflow, which usually leads to the formation of an accretion disk around the compact object. If the companion star is much younger (~few 10^6 yr) and massive (>>1 solar mass), then the system is a high mass X-ray binary, and mass transfer occurs through the wind capture by the compact star. In fact, donor stars in high mass X-ray binaries are typically blue O or B stars whose intense wind can be easily captured by the NS to release X-rays. During the past years, the operation of the present generation of X-ray satellites, such as RXTE, XMM-Newton, Chandra, Swift, and INTEGRAL, has opened a new era in the discovery and study of for X-ray binaries. This provided several breakthroughs as well as surprising new questions on these sources. In this phD thesis we consider both high and low mass NS binaries, and analyze several observational and theoretical aspects of these sources. We use data obtained with the modern X-ray telescopes available on board the present generation satellites in order to investigate the accretion processes in these sources. In particular, we study both disk and wind accretion, and compare the observational results with theoretical expectations. Some improvements in the theory of disk and wind accretion are presented. In Chapter 1 we provide a brief and comprehensive introduction on NS X-ray binaries; all other chapters are based on our original findings. We divided these chapters in two groups. In the first group (Chapters 2, 3, 4, 5, and 6), we concentrate on studies of low mass X-ray binaries. In particular, chapter 2 summarizes the magnetic threaded disk model, that is the most widely accepted model to describe the interaction between a magnetized NS and its surrounding accretion disk. By using quasi-periodic oscillations in X-ray binaries, we present a new method to test the threaded disk model against observations of slow quasi-periodic oscillations in accreting X-ray pulsars contained in high as well as low mass X-ray binaries (Bozzo, E., Stella, L., Vietri, M., et al. 2008, A&A, in press [astro-ph/0811.0049]). We also discuss some improvements on the threaded disk model that we will develop in a future publication. In Chapter 3 we propose the "recycling magnetosphere model" to explain the spin-up/spin-down behaviour of some low mass X-ray binaries that cannot be interpreted within the magnetic threaded disk scenario (Perna, R., Bozzo, E., Stella, L. 2006, ApJ, 639, 363). This model involves an in-depth analysis of the so called propeller mechanism, which is also discussed in more detail in Chapter 4 (Falanga, M., Bozzo, E., Stella, L., et al. 2007, A&A, 464, 807). Chapters 5 and 6 focus on X-ray observations of two low mass X-ray binaries, 4U 2129+47 and XTEJ1701-407. In the case of 4U 2129+47 we present the results of two XMM-Newton observations. Our analysis of these data revealed a delay of ~190 s measured across two eclipses separated by ~22 days. We show that this delay can be naturally explained as being due to the orbital motion of the binary with respect to the center of mass of a triple star and is thus probably the first X-ray signature of the triple nature of an X-ray binary (Bozzo, E., Stella, L., Papitto, A., et al. 2007, A&A, 476, 301). In the case of XTEJ1701-407 we report on the first type I X-ray burst observed from this source and discuss the results of the data analysis in the context of the modern theories of nuclear burning on the NS surface (Falanga, M., Cumming, A., Bozzo, E., et al. 2008, A&A, in press [astro-ph/0901.0314]). In the second part of this thesis (Chapters 7, 8) we concentrate on high mass X-ray binaries. In Chapter 7 we analyze in-depth the accretion process in wind-accreting binaries and apply this scenario to interpret the behaviour of a newly discovered subclass of high mass X-ray binaries, collectively termed supergiant fast X-ray transients. We suggest that these sources might host ultra-magnetized ("magnetar") NSs, and can thus provide the very first opportunity to detect and study magnetars in binary systems (Bozzo, E., Falanga, M., Stella, L. 2008, ApJ, 683, 1031). In Chapter 8 we report on an XMM-Newton observation of the supergiant fast X-ray transient IGRJ16479-4514. The timing, spectral and spatial analysis of this observation revealed a complex phenomenology that could be interpreted in terms of an eclipse by the supergiant companion, with some residual X-ray flux during the eclipse resulting from both scattering local to the source and by an interstellar dust halo along the line of sight to IGR J16479-4514 (Bozzo, E., Stella, L., Israel, G., et al. 2008, MNRAS, 391, L108). Finally, we provide a briefly outline of the possible future development in the research fields of this thesis.

X-ray binaries are binary stars composed of a compact object (a black hole, a neutron star) accreting matter from a companion star. These sources can be considered perfect astrophysical laboratories to test our knowledge of, e.g., General Relativity and Magneto-Hydrodynamics. Accretion is the key phenomenon characterizing these systems, but it is not always completely efficient. In many systems, ejections of matter are also observed, e.g. in the form of jets and winds, or also suggested, e.g. to explain the observed strong orbital expansion of a number of systems. Furthermore accretion and ejection seems to be somehow interconnected but the nature of this correlation is not completely clear. The purpose of this thesis is the study of a number of cases where the accretion is imperfect and mass losses have to be taken into account to correctly model the physical properties of the binaries. In the first of the featured projects, I focus on the spectral study of the accretion flow in the Neutron Star (NS) Low Mass X-ray Binary (LMXB) 1RXS J180408.9-342058, an intriguing system which in the past exhibited “very faint” phases of activity. I performed a spectral analysis of data collected by different X-ray telescopes, i.e. INTEGRAL, Swift and NuSTAR, The study led to several interesting results, in particular the observation of the intermediate spectral state, hard to catch in NS LMXBs because very short-lived, and new constraints on the nature of the companion star, which exclude the hypothesis of a helium dwarf companion as suggested in the past. The second project presents a systematic study of (almost) all known Accreting Millisecond X-ray Pulsars (AMXPs), i.e. LMXBs hosting an X-ray pulsar spinning at millisecond periods, with the aim of looking for indications of non-conservative mass-transfer in this class. Comparing this observed luminosity averaged over twenty years with the one expected from the theory in a conservative scenario, I found that over a sample of 19 sources, around one half of it shows indications for mass losses. The third project in this thesis is dedicated to jets, the most known form of mass ejection in X-ray binaries. Jets are characterized by flat radio-to-mid-IR spectra, which have been modelled in the last few decades using the Internal Shocks model ISHEM. The basic idea of this model consists in using the observed X-ray variability as a proxy for the fluctuations of the Lorentz factor in the ejected shells along the jet. I applied the model on the multi-wavelength data set of the NS LMXB 4U 0614+091. I found that ISHEM describes satisfactorily the data only in two cases: using the X-ray variability but in non-conical geometry or either in conical geometry but using flicker noise instead of the X-ray variability. The final project of my thesis aims at testing a unified accretion-ejection model to the Black Hole LMXB MAXI J1820+070. The model considers the accretion flow in X-ray binaries as two-fold, comprising a truncated geometrically thin disk far from the Black Hole and a so-called jet emitting disk serving as the base of the jet close to the Black Hole. Interestingly, the model allows not only to describe the X-rays data, but also to predict the radio power emitted by the jet. In order to test the model, I used X-rays data from Swift and NuSTAR. The preliminary results of the spectral fitting suggest that the model is indeed effective in describing the observed X-ray spectra. Furthermore, the analysis reveals the need for describing the reflection spectrum with two reflection components instead of one: the origin of such intriguing component, if confirmed, will be object of future investigations.

>Magister Scientiae - MSc
We investigate the link between environment and radiative accretion efficiency using a sample of 8946 radio-loud AGN detected at 1 − 2 GHz in the SDSS Stripe 82 region. We quantify their environments using the surface-density parameter, ƩN, which measures galaxy density based on distances to Nth nearest neighbours. Comparing Ʃ2 and Ʃ5 between AGN and control galaxies, we obtain relative densities that quantify the degree of galaxy clustering around each AGN. Using this, we examine the relation between density and the HERG-LERG dichotomy (accretion-modes) classified using a 1.4 GHz luminosity (L1.4GHz) threshold. Our results indicate that, in the low-redshift interval (0.1 < z < 0.2), LERGs occupy environments denser than the field. At intermediate redshifts (0.2 < z < 1.2), both LERGs and HERGs occupy regions denser than the field. Spearman’s rank tests show that correlations between density and L1.4GHz in both redshift intervals are weak. We conclude that the absence of a strong correlation is confirmation of the idea that galaxy density plays a more secondary role on AGN activity and also, accretion-mode classification (both measured using L1.4GHz). It is likely that the rate of gas accretion or properties of galactic-scale magnetic fields correlate more strongly with L1.4GHz, hence being primarily influential.
National Research Foundation (NRF)

京都大学
0048
新制・課程博士
博士(理学)
甲第22251号
理博第4565号
新制||理||1655(附属図書館)
京都大学大学院理学研究科物理学・宇宙物理学専攻
(主査)教授 嶺重 慎, 准教授 前田 啓一, 教授 長田 哲也
学位規則第4条第1項該当
Doctor of Science
Kyoto University
DFAM

Jatos e discos de acreção associados a objetos galácticos e extragalácticos tais como, microquasares (i.e., buracos negros de massa estelar presentes em alguns sistemas binários estelares), núcleos ativos de galáxias (NAGs) e objetos estelares jovens (OEJs), frequentemente exibem eventos de ejeção de matéria quase periódicos que podem fornecer importantes informações sobre os processos físicos que ocorrem nas suas regiões mais internas. Entre essas classes de objetos, os microquasares com emissão transiente em raios-X vêm sendo identificados em nossa Galáxia desde a última década, e tal como os NAGs e quasares distantes, alguns desses sistemas também produzem jatos colimados com velocidades aparentemente superluminais, não deixando dúvidas de que se tratam de um gás ejetado com velocidades relativísticas. Um exemplo amplamente observado em comprimentos de onda do rádio aos raios-X é o microquasar GRS 1915+105 (e.g., Dhawan et al.,2000), que foi o primeiro objeto galáctico a exibir evidências de um jato com movimento aparentemente superluminal (Mirabel e Rodríguez, 1998, 1994). Um modelo para explicar a origem dessas ejeções superluminais, bem como a emissão rádio sincrotrônica em flares não muito diferentes dos que ocorrem na coroa solar, foi desenvolvido por de Gouveia Dal Pino e Lazarian (2005), onde é invocado um processo de reconexão magnética violenta entre as linhas de campo magnético que se erguem do disco de acreção e as linhas da magnetosfera da fonte central. Em episódios de acreção onde a razão entre a pressão efetiva do disco e a pressão magnética diminui para valores menores ou da ordem de 1 e as taxas de acreção se aproximam da taxa crítica de Eddington, a reconexão pode tornar-se violenta e libera grandes quantidades de energia magnética em pouco tempo. Parte dessa energia aquece o gás, tanto da coroa quanto do disco, e parte acelera as partículas a velocidades relativísticas por um processo de Fermi de primeira ordem, pela primeira vez estudado em zonas de reconexão magnética por esses autores, produzindo um espectro sincrotrônico de lei de potência com índice espectral comparável às observações. Neste trabalho realizamos um estudo complementar, iniciado por Piovezan (2009), no qual generalizamos o modelo acima descrito para o caso dos NAGs. Nesse estudo, constatamos que a atividade de reconexão magnética na região coronal, na base de lançamento do jato, pode explicar a origem das ejeções relativísticas, dos microquasares aos NAGs de baixa luminosidade (tais como galáxias Seyfert e LINERS). A potência liberada em eventos de reconexão magnética em função das massas dos buracos negros dessas fontes, de 5 massas solares a 10^10 massas solares, obedece a uma correlação que se mantém por todo esse intervalo, abrangendo 10^9 ordens de magnitude. Essa correlação implica em uma dependência quase linear (em um diagrama log-log), aproximadamente independente das características físicas locais dos discos de acreção dessas fontes. Além do mais, ela é compatível com o chamado plano fundamental, obtido empiricamente, que correlaciona a emissão rádio e raios-X dos microquasares e NAGs às massas dos seus buracos negros (veja Merloni et al., 2003). Assim, o modelo de de Gouveia Dal Pino e Lazarian (2005), oferece uma interpretação física simples para a existência dessa correlação empírica, como devida à atividade magnética coronal nessas fontes. Já os quasares e NAGs mais luminosos não satisfazem à mesma correlação, possivelmente porque a densidade ao redor da região coronal nessas fontes é tão alta que mascara a emissão devida à atividade magnética. A emissão rádio nesses casos deve-se, possivelmente, a regiões mais externas do jato supersônico, onde ele já expandiu o suficiente para tornar-se opticamente fino e visível, e onde os elétrons relativísticos são possivelmente produzidos em choques (veja também de Gouveia Dal Pino et al., 2010a,b). Paralelamente, investigamos a formação de eventos de reconexão magnética através de simulações magnetohidrodinâmicas axissimétricas (2.5D-MHD), da interação entre o campo magnético poloidal ancorado no disco de acreção viscoso (satisfazendo ao modelo padrão de Shakura e Sunyaev, 1973) e a magnetosfera dipolar da fonte central em rotação. Para esse fim, consideramos condições iniciais semelhantes às dos OEJs. Nos testes preliminares aqui realizados, a reconexão magnética das linhas ocorre em presença de uma resistividade numérica, que não é intensa o bastante para determinar um processo de reconexão a taxas da ordem da velocidade de Alfvén, ou seja, ela é essencialmente lenta. Ainda assim, pudemos identificar alguns dos efeitos previstos pelo modelo de reconexão magnética rápida aqui estudado. Por exemplo, verificamos que a frequência e a intensidade com que eventos de reconexão magnética podem ocorrer é sensível tanto à topologia inicial do campo magnético do sistema quanto às taxas de acreção do disco (como previsto pelo modelo de de Gouveia Dal Pino e Lazarian, 2005), de modo que tais eventos ocorrem de forma mais eficiente em regimes de alta taxa de acreção. Finalmente, além da investigação sobre o desenvolvimento de eventos de reconexão magnética, pudemos também examinar a partir das simulações a formação natural de funis de acreção, os quais são colunas de acreção que conduzem gás do disco para a superfície da fonte central através das linhas do campo magnético. Os resultados desse estudo foram comparados com as observações de funis de acreção de objetos estelares jovens.
Jets and accretion disks associated with galactic and extragalactic objects such as microquasars (i.e., stellar-mass black holes occurring in some binary stellar systems), active galactic nuclei (AGNs) and young stellar objects (YSOs), often exhibit quasi-periodic ejections of matter that may offer important clues about the physical processes that occur in their inner regions. Among these classes of objects, microquasars with transient emission in X-rays have been identified in our Galaxy since the last decade and like AGNs and distant quasars, some of them also produce collimated jets with apparent superluminal speeds, leaving no doubt that we are also dealing with ejected gas with relativistic velocities. One example widely investigated from radio wavelengths to X-rays is the microquasar GRS 1915+105 (e.g., Dhawan et al.,2000), which was the first Galactic object to show evidence of a jet with apparent superluminal motion (Mirabel e Rodríguez, 1998, 1994). A model to explain the origin of the superluminal ejections and the synchrotron radio emission in flares which are not very different from those occurring in the solar corona, was developed by de Gouveia Dal Pino e Lazarian (2005), where they invoked a process of violent magnetic reconnection between the magnetic field lines that arise from the accretion disk and the lines of the magnetosphere of the central source. In accretion episodes where the ratio between the effective disk pressure and magnetic pressure decreases to values smaller than the unity and the accretion rate approaches the critical Eddington rate, the reconnection may become violent and releases large amounts of magnetic energy in a short time. Part of this energy heats the coronal and the disk gas and part accelerates particles to relativistic velocities through a first-order Fermi-like process, which was investigated for the first time in magnetic reconnection by these authors and results a synchrotron radio power-law spectrum that is compatible to the observations. In the present work we conducted a complementary study, initiated by Piovezan (2009), which generalize the model described above for the case of AGNs. We found that the activity due to magnetic reconnection in the coronal region, at the base of the launching jet, can explain the origin of relativistic ejections from microquasars to low luminous AGNs (LLAGNs, such as Seyfert galaxies and LINERs). The power released by magnetic reconnection events as a function of the black hole masses of these sources, between 5 solar mass and 10^10 solar mass, obeys a correlation that is maintained throughout this interval, spanning 10^9 orders of magnitude. This correlation implies an almost linear dependence (in a log-log diagram), which is approximately independent of the physical properties of the accretion disks of these sources. Moreover, it is compatible with the so-called fundamental plan obtained empirically, which correlates the radio and X-rays emission of microquasars and AGNs with the masses of their black holes (see Merloni et al., 2003). Thus, the model of de Gouveia Dal Pino e Lazarian (2005) provides a simple physical interpretation for the existence of this empirical correlation as due to coronal magnetic activity in these sources. More luminous AGNs and quasars do not seem to obey the same correlation, possibly because the density around the coronal region in these sources is so high that it \"masks\" the emission due to the magnetic activity. The radio emission in these cases is possibly due regions further out of the supersonic jet, where it has already expanded enough to become optically thin and visible and where the relativistic electrons are probably accelerated in shocks (see also de Gouveia Dal Pino et al., 2010a,b). In addition, we investigated the development of magnetic reconnection events through axisymmetric magnetohydrodynamic simulations (2.5D-MHD) of the interaction between the poloidal magnetic field that arises from the viscous accretion disk (which satisfies the standard model of Shakura e Sunyaev, 1973) and the dipolar magnetosphere of the rotating central source. To this aim, we considered initial conditions which are compatible to those of YSOs. In the preliminary tests conducted here, magnetic reconnection occurs in the presence of numerical resistivity only, which is not intense enough to determine a process of reconnection with rates of the order of the Alfvén speed, i.e., it is essentially slow. Nevertheless, we were able to identify some of the effects predicted by the model of fast magnetic reconnection studied here. For example, we found that the frequency and strength with which events of magnetic reconnection can occur is sensitive to both the initial topology of the magnetic field of the system and the accretion disk rates (as predicted by the model of de Gouveia Dal Pino e Lazarian, 2005), so that such events occur more efficiently under high accretion rates. Finally, besides the investigation of the development of magnetic reconnection events, we could also examine in our numerical studies the natural formation of funnel flows which are accretion columns that transport gas from the accretion disk to the surface of the central source along the magnetic field lines. The results of these studies were compared with the observations of funnel flows in young stellar objects.

This thesis presents results from two simulation studies of galaxy formation. In the first project, a dark-matter-only HORIZON simulation is used to investigate the environment and redshift dependence of mass accretion onto haloes and subhaloes. It is found that the halo accretion rate varies less strongly with redshift than predicted by the Extended Press--Schechter formalism, and that low accretion events may drive the radio-mode feedback hypothesized for recent galaxy formation models. The subhaloes at $z<0.5$ in the simulation accrete at higher rates than haloes, on average, and it is argued that this is due to their enhanced clustering at small scales. There is no dependence of accretion rate on environment at $zsim2$, but a weak correlation emerges at $zleq0.5$. The results further support previous suggestions that at $z>1$, dark matter haloes and their associated black holes grew coevally, but imply that haloes could be accreting at fractional rates that are up to a factor of 3--4 higher than their associated black holes by the present day. In the second project, outputs from one of the Adaptive Mesh Refinement NUT simulations are analyzed in order to test whether filamentary flows of cold gas are responsible for the build-up of angular momentum within a Milky Way type disk at $zgeq3$. A set of algorithms are presented that use the resolved physical scale of $12,mathrm{pc}$ to identify: (i) the central gas disk and its plane of orientation; (ii) the complex individual filament trajectories that connect to the disk, and; (iii) the infalling satellites. The results suggest that two filaments at $zgtrsim 5.5$, which later merge to form a single filament at $zlesssim 4$, drive the angular momentum and mass budget of the disk between $3lesssim zlesssim 8$, whereas luminous satellite mergers make negligible fractional contributions. These findings hence provide strong quantitative evidence that the growth of thin disks in low mass haloes at high redshift is supported via inflowing streams of cold gas.

Contesto scientifico Le stelle T Tauri classiche (CTTSs) sono oggetti stellari giovani poco massivi che accrescono massa dal loro disco circumstellare. Il disco si estende internamente fino al raggio di tron- camento, ovvero dove il campo magnetico e` abbastanza intenso da sollevare il materiale dal piano del disco e da incanalarlo formando delle colonne di accrescimento (Koenigl 1991). Il materiale incanalato precipita sulla stella e impatta sulla superficie stellare. Gli impatti gener- ano shocks caldi. Le CTTSs sono anche caratterizzate da un alto livello di attivita` coronale, come rivelato dalle osservazioni in banda X (e.g. Favata et al. 2005). Questa attivita` coronale e` prodotta principalmente da flares energetici. Obiettivo di questo lavoro In questo lavoro abbiamo investigato il processo di accrescimento di massa nelle CTTSs. Ab- biamo studiato se l’accrescimento dal disco puo ́ essere il risultato di un’ attivita` coronale e ab- biamo analizzato la struttura e la dinamica della colonna di accrescimento nella regione di im- patto. Abbiamo sviluppato modelli numerici che descrivono: un sistema stella disco sotto gli effetti di un’attivita` coronale in prossimita` della superficie del disco; l’impatto di una colonna di accrescimento su una CTTS. Abbiamo investigato se un’intensa attivita` coronale causata da flares che avvengono in prossimita` del disco di accrescimento puo ́ perturbare la stabilita` del disco interno, distruggere la parte interna del disco e possibilmente innescare fenomeni di accrescimento con tassi di ac- crescimento confrontabili con quelli osservati nelle CTTSs (Colombo et al. 2016). A tal fine, abbiamo sviluppato delle simulazioni 3D magnetoidrodinamiche di una protostella magne- tizzata circondata da un disco di accrescimento. Il modello considera gli effetti della gravita` della stella, della viscosita` nel disco, della conduzione termica (considerando pure gli effetti della saturazione del flusso), delle perdite radiative da parte di plasma otticamente sottile e una funzione parametrizzata per descrivere i flares. Abbiamo esplorato casi caratterizzati da una configurazione di campo magnetico stellare costituita da un ottupolo piu` un dipolo e da diverse densita` o differenti livelli di attivita` di flaring. Per quanto riguarda lo studio degli impatti di accrescimento, abbbiamo analizzato gli ef- fetti della radiazione emergente dal plasma riscaldato dallo shock sulla struttura del materiale pre-shock in caduta. A tal fine abbiamo aggiornato un modulo radiativo implementato nel codice PLUTO (Mignone et al. 2007, 2012) che assume il regime termodinamico locale (LTE). Il modulo e` stato generalizzato anche al caso non-LTE (Colombo et al. 2019a). Abbiamo stu- diato se la radiazione emergente dalla regione di shock viene significativamente assorbita dal materiale ancora non scioccato in caduta libera, e se questo assorbimento puo ́ generare un riscaldamento del materiale pre-shock. Abbiamo sviluppato un modello radiativo magne- toidrodinamico che descrive una colonna di accrescimento che impatta sulla superficie di una CTTS (Colombo et al. 2019b). Il modello considera gli effetti della gravita` della stella, della conduzione termica, delle perdite radiative e anche dell’assorbimento della radiazione dal ma- teriale nel regime non-LTE. Risultati Per quanto riguarda gli effetti sulla stabilita` del disco dell’attivita` di flaring, abbiamo osser- vato, come risultato delle simulazioni di una attivita` flaring intensa, la formazione di diverse loops di plasma caldo confinate dal campo magnetico che collegano la superficie della stella al disco. Tutte queste loops costituiscono una calda corona estesa che produce una luminosita` in banda X confrontabile con i valori tipici osservati nelle CTTSs (Colombo et al. 2019c). L’intensa attivita` di flaring vicina al disco puo ́ perturbarne significativamente la stabilita`. I flares generano un’onda di pressione, la quale viaggia attraverso il disco e ne modifica la con- figurazione. In questo modo, delle colonne di accrescimento vengono generate dall’attivita` di flaring, contribuendo cosi al rate di accrescimento sulla stella. I rates di accrescimento derivati dalle simulazioni assumono valori compresi tra 10−10 e 10−9 M⊙yr−1 (Colombo et al. 2019c). Le colonne di accrescimento possono essere perturbate a loro volta dai flares e interagiscono l’una con l’altra, fondendosi per formare colonne piu` grandi. Il risultato di questa dinamica e` che le colonne di accrescimento hanno una struttura molto disomogenea, costituita da blob densi. Questa disomogeneita` potrebbe essere l’origine della variabilita` osservata nelle strutture di accrescimento (Alencar et al. 2018). Il modulo radiativo in non-LTE che e` stato sviluppato per studiare le dinamica e la strut- tura della regione d’impatto nelle CTTSs e` stato validato utilizzando diversi test. In parti- colare, abbiamo realizzato un modello per analizzare la struttura di un semplice shock radia- tivo come descritto da Ensman (1994). Le nostre soluzioni sono in accordo con le soluzioni semi-analitiche (quando disponibili) con una discrepanza massima del 7%. Inoltre, abbiamo provato che un approccio in non-LTE cambia significativamente la struttura e la dinamica della regione di impatto, dando origine ad un precursore radiativo e ad una maggiore esten- sione della regione post-shock rispetto al caso LTE (Colombo et al. 2019a). Il nostro modello radiativo, che descrive la regione di impatto di una colonna di accresci- mento sulla cromosfera stellare, prova che parte della radiazione emessa dal plasma post-shock (≈ 70%) viene assorbita dal materiale freddo pre-shock in caduta sulla stella. L’irraggiamento riscalda il materiale, in caduta sulla stella, ancora non scioccato fino a temperature di 105K. Il materiale caldo forma un precursore termico che emette nella banda UV. I risultati di questo progetto di dottorato potrebbero aiutare nella soluzione di alcune questioni aperte sulle CTTSs. Per esempio, abbiamo provato che un’intensa attivita` di flaring in prossimita` del disco puo ́ perturbarne la stabilita` e generare colonne di accrescimento con una forte strutturazione in densita` come quelle recentemente osservate da Alencar et al. (2018). Inoltre, utilizzando il nostro modello radiativo possiamo spiegare naturalmente l’origine dei complessi spettri in banda UV provenienti dalle regioni di impatto (Ardila et al. 2013) e an- che il fatto che i rates di accrescimento ottenuti dalle osservazioni in banda UV sono sistemati- camente piu` grandi di quelli ottenuti in banda X (Curran et al. 2011). Infatti, il nostro modello radiativo predice la presenza di un precursore termico che emette in banda UV. Questo pre- cursore: 1) Aumenterebbe il flusso UV prodotto dalla regione di impatto senza assumere un piu` alto rate di accrescimento e 2) potrebbe generare un flusso UV prodotto da plasma a ve- locita` di caduta libera sulla stella, quindi con Doppler shifts molto piu` grandi di quelli generati dalla regione post-shock. Questo potrebbe spiegare le componenti ad alto redshift osservate in banda UV (Ardila et al. 2013).
Context Classical T Tauri Stars (CTTSs) are young low-mass stellar objects that accrete mass from their circumstellar disks. The disks extend internally up to the truncation radius, where the mag- netic field is strong enough to lift up the material from the disk plane and to funnel the mate- rial forming accretion columns (Koenigl 1991). The funneled plasma falls down onto the star and hits the stellar surface. The impacts generate hot shocks. CTTSs are, also, characterized by high levels of coronal activity, as revealed by X-ray observations (e.g. Favata et al. 2005). This coronal activity is mainly produced by energetic flares. Aims of this work In this work we investigated the mass accretion process in CTTSs. We studied if accretion from the disk to the star might occur as a result of a coronal activity, and we analyzed the structure and the dynamics of the accretion column plasma in the impact regions. We de- veloped numerical models that describe: a star-disk system subject to the effects of a coronal activity in proximity of the disk surface; the impact of an accretion column onto the surface of a CTTS. We investigated if an intense coronal activity due to flares that occur close to the accretion disk may perturb the stability of the inner disk, disrupt the inner part of the disk, and possi- bly trigger accretion phenomena with mass accretion rates comparable with those observed in CTTSs (Colombo et al. 2019c). To this end, we modeled a magnetized protostar surrounded by an accretion disk through 3D magnetohydrodynamics simulations. The model takes into account the gravity from the central star, the effects of viscosity in the disk, the thermal con- duction (including the effects of heat flux saturation), the radiative losses from optically thin plasma, and a parameterized heating function to trigger the flares. We explored cases charac- terized by a dipole plus an octupole stellar magnetic field configuration and by either different densities of disk or different levels of flaring activity. As it concerns the study of accretion impacts, we analyzed the effects of radiation emerg- ing from the shock-heated plasma at the base of accretion columns on the structure of the pre- shock downfalling material. To this end, we upgraded a module handling the local thermody- namic equilibrium (LTE) radiation-hydrodynamics (RHD) in the PLUTO code (Mignone et al. 2007, 2012), which we have extended to handle also the non-LTE regime (Colombo et al. 2019a). Then, we investigated if a significant absorption of radiation arising from the shock heated plasma occurs in the unshocked downfalling material, and if it leads to a pre-shock heating of the accreting gas. We developed a radiation hydrodynamics model that describes an accretion column impacting onto the surface of a CTTS (Colombo et al. 2019b). The model takes into account the stellar gravity, the thermal conduction, and the effects of both radia- tive losses and absorption of radiation by matter in non local thermodynamic equilibrium conditions. Results As it concerns the effects of flaring activity on the disk stability, we observed, as a result of the simulated intense flaring activity, the formation of several magnetic loops confining hot plasma that link the star to the disk. All these loops build up a hot extended corona with an X- ray luminosity comparable to typical values observed in CTTSs (Colombo et al. 2019c). The intense flaring activity close to the disk can strongly perturb the disk stability. The flares trig- ger overpressure waves which travel through the disk and modify its configuration. Accretion funnels may be triggered by the flaring activity, thus contributing to the mass accretion rate of the star. Accretion rates derived from the simulations range from 10−10 to 10−9M⊙yr−1 (Colombo et al. 2019c). The accretion columns can be perturbed by the flares and can interact with each other, possibly merging together in larger streams. As a result, the accretion pattern can be rather complex: the streams are highly inhomogeneous, with a complex density struc- ture, and clumped. This inhomogenity may be the origin of the variability observed in the structure of the accretion columns (Alencar et al. 2018). The non-LTE radiation module developed to study the dynamics and structure of the im- pact region of CTTSs has been validated through different tests. In particular, we modeled the structure of a radiative shock, simulating a simple shock case as described by Ensman (1994). The agreement between our solutions and the semi-analytical solutions (when available) is good, with a maximum error of 7%. Moreover, we have proven that a non-LTE approach change significantly the structure and the dynamics of the impact regions, leading to a ra- diative precursor and a greater extension of the post-shock region compared to the LTE case (Colombo et al. 2019a). Our radiative model describing the impact of an accretion column onto the stellar chro- mosphere shows that part of radiation emitted by the post-shock plasma (≈ 70%) is absorbed by the pre-shock accretion column immediately above the slab. The irradiation heats the downfalling unshocked material up to ≈ 105K. This hot material forms a precursor region that emits in the UV band. The results of this PhD project may address some open questions regarding CTTSs. For instance, we proved that an intense flaring activity in proximity of the disk may perturb its stability and may generate accretion columns highly structured in density and characterized by clumps as recently observed, for example, by Alencar et al. (2018). Moreover, with our radiation model we may naturally explain the origin of the complex UV spectra arising from impact regions (Ardila et al. 2013) and the fact that accretion rates derived from UV observations are systematically higher than rates inferred from X-ray ob- servations (Curran et al. 2011). In fact, our model predicts the presence of a precursor region emitting in the UV. This region: 1) would increase the UV flux arising from the impact with- out assuming higher accretion rates and 2) may generate an UV flux produced by plasma at free fall velocity, thus with Doppler shifts stronger than those generated by the post-shock plasma. This may explain the high redshifts and broadening observed in emission lines of UV spectra (Ardila et al. 2013).

Active Galactic Nuclei (AGN) are among the most powerful sources of energy in the Universe. The “central engine” is likely a super massive (M >~ 10^6 M_sun) black hole accreting matter from the nuclei of host galaxies. In order to study the AGN demography, formation, evolution, accretion physics and galaxy feedback processes we need a reliable method to estimate the black hole mass. The most reliable ones (direct methods) can be applied only to a few tens of nearby AGNs, strongly limiting the possibility to perform statistical studies on large samples and high redshift sources. The issue of black hole mass estimation on large samples of Type 1 AGN is addressed using an indirect (hence less reliable) procedure: the Single Epoch Virial (SEV) method. In this thesis I discuss the assumptions, biases and possible systematic errors affecting the SEV estimates, and propose a completely independent method to estimate the Type 1 AGN black hole mass. The method is based on the assumption that accretion occur through a standard Shakura & Sunyaev (1973) accretion disk. The calibration is performed by studying the statistical relationships between the broad–band spectral features of Type 1 AGN and the optical emission line luminosities. I apply the method to a sample of 23 radio–loud narrow–line Seyfert 1 galaxies, for which the SEV masses were suspected to be strongly biased. The resulting black hole mass estimates are significantly greater than SEV ones. I discuss the reliability of these estimates, and the consequence on the physical interpretation of the class of narrow–line Seyfert 1 galaxies in the framework of the AGN unified model.

The scope of this thesis is to investigate the structure and evolution of protoplanetary disk. High resolution observational techniques such as high resolution optical/infrared spectroscopy and infrared interferometry are well suited for this purpose. High resolution spectroscopy allow to resolve the velocity profile of disk emission lines and determine some important parameters such as the disk geometry and the physical conditions of the line emitting region. Infrared interferometry allows to spatially resolve and constraint the disk geometry within the planet forming region. The work presented here aims at contributing to the comprehension of the disk structure and evolution at three different evolutionary stages: 1) the early phase when the system is still (partially) embedded in a remnant of the molecular cloud; 2) the so-called Class II phase (from the classification of Lada 1987). At this stage gas and dust evolve rapidly leading to drastic changes of the disk structure; 3) the transition phase from Class II to Class III when gas and dust are dissipated leaving, eventually, a planetary system. During the early phases of disk evolution the star-disk- envelope system experience powerful instability which are related to rapid enhancement of the mass accretion rate on a timescale of few months. These events are recognizable as so-called FU Orionis outbursts, in which the optical brightness of the system can increase by 4 or more magnitudes. The mass accretion rate increases from 10-7-- 10-8 Myr to 10-3 --10-4 Myr. Statistical studies suggest that young low-mass stars experience several FU Orionis outburst. In late 2003, the young star V1647 Orionis in the L1630 Ori cloud within the Orion B molecular cloud went into outburst. The outburst shares some properties of the FU Orionis outburst. Following spectro-photometric observations confirmed the nature of the outburst as a disk-instability event. We also find, for the first time, probe of a direct link between an accretion event and the ejection of an Herbig-Haro object (HH). During the Class II phase dust coagulation and grain growth occur. This is the first step of planet formation. We applied high resolution optical spectroscopy and infrared interferometry to direct compare gas and dust emission from the disk surface of three protoplanetary disks. This study gives some insight on the relative distribution of gas and dust in disk and on the temporal evolution of the two components. A physical decoupling of gas and dust may occur leading to changes in the relative structure of the two (different scale height) and to rapid settling of dust on the disk midplane. This may increase the dust-to-gas mass ratio in the disk interior and, according to recent simulation, may trigger the formation of planetesimals via gravitational instability. The transition phase from a Class II to a Class III system is characterized by various processes which dissipate the disk material. In particular, viscous accretion and photo-evaporation are very efficient in removing disk material and planet formation is likely in competition with disk dispersion. For this reason, a fundamental quantity is the mass accretion timescale, i.e. the time at which the disk accretion phase ceases. In turn, the time at which the disk accretion phase ceases is a strong constraint on the gas dissipation timescale, relevant for the formation of giant planets. We have observed a number of young stellar clusters of different age aimed at tracing the evolution viscous accretion with time. The preliminary results show that the accretion seems to cease at similar age of the dust dissipation, i.e. within 5 -- 10 Myr.

Etude hydrodynamique du phenomene d'accretion sur un objet compact et stabilite dynamique d'un tel ecoulement. On effectue une simulation numerique d'un flot bidimensionnel keplerien pour permettre d'estimer l'echelle de temps dynamique en fonction de la turbulence

Cette thèse porte sur l'évolution orbitale de planètes dans un disque protoplanétaire constitué de gaz et de poussières autour d'une jeune étoile (les dix premières millions d'années après la formation de l'étoile). L'interaction gravitationnelle entre les planètes et le gaz du disque protoplanétaire change rapidement la distance des planètes à leur l'étoile. C'est ce que l'on appelle la migration planétaire. De nombreuses études ont examiné comment la direction et la vitesse de cette migration planétaire dépendent de la masse de la planète et des propriétés physiques du gaz des disques, afin d'interpréter les propriétés orbitales des exoplanètes. Les poussières sont la plupart du temps négligées dans ce type d'études car elles représentent une masse très inférieure à celle du gaz dans les disques, et ont donc a priori moins d'impact sur la migration planétaire que le gaz. Cependant, la poussière a connu un regain d'intérêt au cours des dernières années en raison du nombre croissant d'observations multi-longueurs d'onde spatialement résolues de l'émission de la poussière des disques protoplanétaires. Cette émission, en particulier l'émission de poussières froides telle qu'observée en radio par l'interféromètre ALMA, peut révéler des structures (spirales, anneaux sombres et brillants, asymétries en forme de croissant...) très similaires à celles engendrées par les interactions disque-planète. Pour cette raison, les structures dans l'émission des poussières sont souvent attribuées à la présence de planètes non-détectées. Ces structures soulignent donc la nécessité de mieux comprendre dans quelle mesure les interactions disque-planète en général, et la migration planétaire en particulier, jouent un rôle dans l'émission des poussières des disques protoplanétaires. Cette problématique constitue la pierre angulaire de mon travail de thèse. Pour ce faire, j'ai réalisé des simulations hydrodynamiques 2D et 3D modélisant à la fois la poussière et le gaz d'un disque protoplanétaire dans lequel une ou plusieurs planètes se forment et migrent. Les résultats des simulations ont ensuite été post-traités par des calculs de transfert radiatif dans la poussière et le gaz afin de calculer des cartes synthétiques de l'émission du disque, directement comparables aux observations. Je montre en particulier que la migration intermittente à grande échelle d'une planète dont la masse est typiquement comprise entre celle de Saturne et celle de Jupiter est capable de générer des anneaux multiples de poussières millimétriques. Ces structures annulaires dans la distribution des poussières prennent en fait la forme d'anneaux sombres et brillants dans l'émission radio, qui ressemblent à ceux observés dans plusieurs disques. Le contraste d'intensité entre les anneaux sombres et brillants est détectable avec la sensibilité actuelle d'ALMA. D'autres aspects explorés dans cette thèse comprennent les signatures cinématiques de la présence d'une planète massive dans l'émission de gaz, et l'émission de poussières aux points de Lagrange d'une planète qui migre
The subject of this PhD thesis is related to the orbital evolution of planets in a protoplanetary disk made of gas and dust around a young star (the first ten million years after the star's formation). The gravitational interaction between planets and the protoplanetary disk gas rapidly changes the distance between the planets and the star. This is known as planetary migration. Many studies have examined how the direction and speed of planetary migration depend on the planet's mass and the physical properties of the disk gas, with the aim to explain the orbital properties of exoplanets. Dust is most often discarded in this kind of studies because its mass content is much smaller than that of the gas and it should therefore have a negligible impact on planetary migration. Yet, dust has gained considerable importance over the past few years owing to the rapidly growing number of multi-wavelength spatially resolved observations of the dust emission in protoplanetary disks. These observations show that dust emission, in particular the cold dust emission at radio wavelengths such as that probed by the ALMA interferometer, can feature structures (spirals, rings and gaps, crescent-shaped asymmetries...) that are very similar to those imparted by disk-planets interactions. For this reason, structures in the dust emission are often interpreted as signatures of the presence of hidden planets. These structures therefore stress the need to better understand how disk-planets interactions generally, and planetary migration more specifically, impact the dust emission in protoplanetary disks. This is the aim of my PhD thesis. To reach this goal, I have carried out 2D and 3D hydrodynamical simulations modeling both the dust and gas of a protoplanetary disk where one or several planets form and migrate. The simulations results have been post-processed by dust and line radiative transfer calculations to compute synthetic maps of the disk emission that are directly comparable to observations. I show that the large-scale intermittent migration towards its star of a planet with a mass typically between those of Saturn and Jupiter is able to generate multiple rings of millimeter dust particles. These annular structures in the dust distribution take the form of bright and dark rings in the radio emission that resemble those observed in several disks. The intensity contrast between bright and dark rings is detectable with ALMA's current sensitivity. Other aspects explored in this thesis include the kinematic signatures of the presence of a massive planet in the gas emission of its disk, and the dust emission at the Lagrange points of a migrating planet

Smoothed Particle Hydrodynamics (SPH) is analysed as the weighted residual method. In particular the analysis focuses on the collocation aspect of the method. Using Monte Carlo experiments we demonstrate that SPH is highly sensitive to node disorder, especially in its symmetrised energy and momentum conserving form. This aspect of the method is related to low [Beta] MHD instabilities observed by other authors. A remedy in the form of the Weighted Differences Method is suggested, which addresses this problem to some extent, but at a cost of losing automatic conservation of energy and momentum. ¶ The Weighted Differences Method is used to simulate propagation of Alfven and magnetosonic wave fronts in [Beta] = 0 plasma, and the results are compared with data obtained with the NCSA Zeus3D code with the Method of Characteristics (MOC) module. ¶ SPH is then applied to two interesting astrophysical situations: accretion on to a white dwarf in a compact binary system, which results in a formation of an accretion disk, and gravitational collapse of a magnetised vortex. Both models are 3 dimensional. ¶ The accretion disk which forms in the binary star model is characterised by turbulent flow: the Karman vortex street is observed behind the stream-disk interaction region. The shock that forms at the point of stream-disk interaction is controlled by the means of particle merges, whereas Monaghan-Lattanzio artificial viscosity is used to simulate Smagorinsky closure. ¶ The evolution of the collapsing magnetised vortex ends up in the formation of an expanding ring in the symmetry plane of the system. We observe the presence of spiralling inward motion towards the centre of attraction. That final state compares favourably with the observed qualitative and quantitative characteristics of the circumnuclear disk in the Galactic Centre. That simulation has also been verified with the NCSA Zeus3D run. ¶ In conclusions we contrast the result of our Monte Carlo experiments with the results delivered by our production runs. We also compare SPH and Weighted Differences against the new generation of conservative finite differences methods, such as the Godunov method and the Piecewise Parabolic Method. We conclude that although SPH cannot match the accuracy and performance of those methods, it appears to have some advantage in simulation of rotating flows, which are of special interest to astrophysics.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2006.
Includes bibliographical references (leaves 61-64).
We consider geometrically thin accretion disks around millisecond X-ray pulsars. We start with the Shakura-Sunyaev thin disk model as a basis and modify the disk equations with a magnetic torque from the central neutron star. Disk solutions are computed for a range of neutron star magnetic fields. We also investigate the effect of different equations of state and opacities on the disk solutions. We show that there are indications of thermal instability in some of the disk solutions, especially for the higher values of 3M. We also explain how the time evolution of the disk solutions can be calculated.
by Antonia Stefanova Savcheva.
S.B.

La gravitation joue un rôle important dans de nombreux domaines de l'astrophysique : elle assure notamment la cohésion et la stabilité des planètes, des étoiles et des disques. Elle est aussi motrice dans le processus d'effondrement de structure et conduit, dès lors qu'un moment cinétique initial est significatif, à la formation d'un disque.Ma thèse est consacrée à l'étude des disques de gaz, et plus particulièrement à la description du potentiel et du champ de gravité qu'ils génèrent dans l'espace et sur eux-mêmes (l'auto-gravitation). Bien que la force de Newton soit connue depuis longtemps, la détermination des interactions auto-gravitantes reste difficile, en particulier lorsque l'on s'écarte significativement de la sphéricité. La principale difficulté tient dans la divergence hyperbolique du Noyau de Green 1/(r'-r) et nécessite un traitement propre. L'approche théorique est intéressante car elle fournit de nouveaux outils (techniques numériques, formules approchées, etc...) qui peuvent aider à produire des solutions de référence et à améliorer les simulations numériques.Dans une première partie, nous introduisons le sujet, les notions et les bases essentielles. Le chapitre $1$ est consacré à une présentation succinte du contexte scientifique et aux motivations de notre travail. Dans le chapitre $2$, nous reproduisons dans ces grandes lignes le cheminement conduisant au développement multipolaire, à partir de l'équation de Poisson et de la formule intégrale de Newton. Il s'agit de l'une des méthodes les plus classiques permettant d'obtenir le potentiel gravitationnel d'un corps. Les deux systèmes de coordonnées les plus utilisées sont mis en avant : sphériques et cylindriques. A travers quelques exemples, nous montrons les limites de cette approche, en particulier dans le cas de l'auto-gravité des disques.Dans une deuxième partie, nous abordons le vif du sujet. Le chapitre $3$ présente l'approche basée sur les intégrales elliptiques que nous retrouverons dans l'ensemble du manuscrit (cas général d'abord, puis cas axi-symétrique). Dans le chapitre $4$, nous établissons un premier résultat concernant le noyau de Green dans des systèmes axi-symétriques et verticalement homogènes : une forme alternative et régulière du noyau, quelque soit le point de l'espace. Nous avons exploité cette nouvelle formule pour déduire une bonne approximation du potentiel des disques géométriquement minces, des anneaux et des systèmes faiblement étendus en rayon. Ceci fait l'objet du chapitre $5$.Dans une troisième partie, nous étudions les effets de bords sur la composante verticale du champ de gravité, $g_z$, causés par un disque mince axi-symétrique. Le chapitre $6$ est dédié à l'approximation de Paczynski \citep{pacz78}, qui permet traditionnellement d'exprimer le champ comme une fonction linéaire de la densité de surface locale. Cette approximation n'est en fait strictement valide que dans le cas du modèle du "plan infini", loin d'un disque réaliste. Près du bord externe des disques où la gravité décroit, l'approximation de Paczynski s'avère assez imprécise (facteur $2$ typiquement), et ne donne pas de bons résultats et doit être corrigée. Toujours dans l'hypothèse d'une homogénéité verticale de la densité, nous avons construit une expression pour $g_z$ qui tient compte de ces effets de bords. Le chapitre $7$ est consacré à ce résultat.Dans une dernière partie, nous relâchons l'hypothèse de symétrie axiale (le disque est discrétisé en cellules cylindriques homogènes). Nous nous sommes inspirés du travail d'\cite{ansorg03} afin d'exprimer, via le théorème de Green, le potentiel d'une cellule cylindrique homogène par une intégrale de contour. Ce résultat s'applique directement aux simulations de disques, où ceux-ci sont découpés en cellules cylindriques, chacune ayant sa propre densité.Une conclusion et quelques perspectives sont données en fin de manuscrit.

Etude des disques d'accretion et de matiere ejectee autour d'etoiles jeunes telles que cep a, w28a2 et g10. 6. Les observations millimetriques a haute resolution font apparaitre des raies qui permettent d'etudier les flots bipolaires des deux premiers objets cites et le disque d'accretion inhomogene en rotation du troisieme. L'observation d'enveloppes froides autour d'objets evolues fait apparaitre une classe d'etoiles peu massives en fin d'evolution, les "pseudo supergeantes f"; une nebuleuse planetaire tres peu evoluee et tres riche en co et la nebuleuse du "lion givre" dont le spectre ir est domine par l'emission de la glace cristalline

The study of high-resolution X-ray spectra of neutron star low-mass X-ray binaries (LMXBs) allows the investigation of the innermost parts of the accretion disk and immediate surroundings of the compact object. The weak magnetic eld of old neutron stars present in such systems allows the accretion disk to approach very close to the compact object, like in black hole X-ray binaries. Using data from X-ray satellites such as XMM-Newton, RXTE, and BeppoSAX, I studied the reection component in two neutron star LMXBs: MXB 1728-34 and 4U 1735-44. I showed that the iron line at 6:4

In this thesis, I present new ultraviolet/optical/infrared photometric and spectroscopic observations of pre-main sequence stars (PMS) that have formed either in metal-poor conditions, or in the vicinity of strong ionising radiation. This includes observations of 235 Classical T Tauri stars in the Lagoon Nebula; 63 Classical T Tauril Herbig Ae stars in the Carina Nebula open cluster Trumpler 14; 24 intermediate mass T Tauri stars in the low-Z Sh 2-284 star-forming region; and one Herbig 8[e] PMS candidate in the metal-poor 30 Doradus region. I measure the accretion rates of these PMS stars using the intensities of the U/Halpha band excess measured through either optical spectra of imaging. Where possible, I use archive infrared photometry in the 1.2-8 micron wavelength range to estimate the PMS disc evolutionary stage. The influence of the surrounding environment on the accretion rate evolution of pre-main sequence stars in these regions is explored using the spatial, and temporal distributions of accretion rate, mass, age and disc stage of PMS stars. In the wide-field photometric data of the Lagoon Nebula, I find that the spatial distributions of PMS stars is a continuum, ranging from dense clustering to relative isolation. Strongly accretion PMS stars are generally clumped together, in close proximity to their natal molecular cloud, whereas weaker, older accretors are relatively space apart. Ionising radiation from early-type stars appears to positively affect accretion rates on scales of 2-3 pc, but no evidence for triggered star formation is found. In addition, the accretion rates measured from Halpha imaging correlate well to those estimated from U-band photometry. In wide-field photometric data of Trumpler 14, I discover a population of PMS candidates nearly 25 Myrs old. I argue that these PMS candidates are a foreground population, approximately 5 Myr old that belong to the Carina Nebula cluster Trumpler 16. Using Halpha spectra of 24 intermediate mass T Tauri stars in Sh 2-284 (Z-0.004), I demonstrate that there is little evidence for a systematic change in accretion rates with metallicity, contrary to previous literature results at Z-0.006-0.002 in the Magellanic Clouds. I suggest that previous studies are likely affected by detection limits and biases. I also present ultraviolet/optical spectra of the Herbig 8[e] PMS candidate VFTS 822 located in the 30 Doradus region of the Large Magellanic Cloud. I discuss the impact of the discovery of VFTS 822 for star formation studies in the Magellanic Clouds, external Galaxies .

Context. Whether or not a self-gravitating accretion disk fragments is still an open issue. There are many different physical and numerical explanations for fragmentation, but simulations often show a non-convergent behavior for ever better resolution. Aims. We aim to investigate the influence of different numerical limiters in Godunov type schemes on the fragmentation boundary in self-gravitating disks. Methods. We have compared the linear and non-linear outcomes in two-dimensional shearingsheet simulations using the VANLE ER and the SUPERBEE limiter. Results. We show that choosing inappropriate limiting functions to handle shock-capturing in Godunov type schemes can lead to an overestimation of the surface density in regions with shallow density gradients. The effect amplifies itself on timescales comparable to the dynamical timescale even at high resolutions. This is exactly the environment in which clumps are expected to form. The effect is present without, but scaled up by, self-gravity and also does not depend on cooling. Moreover it can be backtracked to a well known effect called oversteepening. If the effect is also observed in the linear case, the fragmentation limit is shifted to larger values of the critical cooling timescale.

Using Keck/HIRES spectra (Delta v similar to 7 km s(-1)) we analyze forbidden lines of [O I] 6300 angstrom, [O I] 5577 angstrom. and [S II] 6731 angstrom. from 33 T Tauri stars covering a range of disk evolutionary stages. After removing a high-velocity component (HVC) associated with microjets, we study the properties of the low-velocity component (LVC). The LVC can be attributed to slow disk winds that could be magnetically (magnetohydrodynamic) or thermally (photoevaporative) driven. Both of these winds play an important role in the evolution and dispersal of protoplanetary material. LVC emission is seen in all 30 stars with detected [O. I] but only in two out of eight with detected [S. II], so our analysis is largely based on the properties of the [O. I] LVC. The LVC itself is resolved into broad (BC) and narrow (NC) kinematic components. Both components are found over a wide range of accretion rates and their luminosity is correlated with the accretion luminosity, but the NC is proportionately stronger than the BC in transition disks. The full width at half maximum of both the BC and NC correlates with disk inclination, consistent with Keplerian broadening from radii of 0.05 to 0.5 au and 0.5 to 5 au, respectively. The velocity centroids of the BC suggest formation in an MHD disk wind, with the largest blueshifts found in sources with closer to face-on orientations. The velocity centroids of the NC, however, show no dependence on disk inclination. The origin of this component is less clear and the evidence for photoevaporation is not conclusive.

We develop a simple model to predict the radial distribution of planetesimal formation. The model is based on the observed growth of dust to millimeter-sized particles, which drift radially, pile-up, and form planetesimals where the stopping time and dust-to-gas ratio intersect the allowed region for streaming instability-induced gravitational collapse. Using an approximate analytic treatment, we first show that drifting particles define a track in metallicity-stopping time space whose only substantial dependence is on the disk's angular momentum transport efficiency. Prompt planetesimal formation is feasible for high particle accretion rates (relative to the gas, (M) over dot(p)/(M) over dot greater than or similar to 3 x 10(-2) for alpha = 10(-2)), which could only be sustained for a limited period of time. If it is possible, it would lead to the deposition of a broad and massive belt of planetesimals with a sharp outer edge. Numerically including turbulent diffusion and vapor condensation processes, we find that a modest enhancement of solids near the snow line occurs for centimeter-sized particles, but that this is largely immaterial for planetesimal formation. We note that radial drift couples planetesimal formation across radii in the disk, and suggest that considerations of planetesimal formation favor a model in which the initial deposition of material for giant planet cores occurs well beyond the snow line.

Astrophysikalische Jets sind ausgedehnte, kollimierte Massenausflüsse von verschiedenen astronomischen Objekten. Zeitabhängige magnetohydrodynamische (MHD) Simulationen der Jet-Entwicklung müssen den Akrretionsprozess in der Scheibe berücksichtigen, da der Jet aus der Scheibenmaterie gespeist wird. Allerdings ist die simultane Berechnung der Entwicklung von Scheibe und Jet schwierig, da die charakteristischen Zeitskalen unterschiedlich sind. Selbstähnliche Modelle zeigten, daß eine Beschreibung der Jetentstehung aus einer Akkretionsscheibe durch rein magnetische Prozesse möglich ist.
In this thesis the magnetohydrodynamic jet formation and the effects of magnetic diffusion on the formation of axisymmetric protostellar jets have been investigated in three different simulation sets. The time-dependent numerical simulations have been performed, using the magnetohydrodynamic ZEUS-3D code.

A 1D hydrodynamical code is used to model the viscous evolution of VY Scl stars, which are a subclass of Cataclysmic Variable. Low states arise as a result of occasional drops in the mass transfer rate, which probably result from the passage of starspots across the inner Lagrangian point on the secondary star. The model includes the heating of the accretion disc by irradiation from the white dwarf and shows that outbursts from the low state can be suppressed if the temperature of the white dwarf is sufficiently high (Twd 40 000 K). A magnetic propeller model is used to show that the quiescent value of the viscosity parameter of the accretion disc within WZ Sge is likely to be occo d 0.02, in agreement with estimates of ojcold f r other dwarf novae. Assuming the white dwarf in WZ Sge to be weakly magnetic it is shown that, in quiescence, material close to the white dwarf can be propelled to larger radii, depleting the inner accretion disc. This has the effect of stabilizing the inner disc and allowing the outer disc to accumulate mass. Numerical models yield an estimated recurrence time of rec 30 10 yr, in agreement with the observed recurrence time of trec 33 yr. The model is also used to follow WZ Sge through outburst, producing lightcurves that are in good agreement with observation. Finally, high-speed K-band photometry of WZ Sge is presented. Analysis of the data reveals a strong oscillation at 27.88 0.01 s, along with weaker oscillations at slightly longer periods. The principal oscillation is attributed to the presence of a rapidly rotating weakly magnetic white dwarf, and possible explanations for the weaker oscillations are discussed. The long term brightness variation in the K-band lightcurves is analysed, providing tentative evidence of a precessing, elliptical disc. The observational properties can be explained if the white dwarf possesses a weak magnetic field.

Rapid progress in multi-wavelength observations of Seyfert Galaxies in recent years is providing evidence that X-ray emission in these objects may be produced by magnetic flares occurring above a cold accretion disk. Here we attempt to develop a physically consistent model of accretion disks producing radiation via magnetic flares as well as the optically thick intrinsic disk emission, and apply this model to observations of Active Galactic Nuclei (AGN) and Galactic Black Hole Candidates (GBHCs). The following issues are considered: (1) the pressure equilibrium in the flare region, (2) the reflection and reprocessing of the X-radiation from flares in the underlying disk, (3) the spectra of GBHCs in the context of the model, (4) and the generation of the flares by the disk--the energy budget of the corona. Our results show that: (1) The temperature of the disk atmosphere near active magnetic flares in AGN is in the range 1 - 3 x 10⁵ Kelvin, and that the material is relatively non-ionized. This temperature is in a good agreement with the observed rollover energy in the Big Blue Bump (BBB) of Seyfert 1 Galaxies. We thus suggest that the BBB is simply the X-rays from magnetic flares reprocessed into the X-ray skin of the accretion disk. (2) We suggest an explanation for the recently discovered X-ray Baldwin effect and the controversy over the existence of BBBs in quasars more luminous than typical Seyferts. (3) Due to an ionization instability and much higher X-ray incident flux, we found that the X-ray skin in GBHCs is nearly completely ionized. Using an approximate model to describe this effect, we calculated the reflected/reprocessed spectrum and the resulting corona spectrum simultaneously. We found that the spectrum of GBHCs in their hard state may be explained with this model, with basically the same parameters for magnetic flares as in the AGN case. (4) The magnetic energy transport is shown to be large enough to account for the observed amount of X-rays from Seyferts and GBHCs. We predict that X-ray spectra are hard for accretion rates below the gas-to-radiation transition, and that they are softer above this transition. (5) We collected our results into a diagram that shows how the observational appearance of accreting black holes changes with the accretion rate and the mass of the hole, and compared it with observations of AGN and GBHCs. Our conclusion is that the agreement between theory and observations is very encouraging and we suggest that the physics of magnetic flares is the physics that should be added to the standard accretion disk theory in order to produce a more realistic description of accretion flows with large angular momentum.

Small solids embedded in gaseous protoplanetary disks are subject to strong dust-gas friction. Consequently, tightly coupled dust particles almost follow the gas flow. This near conservation of the dust-to-gas ratio along streamlines is analogous to the near conservation of entropy along flows of (dust-free) gas with weak heating and cooling. We develop this thermodynamic analogy into a framework to study dusty gas dynamics in protoplanetary disks. We show that an isothermal dusty gas behaves like an adiabatic pure gas, and that finite dust-gas coupling may be regarded as effective heating/cooling. We exploit this correspondence to deduce that (1) perfectly coupled, thin dust layers cannot cause axisymmetric instabilities; (2) radial dust edges are unstable if the dust is vertically well-mixed; (3) the streaming instability necessarily involves a gas pressure response that lags behind dust density; and (4) dust-loading introduces buoyancy forces that generally stabilize the vertical shear instability associated with global radial temperature gradients. We also discuss dusty analogs of other hydrodynamic processes (e.g., Rossby wave instability, convective overstability, and zombie vortices) and how to simulate dusty protoplanetary disks with minor tweaks to existing codes for pure gas dynamics.

The circumnuclear disc (CND) orbiting the Galaxy's central black hole is a reservoir of material that can ultimately provide energy through accretion, or form stars in the presence of the black hole, as evidenced by the stellar cluster that is presently located at the CND's centre. In this paper, we report the results of a computational study of the dynamics of the CND. The results lead us to question two paradigms that are prevalent in previous research on the Galactic Centre. The first is that the disc's inner cavity is maintained by the interaction of the central stellar cluster's strong winds with the disc's inner rim, and secondly, that the presence of unstable clumps in the disc implies that the CND is a transient feature. Our simulations show that, in the absence of a magnetic field, the interaction of the wind with the inner disc rim actually leads to a filling of the inner cavity within a few orbital time-scales, contrary to previous expectations. However, including the effects of magnetic fields stabilizes the inner disc rim against rapid inward migration. Furthermore, this interaction causes instabilities that continuously create clumps that are individually unstable against tidal shearing. Thus the occurrence of such unstable clumps does not necessarily mean that the disc is itself a transient phenomenon. The next steps in this investigation are to explore the effect of the magnetorotational instability on the disc evolution and to test whether the results presented here persist for longer time-scales than those considered here.