Дисертації з теми "White dwarf stars Spectra"
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1
Liebert, J., R. A. Saffer, and C. A. Pilachowski. "High Dispersion Observations of H alpha in the Suspected Brown Dwarf, White Dwarf Binary System G29-38." Steward Observatory, The University of Arizona (Tucson, Arizona), 1988. http://hdl.handle.net/10150/623899.
Повний текст джерелаАнотація:
We report on high dispersion spectroscopy of the Ha absorption line of
the cool DA white dwarf G 29 -38. This is the star for which a recently detected
infrared excess has been suggested to be due to a possible brown dwarf companion by
Zuckerman and Becklin (1986, 1987). Three echelle spectra obtained at the Multiple
Mirror Telescope and at the Kitt Peak Mayall 4m telescope in 1987 December show
no evidence for radial velocity variations larger than -'1.1 ± 8.7 km s -1 and are used
to derive a weighted heliocentric radial velocity Vr = 33.7 ± 4.3 km s -1 for the white
dwarf. No emission component from the hypothesized secondary star is detected.
These negative results do not constitute strong evidence against the companion
hypothesis, since the expected orbital velocity of the white dwarf component could
be quite small, and the companion's line emission could be too faint to be detected.
However, the observation of a sharp absorption line core restricts the possible rotation
of the white dwarf to < 40 km s -1 and ensures that any surface magnetic field has
a strength < 105 gauss. These results make it unlikely that the DA white dwarf
has previously been in a cataclysmic variable accretion phase.
2
Bainbridge, Matthew, Martin Barstow, Nicole Reindl, W. Ü. Tchang-Brillet, Thomas Ayres, John Webb, John Barrow, et al. "Probing the Gravitational Dependence of the Fine-Structure Constant from Observations of White Dwarf Stars." MDPI AG, 2017. http://hdl.handle.net/10150/625061.
Повний текст джерелаАнотація:
Hot white dwarf stars are the ideal probe for a relationship between the fine-structure constant and strong gravitational fields, providing us with an opportunity for a direct observational test. We study a sample of hot white dwarf stars, combining far-UV spectroscopic observations, atomic physics, atmospheric modelling, and fundamental physics in the search for variation in the fine structure constant. This variation manifests as shifts in the observed wavelengths of absorption lines, such as quadruply ionized iron (FeV) and quadruply ionized nickel (NiV), when compared to laboratory wavelengths. Berengut et al. (Phys. Rev. Lett. 2013, 111, 010801) demonstrated the validity of such an analysis using high-resolution Space Telescope Imaging Spectrograph (STIS) spectra of G191-B2B. We have made three important improvements by: (a) using three new independent sets of laboratory wavelengths; (b) analysing a sample of objects; and (c) improving the methodology by incorporating robust techniques from previous studies towards quasars (the Many Multiplet method). A successful detection would be the first direct measurement of a gravitational field effect on a bare constant of nature. Here we describe our approach and present preliminary results from nine objects using both FeV and NiV.
3
Levenhagen, Ronaldo S., Marcos P. Diaz, Paula R. T. Coelho, and Ivan Hubeny. "A Grid of Synthetic Spectra for Hot DA White Dwarfs and Its Application in Stellar Population Synthesis." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/624926.
Повний текст джерелаАнотація:
In this work we present a grid of LTE and non-LTE synthetic spectra of hot DA white dwarfs (WDs). In addition to its usefulness for the determination of fundamental stellar parameters of isolated WDs and in binaries, this grid will be of interest for the construction of theoretical libraries for stellar studies from integrated light. The spectral grid covers both a wide temperature and gravity range, with 17,000 K <= T-eff <= 100,000 K and 7.0 <= log g <= 9.5. The stellar models are built for pure hydrogen and the spectra cover a wavelength range from 900 angstrom to 2.5 mu m. Additionally, we derive synthetic HST/ACS, HST/WFC3, Bessel UBVRI, and SDSS magnitudes. The grid was also used to model integrated spectral energy distributions of simple stellar populations and our modeling suggests that DAs might be detectable in ultraviolet bands for populations older than similar to 8 Gyr.
4
Chen, Eugene Yu-Yu. "White dwarf stars with some hydrogen." Diss., Restricted to subscribing institutions, 2008. http://proquest.umi.com/pqdweb?did=1799891611&sid=6&Fmt=2&clientId=1564&RQT=309&VName=PQD.
Повний текст джерела
5
Thompson, A. M. "Energy transport in radially accreting white dwarf stars." Thesis, University of Glasgow, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.234837.
Повний текст джерела
6
Montgomery, Michael Houston. "The evolution and pulsation of crystallizing white dwarf stars /." Full text (PDF) from UMI/Dissertation Abstracts International, 1998. http://wwwlib.umi.com/cr/utexas/fullcit?p9937100.
Повний текст джерела
7
Dickinson, Nathan James. "The metal content of hot DA white dwarf spectra." Thesis, University of Leicester, 2012. http://hdl.handle.net/2381/10919.
Повний текст джерелаАнотація:
In this thesis, a study of the high ionisation-stage metal absorption features in the spectra of hot DA white dwarfs is presented. Metals are present in the photospheres of such stars due to radiative levitation (Chayer et al. 1994, 1995; Chayer Fontaine & Wesemael 1995). However, studies of the patterns between metal abundance and Teff show that, though the broad patterns predicted are seen, individual abundance measurements often do not reflect the predictions of radiative levitation theory (e.g. Barstow et al. 2003b). In this thesis, an analysis of the nitrogen abundance in three stars is performed, where a highly abundant layer of nitrogen was thought to reside at the top of the photospheres of the stars. The nitrogen abundance and distribution in these DAs is found to be homogeneous and of an abundance in keeping with stars of higher Teff. The accretion of metals from circumstellar discs has been shown to be the source of photospheric metals in DAs with Teff < 25,000 K (e.g. Zuckerman et al. 2003), where gravitational diffusion dominates (Koester & Wilken, 2006). In some cases, gaseous components are seen at such discs (e.g. SDSS 122859.93+104032.9; Gänsicke et al. 2006). A survey is made of a sample of hot (19,000 K < Teff < 51,000 K) DAs, where similar accretion may explain the inability of radiative levitation theory alone to account for the detected photospheric metal abundances. No circumstellar gas discs are found, though accretion from as yet undetected circumstellar sources remains an attractive explanation of the photospheric abundances of the stars. Circumstellar absorption is seen in the UV spectra of some hot DA stars (Holberg et al. 1998; Bannister et al. 2003). Sources suggested for this material include circumstellar discs, the ionisation of the ISM, stellar mass loss and planetary nebulae. A re-analysis of this absorption is presented, using a technique that for the first time allows proper modelling of the circumstellar absorption features, and provides column densities for all components. The ionisation of circumstellar discs or planetesimals, the ionisation of the ISM and the ionisation of mass lost by binary companions are put forward as the origin for this circumstellar material.
8
Day-Jones, A. C. "Searching for brown dwarf companions." Thesis, University of Hertfordshire, 2009. http://hdl.handle.net/2299/3475.
Повний текст джерелаАнотація:
In this thesis I present the search for ultracool dwarf companions to main sequence stars, subgiants and white dwarfs. The ultracool dwarfs identified here are benchmark objects, with known ages and distances. The online data archives, the two micron all sky survey (2MASS) and SuperCOSMOS were searched for ultracool companions to white dwarfs, where one M9 1 companion to a DA white dwarf is spectroscopically confirmed as the widest separated system of its kind known to date. The age of the M9 1 is constrained to a minium age of 1.94Gyrs, based on the estimated age of the white dwarf from a spectroscopically derived Teff and log g and an initial-final mass relation. This search was extended using the next generation surveys, the sloan digital sky survey (SDSS) and the UK infrared deep sky survey (UKIDSS), where potential white dwarf + ultracool dwarf binary systems from this search are presented. A handful of these candidate systems were followed-up with second epoch near infrared (NIR) imaging. A new white dwarf with a spectroscopic M4 companion and a possible wide tertiary ultracool component is here confirmed. Also undertaken was a pilot imaging survey in the NIR, to search for ultracool companions to subgiants in the southern hemisphere using the Anglo-Australian telescope. The candidates from that search, as well as the subsequent follow-up of systems through second epoch NIR/optical imaging and methane imaging are presented. No systems are confirmed from the current data but a number of good candidates remain to be followed-up and look encouraging. A search for widely separated ultracool objects selected from 2MASS as companions to Hipparcos main-sequence stars was also undertaken. 16 candidate systems were revealed, five of which had been previously identified and two new L0 2 companions are here confirmed, as companions to the F5V spectroscopic system HD120005 and the M dwarf GD 605. The properties of HD120005C were calculated using the DUSTY and COND models from the Lyon group, and the age of the systems were inferred from the primary members. For GD 605B no age constraint could be placed due to the lack of information available about the primary, but HD120005C has an estimated age of 2-4Gyr. In the final part of this thesis I investigate correlations with NIR broadband colours (J - H, H - K and J - K) with respect to properties, Teff , log g and [Fe/H] for the benchmark ultracool dwarfs, both confirmed from the searches undertaken in this work and those available from the literature. This resulted in an observed correlation with NIR colour and Teff, which is presented here. I find no correlation however with NIR colours and log g or [Fe/H], due in part to a lack of suitable benchmarks. I show that despite the current lack of good benchmark objects, this work has the potential to allow UCD properties to be measured from observable characteristics, and suggest that expanding this study should reveal many more benchmarks where true correlation between properties and observables can be better investigated.
9
Snyder, Lucas A. "On the reliability of 2MASS data in identifying red dwarf stars." Virtual Press, 2004. http://liblink.bsu.edu/uhtbin/catkey/1294901.
Повний текст джерелаАнотація:
Volume-limited samples indicate that red dwarfs are the most abundant stars in the Milky Way and account for most of its mass, despite their low individual masses. However, because of their low luminosity, they are extremely underdetected in magnitude-limited surveys. Complicating the task of identification is the fact that they have the same temperature as red giants and thus are in the same spectral class. We must therefore look for certain spectral features to differentiate between dwarfs and giants. Intermediate-to-broad-band photometry is one method that allows us to perform this task quantitatively. The 2MASS point source catalog contains data for -0.5 billion objects, including photometry in three broadband infrared filters. This paper discusses the manner in which these data can be analyzed to find red dwarfs and the success rate of such analysis.Department of Physics and Astronomy
10
Yeates, Celeste Marie Clemens J. Christopher. "Mode identification from combination frequency amplitudes in pulsating white dwarf stars." Chapel Hill, N.C. : University of North Carolina at Chapel Hill, 2006. http://dc.lib.unc.edu/u?/etd,389.
Повний текст джерелаАнотація:
Thesis (Ph. D.)--University of North Carolina at Chapel Hill, 2006.Title from electronic title page (viewed Oct. 10, 2007). "... in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics & Astronomy." Discipline: Physics and Astronomy; Department/School: Physics and Astronomy.
11
Pelisoli, Ingrid Domingos. "White dwarf and subdwarf stars in the sloan digital sky survey." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2018. http://hdl.handle.net/10183/180510.
Повний текст джерелаАнотація:
Estrelas anãs brancas são o último estágio evolutivo observável de mais de 95% das estrelas e também um resultado comum na evolução de estrelas binárias. O estudo de anãs brancas é, portanto, uma ferramenta poderosa na compreensão da evolução de estrelas simples e binárias, da função de massa inicial local, e da perda de massa após a sequência principal, levando-nos a uma melhor compreensão do histórico de formação e evolução estelar de diferentes populações. Para que esses estudos sejam possíveis, é necessária uma amostra grande e preferencialmente completa de anãs brancas, cobrindo todo o intervalo de parâmetros físicos. A maneira mais simples de obter isso é utilizando dados de grandes projetos de mapeamento. O Sloan Digital Sky Survey já permitiu o aumento do número de anãs brancas conhecidas em cinco vezes até o data release 10. Neste trabalho, estendemos a busca por anãs brancas aos novos objetos no data release 12, descobrindo 3 157 novas anãs brancas e 1 349 novas subanãs. Pela primeira vez, estendemos essa busca para log g < 6.5, correspondente a M < 0.3 M⊙. Anãs brancas abaixo desse limite de massa não podem formar-se em um tempo de Hubble; contudo, se a estrela é parte de um sistema de binárias próximas, a perda de massa pode ser tão intensa que a anã branca resultante tem massa inferior ao limite por evolução simples. Esses objetos são chamados anãs brancas de massa extremamente-baixa (ELMs, do inglês extremely-low mass white dwarfs) Elas têm Teff < 20 000 K e 5.0 . log g . 6.5 e espectros muito similares a estrelas A de sequência principal. Menos de cem são conhecidas, e a maioria dos objetos foi descoberta tendo em conta um critério de seleção tendencioso, que excluiu ELMs frias (Teff < 9 000 K) e com mais baixa massa (M . 0.15 M⊙), tornando difícil verificar modelos e compreender as propriedades das ELMs como classe. Nós identificamos milhares de objetos cujas propriedades físicas, temperatura efetiva e log g, estão no intervalo de produtos de evolução binária, como as ELMs. Nós os chamamos de sdAs, referindo-nos ao seu log g que as coloca abaixo da sequência principal e seu espectro dominado por hidrogênio. As sdAs parecem conter populações estelares sobrepostas, e encontramos que 7% são mais compatíveis com (pre-)ELMs do que com objetos de sequência principal. Obtivemos espectroscopia resolvida temporalmente para 26 objetos e pudemos confirmar que 15 estão em binárias. Um objeto faz parte de um sistema eclipsante, enquanto outro é pulsante — o oitavo membro da classe de ELMs pulsantes. Outros seis objetos também apresentaram pulsações em nossas observações, cinco desses estão na vizinhança da faixa de instabilidade das ELMs. Com estes resultados, aumentamos a população de ELMs por um fator de 20%, elevando a fração de ELMs de 4 para 20%, um valor que é consistente com as previsões de modelos evolucionários.White dwarf stars are the final observable evolutionary state of over 95% of stars and also a common outcome in binary evolution. Therefore, studying white dwarfs is a powerful tool to understand both single and binary stellar evolution, local initial mass function, and post-main sequence mass loss, leading us to a better comprehension of the history of stellar formation and evolution of different stellar populations. In order to make this type of studies possible, a large and preferably complete sample of white dwarf stars, covering the whole range of physical parameters, is required. The simplest way to achieve that is to take advantage of data provided by large surveys. The Sloan Digital Sky Survey has already allowed the increase of the number of known white dwarf stars fivefold up to its data release 10. In this work, we extended the search for white dwarfs to the new objects in the data release 12, discovering 3 157 new white dwarfs and 1 349 new subdwarfs. For the first time, we have extended this search to log g < 6.5, corresponding to M < 0.3 M⊙. White dwarfs below this mass limit cannot be formed through single evolution within a Hubble time; however, if the star is part of a close binary system, the mass loss of the system may be so intense that the resulting white dwarf has mass below the single evolution limit. These objects are known as extremely-low mass white dwarfs (ELMs) They show Teff < 20 000 K and 5.0 . log g . 6.5 and spectra very similar to main sequence A stars. Less than a hundred of them are known, and most objects were discovered relying on biased selection criteria, that excluded cool (Teff < 9 000 K), lowermass (M . 0.15 M⊙) ELMs, making it difficult to validate the models and comprehend the properties of the ELMs as a class. We have identified thousands of objects whose physical properties, effective temperature and surface gravity, place them in the range of by-products of binary interaction such as the ELMs. We have called them sdAs, referring to their sub-main sequence log g and hydrogen dominated spectra. They seem to be composed of overlapping stellar populations, and we found that at least 7% are more likely ELMs or their precursors, the pre-ELMs, than main sequence stars. Obtaining time-resolved spectroscopy for 26 objects, we could confirm 15 to be in close binaries. One of them is also an eclipsing system, while another is a pulsator — the eighth member of the pulsating ELM class. Other six new pulsators were found as part of our follow-up, five of them in the vicinity of the ELM instability strip. With these results, we increase the population of ELMs by 20%, raising the fraction of cool ELMs from 4 to 20%, which is consistent with the predictions from the evolutionary models.
12
Ramos, Gabriel Lauffer. "Evolutionary sequences for H and He atmosphere massive white dwarf stars." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2018. http://hdl.handle.net/10183/181005.
Повний текст джерелаАнотація:
White dwarf stars are the most common final stage of stellar evolution, corresponding to 99% of all stars in the Galaxy. White dwarf models can be used to obtain the age of stellar populations, to build an initial to final mass relation to understand the connection between the properties of white dwarfs and their progenitors, determine the upper mass limit that separates white dwarfs progenitors from Type II supernovae, enhance the comprehension of the physical properties of high density matter and derive ages and masses for observed white dwarfs from the cooling tracks. The literature is populated with low mass and intermediate mass white dwarf models, however the massive white dwarfs are often forgotten and the evolutionary sequences are incomplete. In this dissertation, we compute full evolutionary sequences for massive white dwarfs, exploring the evolution of hydrogen-rich and hydrogen-deficient white dwarfs stars with masses between 1.012 and 1.307 M , and initial metallicity of Z = 0.02. These sequences are the result of main sequence stars with masses between 8.8 and 11.8 M . The simulations were performed with the Modules for Experiments in Stellar Astrophysics - MESA code, starting at the zero-age main sequence, through thermally pulsing and mass-loss phases, ending as the white dwarfs at the cooling sequence. Our simulations are full evolutionary, in which we consider the entire evolutionary history of the progenitors. We present reliable nuclear chemical profiles for the whole mass range considered, covering the different expected central compositions, i.e. C/O, O/Ne and Ne/O/Mg, and their dependence with the stellar mass. In addition, we present detailed chemical profiles of hybrid C/O-O/Ne core white dwarfs, found in the mass range between 1.024 and 1.150 M . We present the initial-to-final mass relation, mass-radius relation, and cooling times with improved crystallization limits, considering the effects of atmosphere and core composition.
13
Kawka, Adela. "A study of white dwarfs in the solar neighbourhood." Thesis, Kawka, Adela (2003) A study of white dwarfs in the solar neighbourhood. PhD thesis, Murdoch University, 2003. https://researchrepository.murdoch.edu.au/id/eprint/120/.
Повний текст джерелаАнотація:
The aim of this thesis is to revisit the properties of white dwarf stars in the Solar neighbourhood (distance > 100 pc), in particular their magnetic fields, the occurrence of binarity and their space density. This thesis presents observations and analysis of a sample of white dwarfs from the southern hemisphere. Over 80 objects were observed spectroscopically, and 65 of these were also observed with a spectropolarimeter. Many of the white dwarfs observed belong to the Solar neighbourhood, and can be used to study the star formation and evolution in this region.
Our spectropolarimetric measurements helped constrain the fraction of magnetic white dwarfs in the Solar neighbourhood. Combining data from different surveys, I found a higher fraction of these objects in the relatively old local population than in other younger selections such as the Palomar-Green survey which suggests magnetic field evolution in white dwarfs, or different sets of progenitors. The progenitors of magnetic white dwarfs have been assumed to be Ap and Bp stars, however I find that the properties and number of Ap and Bp stars would only explain white dwarfs with magnetic fields larger than 100 MG. The number of known white dwarfs is believed to be complete to about 13 pc, however the sample is certainly incomplete to 20 pc from the Sun. To identify new white dwarfs in the Solar neighbourhood, some possibly magnetic or in binaries, numerous candidate white dwarfs from the Revised NLTT catalogue have been observed, which resulted in the discovery of 13 new white dwarfs, with 4 of these having a distance that places them within 20 pc of the Sun. The candidates were selected using a V - J reduced-proper-motion diagram and optical-infrared diagram. A total of 417 white dwarf candidates were selected, 200 of these have already been spectroscopically confirmed as white dwarfs.
Spectroscopic confirmation is required for the remaining 217 candidates, many of these are likely to belong to the Solar neighbourhood. Four close binaries consisting of a white dwarf and a cool companion were also observed, for which atmospheric and orbital parameters were obtained. The photometry for two of these binary systems, BPM 71214 and EC 13471-1258 shows that the secondary stars are filling their Roche lobes, and combined with their orbital parameters, these systems are very good candidates for hibernating novae. The time of their previous interaction or the extent of this interaction are unknown. The two other binary systems, BPM 6502 and EUVE J0720-31.7 are post-common envelope binaries. BPM 6502 is not expected to interact within a Hubble time, however EUVE J0720-31 is expected to become a cataclysmic variable within a Hubble time. The atmospheric parameters of the white dwarfs were determined using model atmosphere codes which were modified for the present study to include convective energy transfer, self-broadening and Lyman satellite features.
14
Kawka, Adela. "A study of white dwarfs in the solar neighbourhood." Kawka, Adela (2003) A study of white dwarfs in the solar neighbourhood. PhD thesis, Murdoch University, 2003. http://researchrepository.murdoch.edu.au/120/.
Повний текст джерелаАнотація:
The aim of this thesis is to revisit the properties of white dwarf stars in the Solar neighbourhood (distance > 100 pc), in particular their magnetic fields, the occurrence of binarity and their space density. This thesis presents observations and analysis of a sample of white dwarfs from the southern hemisphere. Over 80 objects were observed spectroscopically, and 65 of these were also observed with a spectropolarimeter. Many of the white dwarfs observed belong to the Solar neighbourhood, and can be used to study the star formation and evolution in this region.
Our spectropolarimetric measurements helped constrain the fraction of magnetic white dwarfs in the Solar neighbourhood. Combining data from different surveys, I found a higher fraction of these objects in the relatively old local population than in other younger selections such as the Palomar-Green survey which suggests magnetic field evolution in white dwarfs, or different sets of progenitors. The progenitors of magnetic white dwarfs have been assumed to be Ap and Bp stars, however I find that the properties and number of Ap and Bp stars would only explain white dwarfs with magnetic fields larger than 100 MG. The number of known white dwarfs is believed to be complete to about 13 pc, however the sample is certainly incomplete to 20 pc from the Sun. To identify new white dwarfs in the Solar neighbourhood, some possibly magnetic or in binaries, numerous candidate white dwarfs from the Revised NLTT catalogue have been observed, which resulted in the discovery of 13 new white dwarfs, with 4 of these having a distance that places them within 20 pc of the Sun. The candidates were selected using a V - J reduced-proper-motion diagram and optical-infrared diagram. A total of 417 white dwarf candidates were selected, 200 of these have already been spectroscopically confirmed as white dwarfs.
Spectroscopic confirmation is required for the remaining 217 candidates, many of these are likely to belong to the Solar neighbourhood. Four close binaries consisting of a white dwarf and a cool companion were also observed, for which atmospheric and orbital parameters were obtained. The photometry for two of these binary systems, BPM 71214 and EC 13471-1258 shows that the secondary stars are filling their Roche lobes, and combined with their orbital parameters, these systems are very good candidates for hibernating novae. The time of their previous interaction or the extent of this interaction are unknown. The two other binary systems, BPM 6502 and EUVE J0720-31.7 are post-common envelope binaries. BPM 6502 is not expected to interact within a Hubble time, however EUVE J0720-31 is expected to become a cataclysmic variable within a Hubble time. The atmospheric parameters of the white dwarfs were determined using model atmosphere codes which were modified for the present study to include convective energy transfer, self-broadening and Lyman satellite features.
15
Toloza, O., B. T. Gänsicke, J. J. Hermes, D. M. Townsley, M. R. Schreiber, P. Szkody, A. Pala, et al. "GW Librae: a unique laboratory for pulsations in an accreting white dwarf." OXFORD UNIV PRESS, 2016. http://hdl.handle.net/10150/621433.
Повний текст джерелаАнотація:
Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in atmospheres with mixed compositions of hydrogen, helium, and metals, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 accretion outburst. Our analysis of Hubble Space Telescope (HST) ultraviolet spectroscopy taken in 2002, 2010, and 2011, showed that pulsations produce variations in the white dwarf effective temperature as predicted by theory. Additionally in 2013 May, we obtained new HST/Cosmic Origin Spectrograph ultraviolet observations that displayed unexpected behaviour: besides showing variability at a parts per thousand integral 275 s, which is close to the post-outburst pulsations detected with HST in 2010 and 2011, the white dwarf exhibits high-amplitude variability on an a parts per thousand integral 4.4 h time-scale. We demonstrate that this variability is produced by an increase of the temperature of a region on white dwarf covering up to a parts per thousand integral 30 per cent of the visible white dwarf surface. We argue against a short-lived accretion episode as the explanation of such heating, and discuss this event in the context of non-radial pulsations on a rapidly rotating star.
16
Camarota, Lawrence Francis, and Lawrence Francis Camarota. "Determining the Pressure Shift of Helium I Lines Using White Dwarf Stars." Diss., The University of Arizona, 2017. http://hdl.handle.net/10150/625365.
Повний текст джерелаАнотація:
This dissertation explores the non-Doppler shifting of Helium lines in the high pressure conditions of a white dwarf photosphere. In particular, this dissertation seeks to mathematically quantify the shift in a way that is simple to reproduce and account for in future studies without requiring prior knowledge of the star’s bulk properties (mass, radius, temperature, etc.). Two main methods will be used in this analysis. First, the spectral line will be quantified with a continuous wavelet transformation, and the components will be used in a χ^2 minimizing linear regression to predict the shift. Second, the position of the lines will be calculated using a best-fit Levy-alpha line function. These techniques stand in contrast to traditional methods of quantifying the center of often broad spectral lines, which usually assume symmetry on the parts of the lines.
17
Preval, Simon Paul. "Heavy metal opacity and line blanketing in hot DA white dwarf stars." Thesis, University of Leicester, 2015. http://hdl.handle.net/2381/32281.
Повний текст джерелаАнотація:
This thesis concerns atomic data, and it’s impact upon white dwarf model atmosphere calculations, and consequentially, the measurements made using such models. The thesis begins with a brief introduction to the history and properties of white dwarfs, the theory of radiative transfer, and the uses of white dwarfs in astronomy. A detailed spectroscopic survey of WD0501+524 (G191-B2B) is presented. 976 absorption features were detected, 947 of which have been successfully identified. ∼ 60% of the identified features were found to pertain to Fe and Ni iv-vi transitions. The potential consequences of using a limited atomic data set in model atmospheres are discussed, and other possible identifications to the unknown absorption features are queried. The Kurucz (1992) (Ku92) atomic database (containing ∼ 9, 000, 000 transitions) is supplemented by photoionisation (PI) cross section data from the Opacity Project for use in stellar atmosphere calculations. The more recent Kurucz (2011) (Ku11) database (containing ∼ 160, 000, 000 transitions) is not accompanied by PI cross section data. Calculations performed to create this data are described, and their effects on model atmosphere calculations are discussed. The Lyman/Balmer line problem, a discrepancy between the measured effective temperatures (Teff) from the Lyman/Balmer line series is considered. Teff and surface gravity (log g) measurements are made of 24 DA white dwarfs using model atmosphere grids utilising two atmospheric compositions, two Stark broadening tables, and the Ku92 and Ku11 atomic data sets. It is shown that the average opacity contributed by all metal species in the atmosphere drives the discrepancy between the measured Lyman/Balmer Teff. Analysis of Extreme Ultraviolet Explorer (EUVE) data for seven metal rich white dwarf stars is presented. Four model atmosphere grids were calculated using two atmospheric compositions (Preval et al. 2013; Barstow et al. 2003) and the Ku92 and Ku11 atomic datasets. Improved fits for wavelengths shortward of 230°A are obtained for all stars except WD0501+524, where significant discrepancies remain.
18
Saffer, R. A., and J. Liebert. "Search for Close Binary Evolved Stars." Steward Observatory, The University of Arizona (Tucson, Arizona), 1988. http://hdl.handle.net/10150/623897.
Повний текст джерелаАнотація:
We report on a search for short -period binary systems composed of pairs of
evolved stars. The search is being carried out concurrently with a program to characterize
the kinematical properties of two different samples of stars. Each sample has produced one
close binary candidate for which further spectroscopic observations are planned. We also
recapitulate the discovery of a close detached binary system composed of two cool DA
white dwarfs, and we discuss the null results of Ha observations of the suspected white
dwarf /brown dwarf system G 29-38.
19
West, Steven Charles. "The development of a near-infrared polarimeter and its application to the continuum polarization of magnetic white dwarfs." Diss., The University of Arizona, 1987. http://hdl.handle.net/10150/184285.
Повний текст джерелаАнотація:
A new type of astronomical polarimeter is presented that incorporates a fused silica stress-birefringent modulator which simultaneously allows the selection of chopping frequency (≤ 5 Hz) and spectral tuning (1 ≤ λ ≤ 2.5 μm) with very high modulation efficiency. Two integrating detector packages are discussed. The first uses a single germanium photodiode and HR polaroid analyzer and achieves σᵥ = 1% for a J = 14.5 object in 0.5 hr with the MMT. The second package uses a 12 x 30 pixel HgCdTe array and MgF₂ Wollaston prism to provide four simultaneous polarimetric channels that properly eliminate the systematic errors resulting from "staring" mode operation. The instrumental polarizations of the MMT are calculated using Mueller calculus and experimentally verified. It is found that the hexagonal symmetry of the telescope cancels the large amplitude spurious polarizations that arise from the single "arms" and results in a polarimetric efficiency term that is a function only of spectral bandpass and not sky position. Therefore, the MMT behaves essentially like a clean Cassegrain telescope. The continuum polarizations of five isolated highly magnetic white dwarfs are investigated both observationally and theoretically. The most complete broad-band polarimetric survey throughout the spectral region 0.35 ≤ λ ≤ 1.65 μm is performed. An apparent correlation in the linear polarizations of Grw + 70°8247, GD229, and G240-72 is discovered. In addition, no evidence for time-dependent rotation of position angle is found for any of the stars. Hydrogen Balmer photoionization occurring from the magnetically-perturbed bound states into the Coulomb-Lorentz mixed quasi-Landau continuum is investigated with the aid of recent high field calculations. Finally, the continuum polarization of Grw + 70°8247 is compared to models for cyclotron and inverse magnetobremsstrahlung absorptions in a dipolar field. The conclusion is that the continuum polarizations of these objects still eludes an exact description. New observations of BG C Mi reveal the first definitive discovery of polarized cyclotron emission in any intermediate polar and confirms that the long-held basic model of a magnetically accreting white dwarf is correct. The wavelength dependence of circular polarization is found to increase rapidly into the near-infrared, from V(I) = -0.25±0.06% to V(J) = -1.74±0.26% and suggests a field strength in the cyclotron emission region near 5-10 MG if the system scales directly with the AM Her stars.
20
Liebert, J., C. C. Dahn, and D. G. Monet. "Luminosity Function of White Dwarfs in the Local Disk and Halo." Steward Observatory, The University of Arizona (Tucson, Arizona), 1988. http://hdl.handle.net/10150/623908.
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21
Farihi, J., D. Koester, B. Zuckerman, L. Vican, B. T. Gänsicke, N. Smith, G. Walth, and E. Breedt. "Solar abundances of rock-forming elements, extreme oxygen and hydrogen in a young polluted white dwarf." OXFORD UNIV PRESS, 2016. http://hdl.handle.net/10150/622949.
Повний текст джерелаАнотація:
The T-eff = 20 800 K white dwarf WD 1536+520 is shown to have broadly solar abundances of the major rock-forming elements O, Mg, Al, Si, Ca, and Fe, together with a strong relative depletion in the volatile elements C and S. In addition to the highest metal abundances observed to date, including log (O/He) = -3.4, the helium-dominated atmosphere has an exceptional hydrogen abundance at log (H/He) = -1.7. Within the uncertainties, the metal-to-metal ratios are consistent with the accretion of an H2O-rich and rocky parent body, an interpretation supported by the anomalously high trace hydrogen. The mixed atmosphere yields unusually short diffusion time-scales for a helium atmosphere white dwarf, of no more than a few hundred years, and equivalent to those in a much cooler, hydrogen-rich star. The overall heavy element abundances of the disrupted parent body deviate modestly from a bulk Earth pattern, and suggest the deposition of some core-like material. The total inferred accretion rate is 4.2 x 10(9) g s(-1), and at least four times higher than for any white dwarf with a comparable diffusion time-scale. Notably, when accretion is exhausted in this system, both metals and hydrogen will become undetectable within roughly 300 Myr, thus supporting a scenario where the trace hydrogen is related to the ongoing accretion of planetary debris.
22
Munn, Jeffrey A., Hugh C. Harris, Hippel Ted von, Mukremin Kilic, James W. Liebert, Kurtis A. Williams, Steven DeGennaro, et al. "A DEEP PROPER MOTION CATALOG WITHIN THE SLOAN DIGITAL SKY SURVEY FOOTPRINT. II. THE WHITE DWARF LUMINOSITY FUNCTION." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/622633.
Повний текст джерелаАнотація:
A catalog of 8472 white dwarf (WD) candidates is presented, selected using reduced proper motions from the deep proper motion catalog of Munn et al. Candidates are selected in the magnitude range 16 < r < 21.5 over 980 square degrees, and 16 < r < 21.3 over an additional 1276 square degrees, within the Sloan Digital Sky Survey (SDSS) imaging footprint. Distances, bolometric luminosities, and atmospheric compositions are derived by fitting SDSS ugriz photometry to pure hydrogen and helium model atmospheres (assuming surface gravities log g = 8). The disk white dwarf luminosity function (WDLF) is constructed using a sample of 2839 stars with 5.5 < M-bol < 17, with statistically significant numbers of stars cooler than the turnover in the luminosity function. The WDLF for the halo is also constructed, using a sample of 135 halo WDs with 5 < M-bol < 16. We find space densities of disk and halo WDs in the solar neighborhood of 5.5 +/- 0.1 x 10(-3) pc(-3) and 3.5 +/- 0.7 x 10(-5) pc(-3), respectively. We resolve the bump in the disk WDLF due to the onset of fully convective envelopes in WDs, and see indications of it in the halo WDLF as well.
23
Sing, David Kent. "Post Common Envelope Pre-Cataclysmic and Cataclysmic Variable Binaries." Diss., Tucson, Arizona : University of Arizona, 2005. http://etd.library.arizona.edu/etd/GetFileServlet?file=file:///data1/pdf/etd/azu%5Fetd%5F1400%5F1%5Fm.pdf&type=application/pdf.
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Zong, Weikai. "Amplitude and frequency modulations of oscillation modes in hot B subdwarf and white dwarf stars from Kepler photometry." Thesis, Toulouse 3, 2016. http://www.theses.fr/2016TOU30306/document.
Повний текст джерелаАнотація:
Les interactions non linéaires entre modes de pulsation, induisant des modulations d'amplitude et de fréquence, sont difficiles à mettre en évidence avec les télescopes au sol en raison des temps caractéristiques en jeu, de l'ordre de la semaine, du mois, ou même de l'année. L'avènement des télescopes spatiaux comme KEPLER (opéré par la NASA) a considérablement changé la donne en apportant de nouvelles données pour ce domaine de recherche. Dans cette thèse, nous analysons les données photométriques obtenues avec KEPLER pour 24 étoiles compactes pulsantes, incluant 18 étoiles sous-naines de type B (sdB) et 6 naines blanches. Nous établissons que les modulations d'amplitude et de fréquence des modes d'oscillation sont un phénomène courant dans ces étoiles. Nous étudions en particulier deux étoiles : KIC 0862602, une naine blanche pulsante de type DB, et KIC 10139564, une étoile sdB variable à courtes périodes. KIC 0862602 et KIC 10139564 ont été observées sans interruption par KEPLER en cadence rapide pendant deux années pour la première et plus de trois ans pour la seconde. En analysant en détail ces données photométriques de très haute précision, nous mettons en évidence différents types de comportements affectant les composantes de triplets induits par la rotation stellaire. Les fréquences et amplitudes de ces modes montrent des variations soi périodiques soi irrégulières, ou demeurent constantes. Ces comportements peuvent être connectés à ceux prédits par les équations d'amplitude dans le cas de couplages non linéaires résonants entre modes, en particulier pour les temps caractéristiques des modulations. De plus, nous montrons que les modes en résonance constituant les triplets peuvent également interagir avec des modes extérieurs par le biais d'autres formes de résonances telle que la résonance à trois modes v 0 ~ v 1 + v 2 , une situation qui n'est pas prise en compte dans le cadre théorique existant. Ces études apportent pour la première fois une preuve claire de l'existence de mécanismes de couplages non linéaires entre modes d'oscillations dans les pulsateurs compacts. Cette découverte résonne comme un avertissement pour les projets visant à mesurer les taux de changement des périodes dus à l'évolution dans les étoiles compactes en général. Les modulations de fréquence d'origine non linéaire peuvent potentiellement empêcher toute mesure fiable de ces taux, à moins de corriger ces effets auparavant. Les modulations observées caractérisées dans cette thèse apportent un regard nouveau sur "l'astérosismologie non linéaire" et appellent à revisiter les méthodes d'analyse des courbes de lumière pour en extraire les modes d'amplitude et de fréquence variables. Dans un futur proche, nous anticipons davantage d'attention portée à ces phénomènes non inéaires dans différents types d'étoiles pulsantes observées depuis l'espace, ainsi qu'un regain d'intérêt pour la théorie non linéaire des oscillations stellaires en généralNonlinear mode interactions, inducing amplitude and frequency modulations, are difficult to observe from ground-based telescopes as these typical timescales of the modulations are of the order of weeks, months, or even years. The launch of space telescopes such as Kepler (operated by NASA) has tremendously changed the situation by providing new data for this research field. In this thesis, we analyze the Kepler photometric data observed for 24 compact pulsators, including 18 hot B subdwarf (sdB) stars and six white dwarf stars. We find that it is a common phenomenon that oscillation modes in these pulsating stars show amplitude and/or frequency variations. We focus in particular on two stars, KIC 08626021, a DB white dwarf, and KIC 10139564, a short period sdB star. KIC 08626021 and KIC 10139564 have been monitored by Kepler in short-cadence mode for nearly two years and more than three years without interruption, respectively. By analyzing in depth these high-quality photometric data, we find that the modes within triplets induced by rotation clearly reveal different types of behaviors : their frequency and amplitude may exhibit either periodic or irregular modulations, or remain constant. These various behaviors can be linked to those predicted within the amplitude equation formalism in the case of the nonlinear resonant mode coupling mechanism, particularly for the modulation timescales. Furthermore, we find that the triplet resonance modes can also interact with outside modes through other types of resonances such as the three-mode resonance v 0 ~ v 1 + v 2 , which is not considered within the current nonlinear theoretical frameworks. These findings constitute the first clear evidence of nonlinear resonant mode couplings occurring in compact pulsators. This should resonate as a warning to projects aiming at measuring the evolutionary change rate of pulsation periods in compact stars in general. Nonlinear modulations of the frequencies can potentially jeopardize any attempt to measure reliably such rates, unless they can be corrected beforehand. The observed modulations characterized in this thesis provide new insights to "nonlinear asteroseismology" and call for new methods to process the signals of variable modes from the observed light curves. We foresee that increasing attention will focus on these nonlinear phenomena in various types of pulsating stars observed from space in the near future, thus reviving interest in the nonlinear oscillation theory in general
25
Voss, Björn. "White dwarfs in the SPY and HQS surveys an analysis of a large set of white dwarf spectra and a search for variable ZZ Ceti white dwarfs /." [S.l.] : [s.n.], 2006. http://e-diss.uni-kiel.de/diss_1796/d1796.pdf.
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26
Agüeros, Marcel A. "Candidate isolated neutron stars and other stellar x-ray sources from the ROSAT all-sky and Sloan Digital Sky Surveys /." Thesis, Connect to this title online; UW restricted, 2006. http://hdl.handle.net/1773/5436.
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27
Pudas, Markus. "Identification of elements and molecules in the spectra of an M dwarf star using high resolution infrared spectroscopy." Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-325908.
Повний текст джерелаАнотація:
M dwarfs are abundant and long-lived stellar objects. The formation of planets around stars in stellar systems is believed to be metallicity dependent. To determine the metallicity with synthetic spectrum analysis, the elements producing the absorption lines ofthe spectra first have to be identified. The aim of this thesis is to identify and list the elements or molecules that produce the absorption lines in the spectra of Barnard's star. This thesis was done at the Division for Astronomy and Space Physics at Uppsala University. High resolution infrared spectral data recorded in the J band 1.1–1.4 μm was downloaded from the CRIRES-POP database. The data had to be wavelength corrected due to the effects of Doppler shift. A modified IDL program was used to read the data files,normalize the flux to unity and plot the spectra. This procedure was also done with the telluric spectra using data from a solar flux atlas. The IDL program plotted the normalized spectra together in the same plot. With this procedure the absorption features originating from the earth’s atmosphere could be identified and discarded. The analysis of the spectral lines resulted in wavelength values which were tested against the VALD3 database to determine what elements were possibly responsible for the absorption features. The results are presented in a line list. It can be used with other software programs to determine the metallicity. The identified elements and molecules agrees in part with earlier measurements of stellar spectra from M dwarf stars except for a number of lines where no matching elements were found in the VALD3 database. A line list with possible elements in the photosphere of Barnard’s star can be constructed from the spectra using high-resolution infrared spectroscopy.M dvärgstjärnor är de mest förekommande stjärnorna i vår galax. De har en mycket långlivslängd, vissa tusen gånger längre än vår sol. Det finns teorier om att planetbildning runt stjärnor styrs av halten av ämnen som inte är väte eller helium. Denna halt kallas metallicitet. För att på konstgjord eller syntetisk väg bestämma metalliciteten i Barnard’s stjärna, en M dvärg, behöver de ämnen som bidrar till absorptionslinjerna i fotosfären först identifieras. Målsättningen med detta arbete var att identifiera de grundämnen och eventuella molekyler som skapar absorptionslinjerna i spektrumet till Barnard’s stjärna. Detta arbete utfördes på institutionen för fysik och astronomi. Metoden använde ett modifierat IDL program för att läsa och plotta data. Högupplöst infraröd spektraldata från Jbandet (1.1–1.4 μm) till Barnard’s stjärna hämtades från CRIRES-POP databasen och data för det telluriska spektrumet från en databas med telluriska linjer. Därefter plottades de samtidigt i ett våglängdsöverlappande normaliserat spektra. I programmet gick absorptionslinjer som inte hade sitt ursprung i jordens atmosfär att urskilja manuellt. Då våglängderna för absorptionlinjerna bestämts, matades värden in i databasen VALD3. Analysen av de returnerade värdena från VALD3 genererade en resultatlista med de mest sannolika elementen för de olika absorptionsvåglängderna. Resultatlistan kan användas som ingångsvärde till program som syntetiskt beräknar metalliciteten. Resultaten överensstämmer till viss del med tidigare mätningar. Slutsatsen är att metoden med högupplöst infraröd spektral data kan användas för att bestämma en lista med element och molekyler från fotosfären i Barnard’s stjärna.
28
Jadhav, Yashashree Shirish. "Identifying Cataclysmic Variables in Sparsely Sampled Pan-STARRS1 data." Ohio University Honors Tutorial College / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=ouhonors1400538970.
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29
Hallakoun, N., (许偲艺) S. Xu, D. Maoz, T. R. Marsh, V. D. Ivanov, V. S. Dhillon, M. C. P. Bours, et al. "Once in a blue moon: detection of ‘bluing' during debris transits in the white dwarf WD 1145+017." OXFORD UNIV PRESS, 2017. http://hdl.handle.net/10150/625505.
Повний текст джерелаАнотація:
The first transiting planetesimal orbiting a white dwarf was recently detected in K2 data of WD 1145+017 and has been followed up intensively. The multiple, long and variable transits suggest the transiting objects are dust clouds, probably produced by a disintegrating asteroid. In addition, the system contains circumstellar gas, evident by broad absorption lines, mostly in the u' band, and a dust disc, indicated by an infrared excess. Here we present the first detection of a change in colour of WD 1145+017 during transits, using simultaneous multiband fast-photometry ULTRACAM measurements over the u'g'r'i' bands. The observations reveal what appears to be 'bluing' during transits; transits are deeper in the redder bands, with a u' - r' colour difference of up to similar to-0.05 mag. We explore various possible explanations for the bluing, including limb darkening or peculiar dust properties. 'Spectral' photometry obtained by integrating over bandpasses in the spectroscopic data in and out of transit, compared to the photometric data, shows that the observed colour difference is most likely the result of reduced circumstellar absorption in the spectrum during transits. This indicates that the transiting objects and the gas share the same line of sight and that the gas covers the white dwarf only partially, as would be expected if the gas, the transiting debris and the dust emitting the infrared excess are part of the same general disc structure (although possibly at different radii). In addition, we present the results of a week-long monitoring campaign of the system using a global network of telescopes.
30
Rodigas, Timothy J., P. Bergeron, Amélie Simon, Pamela Arriagada, Jacqueline K. Faherty, Guillem Anglada-Escudé, Eric E. Mamajek, et al. "MagAO IMAGING OF LONG-PERIOD OBJECTS (MILO). II. A PUZZLING WHITE DWARF AROUND THE SUN-LIKE STAR HD 11112." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/622154.
Повний текст джерелаАнотація:
HD 11112 is an old, Sun-like star that has a long-term radial velocity (RV) trend indicative of a massive companion on a wide orbit. Here we present direct images of the source responsible for the trend using the Magellan Adaptive Optics system. We detect the object (HD 11112B) at a separation of 2 2 (100 au) at multiple wavelengths spanning 0.6-4 mu m. and show that it is most likely a gravitationally bound cool white dwarf. Modeling its spectral energy distribution suggests that its mass is 0.9-1.1M(circle dot), which corresponds to very high eccentricity, near edge-on orbits from a. Markov chain Monte Carlo analysis of the RV and imaging data together. The total age of the white dwarf is > 2 sigma, which is discrepant with that of the primary star under most assumptions. The problem can be resolved if the white dwarf progenitor was initially a double white dwarf binary that then merged into the observed high-mass white dwarf. HD 11112B is a unique and intriguing benchmark object that can be used to calibrate atmospheric and evolutionary models of cool white dwarfs and should thus continue to be monitored by RV and direct imaging over the coming years.
31
Putney, Angela. "Magnetic white dwarf stars." Thesis, 1996. https://thesis.library.caltech.edu/3770/1/Putney_a_1996.pdf.
Повний текст джерелаАнотація:
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Three aspects of magnetic white dwarf stars are studied to aid in the understanding of stellar evolution.
A survey of ~ 50 DC white dwarf stars was conducted in circular spectropolarimetry to search for magnetic fields [...] 30 kG. Four DC stars were discovered with magnetic fields above 30 kG: G 111-49 with B[subscript e] ~ —220 MG, G 183-35 with B[subscript e] = +6.8 ± 0.5 MG, G 256-7 with B[subscript e] = +4.9 ± 0.5 MG, and G 234-4 with B[subscript e] = +39.6 ± 11.6 kG. A new magnetic DB white dwarf was also discovered, LB 8827 with B[subscript e] = 1.0 ± 0.5 MG. A total of 15% of the white dwarfs in the survey have a magnetic field > 30 kG. This value is far larger than the 2% of DA stars, but more than half of the DC stars were originally misclassified. Only 5% of the re-classified DC stars have magnetic fields above 30 kG.
Three magnetic stars from the DC white dwarf survey were re-observed to investigate the possibility of rotation. Two are definitely rotating: LHS 1734 with 16 min [...] P < 1 yr and G 158-45 (=LHS 1044) with a probable period P ~ 11 hr but a definite period P [...] 1 d or P ~ a few days. G 183-35 might be rotating with 50 min [...] P < a few yr. From all the white dwarf rotations known, it is clear that angular momentum is lost before a star becomes a white dwarf, but not clear that the loss is greatly enhanced by magnetic fields.
The isolated magnetic white dwarfs G 99-47, KUV 813-14 (KUV 23162-1220), and G 227-35 were observed in linear and circular spectropolarimetry and then compared to calculated theoretical spectra to find a model for the magnetic field strength and structure. The comparisons were to Stokes' V/I (circular polarization) spectra in addition to total flux F[...], and these add many constraints to the possible solutions. An off-centered dipole or a dipole+quadrupole configuration best fits the observations.
The results of the survey and the modeling are consistent with the theory that the magnetic Ap stars are the predecessors of magnetic white dwarfs.
32
Richer, Harvey. "Lives of White Dwarf Stars." 2008. http://hdl.handle.net/2429/589.
Повний текст джерелаАнотація:
White dwarf stars are the burnt out remnants that remain after a
star like the Sun has completed its nuclear evolution. In such a
star there are no remaining nuclear energy sources, so the star
evolves by simply radiating its stored thermal energy out into
space. This may seem rather uninteresting, but in fact there is a
wealth of physical phenomena that occur during this part of a
star's life - from getting kicked at birth, to neutrino emission
in early life, to some interesting high density physics, through
to functioning as precise clocks that can provide an age for some
of the oldest know stars in the Universe. Some of these phases
will be illustrated with detailed observations taken recently with
the Hubble Space Telescope.
33
Ruan, Kui. "Photospheric studies of M dwarf stars." Phd thesis, 1991. http://hdl.handle.net/1885/138478.
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34
Mullally, Fergal Robert 1979. "Substellar companions to white dwarves." Thesis, 2007. http://hdl.handle.net/2152/3375.
Повний текст джерелаАнотація:
We search for planets and brown dwarves around white dwarves (WDs). Finding extra-solar planets is the first step toward establishing the existence and abundance of life in the Universe. The low mass and luminosity of WDs make them ideal stars to search for low mass companion objects. Theoretical predictions generally agree that a star will consume and destroy close-in, low mass planets as it ascends the red giant and asymptotic giant branch evolutionary tracks, but larger mass objects and those further out will survive. The matter ejected from the star as it evolves into a white dwarf may also be accreted onto daughter planets, or may coalesce into a disk from which planets can form. We employ two techniques to search for planets and brown dwarves (BDs) around WDs. A subset of pulsating white dwarf stars have a pulsational stability that rivals pulsars and atomic clocks. When a planet is in orbit around a such a star the orbital motion of the star around the centre of mass is detectable as a change in arrival times of the otherwise stable pulsations. We search for, and find, a sample of suitable pulsators, monitor them for between three and four years, and place limits on companions by constraining the variation consistent with a 2.4M[subscript J] planet in a 4.6 year orbit. We also observe a large sample of WDs to search for a mid-infrared excess caused by the presence of sub-stellar companions. We present evidence for a potential binary system consisting of a WD and a BD on the basis of an observed excess flux at near and mind-infrared wavelengths. We also place limits on the presence of planetary mass companions around those stars and compare our results to predictions of planetary survival theories. Our findings do not support suggestions of planet formation or accretion of extra mass during stellar death.
35
Allers, Katelyn Natalie. "Disks and dissociation regions: the interaction of young stellar objects with their environments." Thesis, 2005. http://hdl.handle.net/2152/2234.
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36
Kim, Agnès. "Probing exotic physics with pulsating white dwarfs." Thesis, 2007. http://hdl.handle.net/2152/3114.
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37
Kim, Agnès 1975. "Probing exotic physics with pulsating white dwarfs." 2007. http://hdl.handle.net/2152/13301.
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38
Paradis, Dominique. "The effects of crystallization on the pulsations of white dwarf stars." Thèse, 2004. http://hdl.handle.net/1866/14763.
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39
"Physical properties of white dwarf with a dark matter core." 2011. http://library.cuhk.edu.hk/record=b5894665.
Повний текст джерелаАнотація:
Wong, Ka Wing = 含有暗物質核心的白矮星的物理性質 / 黃家榮.Thesis (M.Phil.)--Chinese University of Hong Kong, 2011.
Includes bibliographical references (leaves 81-86).
Abstracts in English and Chinese.
Wong, Ka Wing = Han you an wu zhi he xin de bai ai xing de wu li xing zhi / Huang Jiarong.
Abstract --- p.ii
Acknowledgement --- p.iv
List of Abbreviations --- p.v
List of Figures --- p.vi
Table of Contents --- p.xi
Chapter 0 --- Introduction --- p.1
Chapter 1 --- White Dwarfs --- p.5
Chapter 1.1 --- Introduction --- p.5
Chapter 1.2 --- Observation --- p.6
Chapter 1.3 --- Mass-Radius Relationship & Mass Limit --- p.9
Chapter 1.4 --- Type Ia Supernovae --- p.14
Chapter 2 --- Dark Matter --- p.16
Chapter 2.1 --- Introduction --- p.16
Chapter 2.2 --- Observational Evidences --- p.17
Chapter 2.3 --- Dark Matter Candidates and their Categorization --- p.21
Chapter 3 --- Moon-sized White Dwarf with a Dark Matter Core --- p.26
Chapter 3.1 --- Introduction --- p.26
Chapter 3.2 --- Model --- p.29
Chapter 3.3 --- Results --- p.32
Chapter 3.3.1 --- Typical Density Profile --- p.32
Chapter 3.3.2 --- M-R Curves --- p.33
Chapter 3.3.3 --- Scaling in n --- p.35
Chapter 3.3.4 --- BPS Equation of State --- p.40
Chapter 3.4 --- Discussion --- p.41
Chapter 4 --- Newtonian Hydrodynamic Simulation of a Spherically Symmetric Star --- p.44
Chapter 4.1 --- Introduction --- p.44
Chapter 4.2 --- WENO Scheme --- p.46
Chapter 4.3 --- Runge-Kutta Time Discretization --- p.48
Chapter 4.4 --- Fluid Dynamics without Gravity in One Dimension --- p.49
Chapter 4.4.1 --- Riemann Problem Tests --- p.50
Chapter 4.5 --- Spherically Symmetric Fluid Dynamics without Gravity --- p.54
Chapter 4.5.1 --- Diffusion Problem --- p.55
Chapter 4.6 --- Spherically Symmetric Star --- p.57
Chapter 4.6.1 --- Radial Oscillation of a White Dwarf --- p.58
Chapter 4.6.2 --- Radial Oscillation of a White Dwarf with a Point-Sized Dark Matter Core --- p.64
Chapter 4.7 --- Discussion --- p.68
Chapter 5 --- Newtonian Hydrodynamic Simulation of a Spherically Symmetric Two-Fluid Star --- p.69
Chapter 5.1 --- Introduction --- p.69
Chapter 5.2 --- Spherically Symmetric Two-Fluid Stars --- p.70
Chapter 5.2.1 --- A White Dwarf with a Dark Matter Core --- p.71
Chapter 5.3 --- Discussion --- p.77
Chapter 6 --- Summary --- p.79
Bibliography --- p.81
40
Andrews, Jeffrey. "Double White Dwarfs as Probes of Single and Binary Star Evolution." Thesis, 2016. https://doi.org/10.7916/D8VH5NQ4.
Повний текст джерелаАнотація:
As the endpoints of stars less massive than roughly eight solar masses, the population of Galactic white dwarfs (WD) contain information about complex stellar evolution processes. Associated pairs of WDs add an extra degree of leverage; both WDs must have formed and evolved together. The work presented in this dissertation uses various populations of double WDs (DWD) to constrain evolution of both single and binary stars.
One example is the set of low-mass WDs with unseen WD companions, which are formed through a dynamically-unstable mass loss process called the common envelope. To work toward a quantitative understanding of the common envelope, we develop and apply a Bayesian statistical technique to identify the masses of the unseen WD companions. We provide results which can be compared to evolutionary models and hence a deeper understanding of how binary stars evolve through a common envelope. The statistical technique we develop can be applied to any population of single-line spectroscopic binaries.
Binaries widely separated enough that they avoid any significant interaction independently evolve into separate WDs that can be identified in photometric and astrometric surveys. We discuss techniques for finding these objects, known as wide DWDs. We present a catalog of 142 candidate wide DWDs, combining both previously detected systems and systems we identify in our searches in the Sloan Digital Sky Survey. Having been born at the same time, the masses and cooling ages of the WDs in wide DWDs, obtained with our spectroscopic follow-up campaign can be used to constrain the initial-final mass relation, which relates a main sequence star to the mass of the WD into which it will evolve. We develop a novel Bayesian technique to interpret our data and present our resulting constraints on this relation which are particularly strong for initial masses between two and four solar masses.
During this process, we identified one wide DWD, HS 2220+2146, that was peculiar since the more massive WD in this system evolved second. We construct an evolutionary formation scenario in which the system began as a hierarchical triple in which the inner binary merged (possibly due to Kozai-Lidov oscillations) forming a post-blue straggler binary. The system then evolved into the DWD we observe today. We further discuss the potential for identifying more wide DWDs, including peculiar systems like HS 2220+2146, in future surveys such as Gaia.
41
Weston, Jennifer Helen Seng. "Radio Observations as a Tool to Investigate Shocks and Asymmetries in Accreting White Dwarf Binaries." Thesis, 2016. https://doi.org/10.7916/D87M082M.
Повний текст джерелаАнотація:
This dissertation uses radio observations with the Karl G. Jansky Very Large Array (VLA) to investigate the mechanisms that power and shape accreting white dwarfs (WD) and their ejecta. We test the predictions of both simple spherical and steady-state radio emission models by examining nova V1723 Aql, nova V5589 Sgr, symbiotic CH Cyg, and two small surveys of symbiotic binaries.
First, we highlight classical nova V1723 Aql with three years of radio observations alongside optical and X-ray observations. We use these observations to show that multiple outflows from the system collided to create early non-thermal shocks with a brightness temperature of ⪆10⁶ K. While the late-time radio light curve is roughly consistent an expanding thermal shell of mass 2x10⁻⁴ M ⊙ solar masses, resolved images of V1723 Aql show elongated material that apparently rotates its major axis over the course of 15 months, much like what is seen in gamma-ray producing nova V959 Mon, suggesting similar structures in the two systems. Next, we examine nova V5589 Sgr, where we find that the early radio emission is dominated by a shock-powered non-thermal flare that produces strong (kTₓ > 33 keV) X-rays. We additionally find roughly 10⁻⁵ M⊙ solar masses of thermal bremsstrahlung emitting material, all at a distance of ~4 kpc. The similarities in the evolution of both V1723 Aql and V5589 Sgr to that of nova V959 Mon suggest that these systems may all have dense equatorial tori shaping faster flows at their poles.
Turning our focus to symbiotic binaries, we first use our radio observations of CH Cyg to link the ejection of a collimated jet to a change of state in the accretion disk. We additionally estimate the amount of mass ejected during this period (10⁻⁷ M⊙ masses), and improve measurements of the period of jet precession (P=12013 +/- 74 days). We then use our survey of eleven accretion-driven symbiotic systems to determine that the radio brightness of a symbiotic system could potentially be used as an indicator of whether a symbiotic is powered predominantly by shell burning on the surface of the WD or by accretion. We additionally make the first ever radio detections of seven of the targets in our survey. Our survey of seventeen radio bright symbiotics, comparing observations before and after the upgrades to the VLA, shows the technological feasibility to resolve the nebulae of nearby symbiotic binaries, opening the door for new lines of research. We spatially resolve extended structure in several symbiotic systems in radio for the first time. Additionally, our observations reveal extreme radio variability in symbiotic BF Cyg before and after the production of a jet from the system. Our results from our surveys of symbiotics provide some support for the model of radio emission where the red giant wind is photoionized by the WD, and suggests that there may be a greater population of radio faint, accretion driven symbiotic systems. This work emphasizes the powerful nature of radio observations as a tool for understanding eruptive WD binaries and their outflows.
42
Ferrario, L. "Accretion processes in AM Herculis systems." Phd thesis, 1989. http://hdl.handle.net/1885/136627.
Повний текст джерелаАнотація:
We begin by presenting a theoretical model for the radial velocity and
velocity dispersion of the broad emission line component in AM Herculis
systems assuming that it originates mainly in the gas which is diverted out
of the orbital plane and funneled on to the white dwarf surface along magnetic
field lines. The model is used to locate the line forming region in three AM
Her variables using as constraints the radial velocity and velocity dispersion
data. This work shows that the material is threaded by the magnetic field
in a very azimuthally extended coupling region located 0.5 - 0. 75 of the way
between the white dwarf and the inner Lagrange point. This implies that
the cyclotron emission region on the stellar surface is extended, probably
arc-shaped, and offset from the magnetic pole.
Models for cylindrically extended and arc-shaped cyclotron emission regions
displaced from the magnetic poles are then generated, taking into account
the effects of field spread across the shock. The properties of cylindrically
extended models with density and temperature structure and geometrical
extension are also investigated. These models are an improvement
on previous point source models and have characteristics that are in better
overall agreement with the properties of AM Herculis systems.
Phas dependent spectropolarimetric observations of VV Puppis which
show, for the first time, cyclotron lines from both poles are presented. The
rnain and secondary emission regions have fields of 30.5 MG and 56 MG
respectively. The observations have been interpreted in terms of a dipole
which is offset by ~ 0.1 white dwarf radii from the center of the star in the
direction of the dipole axis with the emission regions located near the foot
points of closed field lines.
Spectroscopic and circular polarization observations of the soft X-ray
clipsing binary EXO 033319 - 2554.2 which confirm it as a member of the AM Herculis class are presented. Broad resolvable and variable cyclotron
harmonics from both poles are seen during the bright phase corresponding
to magn tic fields of 56 MG (main region) and 28 MG (secondary region).
As is the case for VV Puppis, the large difference in magnetic field str ngth
of the two regions implies a more complex field structure than that of a cent
red dipole field distribution, if it is assumed that the regions are connected
by closed field lines.
U BV RI and long term plate photometry of the faint and highly erratic
cataclysmic variable discovered from a sequence of UK Schmidt measurements
are presented. We show that this object ( designated Grus Vl) is
almost certainly a new AM Herculis variable, and develop a model in which
the light curves are explained as originating from two nonpolar cyclotron
em1ss1on regions.
43
"Properties of strange stars." 2003. http://library.cuhk.edu.hk/record=b5891677.
Повний текст джерелаАнотація:
Wong Ka Wah = 奇異星的特性 / 黃嘉華.Thesis (M.Phil.)--Chinese University of Hong Kong, 2003.
Includes bibliographical references (leaves 98-101).
Text in English; abstracts in English and Chinese.
Wong Ka Wah = Qi yi xing de te xing / Huang Jiahua.
Chapter 1 --- Introduction --- p.1
Chapter 1.1 --- History of Compact Stars --- p.1
Chapter 1.2 --- The Proposal of Strange Quark Stars --- p.2
Chapter 1.3 --- Outline of the Thesis --- p.3
Chapter 2 --- Cold Equation of State from Perturbative QCD --- p.6
Chapter 2.1 --- Description of Strange Quark Matter --- p.7
Chapter 2.2 --- MIT Bag Model --- p.8
Chapter 2.3 --- Perturbative QCD --- p.10
Chapter 2.4 --- Comparison with MIT Bag Model --- p.11
Chapter 3 --- Static Structure of Strange Stars --- p.16
Chapter 3.1 --- Static Equilibrium --- p.16
Chapter 3.2 --- Models --- p.18
Chapter 3.3 --- Results of Global Properties and Discussions --- p.18
Chapter 4 --- Stability of Strange Quark Matter --- p.25
Chapter 4.1 --- Absolute Stable Condition --- p.25
Chapter 4.2 --- Weak Stable Condition --- p.26
Chapter 4.3 --- Stability Condition Compared to Neutron Stars --- p.27
Chapter 4.4 --- Conclusion --- p.28
Chapter 5 --- Effect of Massive Strange Quarks --- p.31
Chapter 5.1 --- Numerical Analysis of the Effect of Strange Quark Mass on the EOS --- p.33
Chapter 5.2 --- Structure of Strange Stars with Strange Quark Mass --- p.37
Chapter 5.3 --- Conclusion --- p.38
Chapter 6 --- QCD Phase Transition in a Compact Star --- p.46
Chapter 6.1 --- Cooling Properties --- p.47
Chapter 6.1.1 --- Heat capacity of quark stars --- p.49
Chapter 6.1.2 --- Luminosity of quark stars --- p.50
Chapter 6.1.3 --- Microphysics of the neutron star cooling --- p.54
Chapter 6.2 --- Handling of the Phase Transition --- p.56
Chapter 6.3 --- The Models --- p.59
Chapter 6.4 --- Results --- p.60
Chapter 6.4.1 --- Method 1 --- p.61
Chapter 6.4.2 --- Method 2 --- p.66
Chapter 6.5 --- Discussion and Conclusion --- p.66
Chapter 7 --- Formation of a Strange Star --- p.73
Chapter 7.1 --- Formalism of the Problem --- p.73
Chapter 7.2 --- Lagrangian Hydrodynamics --- p.74
Chapter 7.3 --- Hot Equation of State --- p.75
Chapter 7.3.1 --- Nuclear Matter EOS --- p.75
Chapter 7.3.2 --- Quark Matter EOS --- p.77
Chapter 7.3.3 --- Mixed Phase --- p.78
Chapter 7.4 --- Initial Models --- p.78
Chapter 7.5 --- Results --- p.80
Chapter 7.6 --- Discussion and Conclusion --- p.81
Chapter 8 --- Conclusion --- p.95
Bibliography --- p.98
Chapter A --- Solving the EOS --- p.102
Chapter B --- Solving C from Eq. (7.10) --- p.105
44
Kilic, Mukremin. "Cool white dwarfs and the age of the galaxy." Thesis, 2006. http://hdl.handle.net/2152/2733.
Повний текст джерела
45
Falcon, Ross Edward. "Creating and measuring white dwarf photospheres in a terrestrial laboratory." Thesis, 2014. http://hdl.handle.net/2152/25906.
Повний текст джерелаАнотація:
As the ultimate fate of nearly all stars, including our Sun, white dwarfs (WDs) hold rich and informative histories in their observable light. To determine a fundamental parameter of WDs, mass, we perform the first measurement of the average gravitational redshift of an ensemble of WDs. We find a larger mean mass than that determined from the primary and expansive technique known as the spectroscopic method. The potential inaccuracy of this method has broad astrophysical implications, including for our understanding of Type 1a supernova progenitors and for constraining the age of the Universe. This motivates us to investigate the WD atmosphere models used with the spectroscopic method, particularly the input theoretical line profiles, by developing a new experimental platform to create plasmas at WD photospheric conditions (T_e ~ 1 eV, n_e ~ 10^17 cm^-3). Instead of observing WD spectra to infer the plasma conditions at the surface of the star, we set the conditions and measure the emergent spectra in the laboratory. X-rays from a z-pinch dynamic hohlraum generated at the Z Pulsed Power Facility at Sandia National Laboratories irradiate a gas cell to initiate formation of a large (120x20x10 mm or 24 cm^3) plasma. We observe multiple Balmer lines from our plasma in emission and in absorption simultaneously along relatively long (~120 mm) lines of sight perpendicular to the heating radiation. Using a large, radiation-driven plasma aides us to achieve homogeneity along our observed lines of sight. With time-resolved spectroscopy we measure lines at a range of electron densities that spans an order of magnitude, and we do this within one pulsed power shot experiment. Observing our plasma in absorption not only provides the signal-to-noise to measure relative line shapes, it allows us to measure relative line strengths because the lines share the same lower level population. This constrains the theoretical reduction factors used to describe ionization potential depression or the occupation probabilities associated with these Balmer lines. We compare our measured line shapes with the theoretical ones used in WD atmosphere models as part of the first fruits of this rich experimental platform.text
46
Mukadam, Anjum Shagufta. "Ensemble characteristics of the ZZ Ceti stars." Thesis, 2004. http://hdl.handle.net/2152/1278.
Повний текст джерела
47
Mukadam, Anjum Shagufta Winget Donald Earl. "Ensemble characteristics of the ZZ Ceti stars." 2004. http://wwwlib.umi.com/cr/utexas/fullcit?p3143434.
Повний текст джерела
48
Hermes, James Joseph Jr. "A search for periodic variations in pulse arrival times in DA white dwarfs." Thesis, 2010. http://hdl.handle.net/2152/ETD-UT-2010-08-1998.
Повний текст джерелаАнотація:
We present updated observations of a pilot survey of 14 pulsating DA white dwarfs, monitored for evidence of center-of-mass motion caused by a planetary companion. We have nearly doubled the number of periodicites for which we can produce O-C diagrams that document pulse arrival times from
our stars, and have implemented a method to minimize the apertures we use in our reductions in order to reduce sky noise. In addition to a previously published candidate, GD66, we have identi fed at least four more systems worthy
of rigorous observational follow-up. We have also implemented a method,
a generalized Lomb-Scargle periodogram, that takes into account weighted points in order to characterize any periodic behavior present in our O-C
diagrams.
For at least one DAV within this same sample, we have found strong
observational evidence for an evolutionary time scale (via the rate of period change) that is inconsistent with cooling alone. In that star, WD0111+0018, we report for the first time measurement of the rate of period change of nonlinear combination frequencies in a pulsating white dwarf. We speculate that this may be caused by a changing rotation rate that aff ects only modes with
m not equal to 0.text
49
Stephens, David. "3D1D modeling of the convective-reactive mixing in rapidly accreting white dwarfs." Thesis, 2019. http://hdl.handle.net/1828/11414.
Повний текст джерелаАнотація:
1D stellar evolution and nucleosynthesis simulations have traditionally modeled the mixing within convection zones as a diffusive process. The fluids within a convection zone are advecting and do not diffuse. However the diffusive approximation is valid when the burning timescale of an exothermic reaction is longer than the convective turn over timescale to which the mixing of those species is approximated over. Since it is 1D, it also assumes that the material is isotropically distributed within the convection zone. In the He-flash convection zones of rapidly accreting white dwarfs (RAWD) H is ingested and burned well within the convective turn over time of 38 minutes. The H is burned through the exothermic 12C(p,γ)13N reac- tion, Q = 1.944 MeV, and then the unstable 13N, with a half-life of 9.6 minutes, will decay to 13C which will undergo the 13C(α,n)16O reaction releasing neutrons. The neutron densities, depending on the H-ingestion rates and mixing details, reach Nn ≈ 1013 − 1015 cm−3 which starts the i-process within the convection zone. The H burning provides energy to the flow leading to the dynamic details of the flow being important for the mixing of the H and thus the i-process nucleosynthesis. This is a convective-reactive environment. The isotropic, well mixed over many convective turn over timescales, and long burning timescale assumptions for H in the diffusive approximation are broken in the convective-reactive environment of a He-shell flash convection zone in a RAWD.
To more accurately model convective-reactive mixing environments, a 1D two stream advective mixing model is formulated. A downstream advects H-rich material from the top of the convection zone down to the H-burning region while the upstream advects H-poor material back up to the upper convective boundary. The mixing model includes a horizontal mass flux, γ, which describes the efficiency to which mass is mixed between the two streams. This predominately causes the homogenization of the material between the two streams. The radial mass flux, α, and the horizontal mass flux, γ, are calibrated from 3D hydrodynamic simulations of the RAWD in order to model the mixing within the He-flash shell convection zone.
The downsampled 3D cartesian data output, the briquette data, from the 3D hy- drodynamic simulations is used to compute γ. This required using numerical tools to interpolate quantities onto spherical shells from 3D cartesian data and to decompose the radial velocity field into its spherical harmonic modes. Trilinear interpolation is the simplest 3D interpolation method that was tested and it was the interpolation method of choice due to the constraints it has on the interpolating function. The validity of using higher order methods on the briquette data was studied in detail but was determined to not be usable due to the computational effort and constraints of the methods.
The two stream model post-processing of the H burning within the 3D hydro- dynamic simulations of the RAWD showed excellent agreement in the metrics of the total mass of H burned, the burning rate and burning location of H. This includes two models which undergo dramatic H-ingestion and burning events caused by a GOSH, Global Oscillations of Shell H-ingestion. By adding a network containing 1000’s of species to the 1D advective mixing model, the i-process from the RAWD is simulated and compared with a traditional 1D diffusive mixing model. The resulting neutron densities between the two models are comparable however the efficiency to which each produce the heaviest stable elements are different. To reproduce the elemental abun- dance distribution of the CEMP-r/s star CS31062-050, the diffusive model is run for 15 days of stellar time while the advective model is run for 20 days. The H-ingestion into the He-shell as predicted by the stellar evolution calculations lasts 30 days. The i-process material within the RAWD can be removed from it and participate in the galactic chemical evolution of the galaxy that it resides in. This is due to the RAWD possibly reaching the Chandrasekhar mass and from the loss of material through stellar winds and common envelope interactions with its nearby companion star.Graduate
50
Lucy, Adrian B. "The Detection and Description of Symbiotic Accretion From Cool Evolved Stars." Thesis, 2021. https://doi.org/10.7916/d8-352d-xr22.
Повний текст джерелаАнотація:
Symbiotic stars are binaries consisting of a cool evolved G-M/S/C I-III star accreting onto a smaller companion---but the accretion disk itself is rarely detected. Accretion signatures like hard X-rays and optical/ultraviolet flickering are usually suppressed or outshone by shell burning on the accreting white dwarf, the luminous giant, and the giant's wind nebula. In Chapters 2 and 3, we present a new way to find symbiotics that is less biased against accreting-only, non-burning symbiotics with directly detectable accretion disks. Our search methodology is based on finding outliers in SkyMapper Southern Sky Survey broad-band and intermediate-band photometry, using a parameter space built from reconstructed u-g u-v snapshot colors and rapid variability between the three exposures of a 20-minute SkyMapper Main Survey filter sequence, from a sample of luminous red objects selected with 2MASS and Gaia.
In a pilot survey employing this new search design, we discovered 12 new symbiotics, including four symbiotics with optical accretion disk flickering and at least two with boundary-layer hard X-rays, as well as 10 new symbiotic candidates. We also discovered optical flickering in the known symbiotic V1044 Cen (CD-36 8436). We conclude that at least 20% of the true population of symbiotics exhibits detectable optical flickering from the inner accretion disk, a substantial fraction of which would not meet the usual H-alpha equivalent width detection thresholds typically used to find symbiotics with traditional narrow-band emission line photometry surveys. There is a significant population of optically-flickering symbiotics hidden both within and beyond the known catalogs of symbiotic stars---however, the question of whether the true population of accreting-only symbiotics is larger than the population of burning symbiotics remains unanswered. We also find that our methods probe a completely different region of parameter space than recent work by the Munari et al. (2021) search for accreting-only symbiotics, while being surprisingly in harmony with the Akras et al. (2019) infrared selection criteria.
As an intermediate step in our pilot survey, we explored several outlying regions in our SkyMapper parameter space with optical spectroscopy of 234 luminous red objects, which we present in a 248-page spectral atlas. Our results identify a zone of the u-g u-v snapshot color-color diagram in which virtually all objects are symbiotics. When all-sky uvg colors become available through future DRs of SkyMapper and MEPHISTO, between about 51 and 117 symbiotics missed by previous surveys (of which 11 to 17 have been reported in this work) will be discoverable using only this mostly-symbiotic zone of the color-color diagram, with a near-zero contamination rate. Main Survey filter-sequence variability is also a powerful tool for finding hidden, flickering symbiotics both inside and outside of the mostly-symbiotic color-color zone, but variability must still be used in conjunction with color; there must be enough of an accretion disk contribution to the u-band for it to exhibit detectable variability. We show that yellow post-AGB stars with strong Balmer jump absorption (along with the symbiotic Southern Crab) are outliers with large positive u-v, while some S and carbon stars are outliers with large negative u-v. We also show that it is important to correct the results of SkyMapper's catalog pipeline for variability when dealing with samples containing large-amplitude pulsating stars.
In Chapters 4 and 5, we present an in-depth study of one of the few optically-flickering symbiotic stars previously known, MWC 560 (V694 Mon). The persistent outflow from MWC 560 is known to manifest as broad absorption lines (BALs), most prominently at the Balmer transitions. In Chapter 4, we report the detection of high-ionization BALs from C IV, Si IV, N V, and He II in International Ultraviolet Explorer spectra obtained on 1990 April 29-30, when an optical outburst temporarily erased the obscuring "iron curtain" of absorption troughs from Fe II and similar ions. The C IV and Si IV BALs reached maximum radial velocities at least 1000 km/s higher than contemporaneous Mg II and He II BALs; the same behaviors occur in the winds of quasars and cataclysmic variables. An iron curtain lifts to unveil high-ionization BALs during the P Cygni phase observed in some novae, suggesting by analogy a temporary switch in MWC 560 from persistent outflow to discrete mass ejection. At least three more symbiotic stars exhibit broad absorption with blue edges faster than 1500 km/s; high-ionization BALs have been reported in AS 304 (V4018 Sgr), while transient Balmer BALs have been reported in Z And and CH Cyg. These BAL-producing fast outflows can have wider opening angles than has been previously supposed. BAL symbiotics are short-timescale laboratories for their giga-scale analogs, broad absorption line quasars (BALQSOs), which display a similarly wide range of ionization states in their winds.
In Chapter 5, we investigate how the accretion disc of MWC 560 is affected by its outflow. We performed optical, radio, X-ray, and ultraviolet observations of MWC 560 during its 2016 optical high state. We tracked multi-wavelength changes that signalled an abrupt increase in outflow power at the initiation of a months-long outflow fast state, just as the optical flux peaked: (1) an abrupt doubling of Balmer absorption velocities; (2) the onset of a 20 𝜇Jy/month increase in radio flux; and (3) an order-of-magnitude increase in soft X-ray flux. Juxtaposing to prior X-ray observations and their coeval optical spectra, we infer that both high-velocity and low-velocity optical outflow components must be simultaneously present to yield a large soft X-ray flux, which may originate in shocks where these fast and slow absorbers collide. Our optical and ultraviolet spectra indicate that the broad absorption-line gas was fast, stable, and dense (⪞10⁶.⁵ cm⁻³) throughout the 2016 outflow fast state, steadily feeding a lower-density (⪝10⁵.⁵ cm⁻³) region of radio-emitting gas. Persistent optical and ultraviolet flickering indicate that the accretion disc remained intact. The stability of these properties in 2016 contrasts to their instability during MWC 560's 1990 outburst, even though the disc reached a similar accretion rate. We propose that the self-regulatory effect of a steady fast outflow from the disc in 2016 prevented a catastrophic ejection of the inner disc. This behaviour in a symbiotic binary resembles disc/outflow relationships governing accretion state changes in X-ray binaries.