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1
Beletsky, Yuri. "Extragalactic molecular clouds and chemistry of diffuse interstellar clouds". Diss., lmu, 2009. http://nbn-resolving.de/urn:nbn:de:bvb:19-105670.
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2
Vichetti, Rafael Mário [UNESP]. "Síntese dos isótopos do monóxido de carbono no meio interestelar". Universidade Estadual Paulista (UNESP), 2009. http://hdl.handle.net/11449/91889.
Testo completoAbstract (sommario):
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De acordo com os resultados observacionais de condensações de nuvens moleculares escuras, grandes variações na razão 13CO/C18O são observadas quando se comparam os resultados obtidos nas condensações situadas dentro da mesma nuvem, bem como de nuvem para nuvem. O valor médio dessa razão na condensação principal de Ophiuchus é inferior a 5. Por outro lado, o valor encontrado nas condensações que estão situadas ao norte de Oph é maior que 10. Grandes diferenças também são encontradas quando se comparam os resultados observacionais de diferentes nuvens escuras, tais como Ophiuchus e Taurus, onde são observados também um decréscimo da razão C18O/C17O com o aumento da densidade. Os processos químicos e físicos que governam essas variações ainda não estão claros. Nesse sentido, o objetivo da presente proposta é analisar a influência do colapso gravitacional de condensações de nuvens moleculares escuras na síntese das moléculas CO, C17O, C18O, 13CO, 13C17O e 13C18O. Tal análise é feita com base em comparações entre modelos que consideram diferentes condições entre si, tais como, tamanho da cadeia química, velocidade de colapso, densidade inicial e processos de congelamento de espécies químicas na superfície de grãos de poeira. Os resultados obtidos mostram que o tamanho da cadeia química tem influência nas razões 13CO/C18O e C18O/C17O, mas não tanto quanto a densidade inicial e a velocidade do colapso. Além disso, o congelamento das espécies químicas nos grãos é mais significativo nos estágios mais avançados da evolução da condensação. Os modelos de condensações escuras que sofrem colapso gravitacional lento e em queda livre reproduzem satisfatoriamente as razões 13CO/C18O e C18O/C17O observadas, o que permite concluir que o colapso gravitacional pode ter um importante efeito nas referidas razões.
According to the observational results of dark molecular clouds condensations, large variations in the ratio 13CO/C18O are observed when comparing the results obtained in the condensations located within the same cloud and cloud to cloud. The average value of this ratio in the main condensation of Ophiuchus is below 5. On the other hand, the value found in the condensations that are located north of Oph is larger than 10. Large differences are also found when comparing the observational results of different dark clouds such as Ophiuchus and Taurus, in which are also found a decrease of the C18O/C17O ratio with increasing density. The chemical and physical processes that govern these variations are still unclear. In this sense, the objective of this proposal is to analyze the influence of the gravitational collapse of centrally condensed clumps of dense molecular gas in the synthesis of the CO, C17O, C18O, 13CO, 13C17O and 13C18O molecules. This analysis is based on comparisons among models that consider different condition, such as, chemical chain, initial density, speed of collapse and freezing processes of the chemical species on the surface of dust grains. The results show that the size of the chemical chain has influence on the 13CO/C18O and C18O/C17O ratios, but they are not as important as the initial density and the speed of the collapse. Furthermore, the freezing of chemical species on the grains occurs at later times of the collapse. The models of a gravitational free-fall collapsing core and of slowly contracting core with higher initial density are consistent with observations. These results indicate that the gravitational collapse of molecular cores can have an important effect in the 13CO/C18O and C18O/C17O ratios.
3
Brown, Ian David. "The velocity of molecular clouds". Thesis, University of Cambridge, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.293612.
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4
Bretherton, Derek. "Star formation in molecular clouds". Thesis, Liverpool John Moores University, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.402927.
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5
Rowles, Jonathan Henry. "The structure of molecular clouds". Thesis, University of Kent, 2011. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.544095.
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6
Richardson, Kevin John. "Submillimetre molecular line observations and modelling of molecular clouds". Thesis, Queen Mary, University of London, 1985. http://qmro.qmul.ac.uk/xmlui/handle/123456789/1705.
Testo completoAbstract (sommario):
Submillimetre molecular line observations of molecular clouds in our galaxy are presented, and the data analysed using various alternative cloud models. A critical review is given of the methods commonly used to interpret molecular line data, including both theoretical considerations and issues relating to calibration and comparability of results obtained with different telescopes. A detailed comparison is made between results predicted from large velocity gradient (LVG) models, including the generalisation to non-monotonic velocity flows, and those given by "microturbulent" clouds. An LVG model is employed in an investigation of conditions in the molecular outflows frequently found in star formation regions, for which observations in the CO J=3-2 rotational transition at 345 GHz are presented. These are combined with lower frequency data from the literature to derive various properties of the outflows for a sample of 13 sources. The most important result is that local H2 densities exist in the outflows which are higher, typically by an order of magnitude, than previously derived average values obtained using only lower frequency data. Observations are presented of the S255 and DR21 clouds in the transitions CO J=2-1, CO J=3-2, CS J=7-6, HCN J=4-3, HCO+ J=4-3 and -3- H13CO+ J=4-3 and are supplemented by continuum data at 350 s. n and (for DR21) at 20 pm. It is shown that, although some features of the data can be understood in terms of an LVG model, there is compelling evidence for fragmentation of the clouds on length scales much smaller than the cloud sizes. The data are used to constrain the local H2 densities and relative molecular abundances in the clumpy cloud cores, and compared with lower frequency results from the literature. The implications of these results for the star formation environment are discussed, and an assessment made of possible strategies for their further investigation.
7
McElroy, Daniel. "Grain surface chemistry in molecular clouds". Thesis, Queen's University Belfast, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.602462.
Testo completoAbstract (sommario):
This work ia a study of chemistry in molecular clouds. I begin by describing the improvements made to gas phase chemical reaction data in the recent release of the UMIST database for astrochemistry (Rate 12). Improvements to the reaction network include the addition of anions, new reaction rate coefficient and branching rate measurements across all reactions types and newly calculated photodissociation and photoionisation rates.
8
Morisawa, Yusuke. "Spectroscopic study of some chemically significant molecules in molecular clouds". 京都大学 (Kyoto University), 2005. http://hdl.handle.net/2433/144599.
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García, Fuentes Pablo Fernando. "Giant Molecular Clouds in the Southern Milky Way". Tesis, Universidad de Chile, 2007. http://www.repositorio.uchile.cl/handle/2250/104575.
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Dobbs, Clare Louise. "The formation of molecular clouds in spiral galaxies /". St Andrews, 2007. http://hdl.handle.net/10023/214.
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Fiege, Jason D. "Filamentary molecular clouds and their prolate cores". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape9/PQDD_0032/NQ66208.pdf.
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Hobson, Michael Paul. "The small-scale structure of molecular clouds". Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.282113.
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Visser, Anja Eveline. "Star forming cores in dark molecular clouds". Thesis, University of Cambridge, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.620970.
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14
Penaloza, Cabrera Camilo. "Giant molecular clouds : a view through molecular tracers and synthetic images". Thesis, Cardiff University, 2018. http://orca.cf.ac.uk/116132/.
Testo completoAbstract (sommario):
Line emission is strongly dependent on the local environmental conditions in which the emitting tracers reside. In this work, we focus on modelling the CO emission from simulated giant molecular clouds (GMCs), and study the variations in the resulting line ratios arising from the emission from the J = 1 − 0, J = 2 − 1 and J = 3 − 2 transitions. We first study the ratio (R2−1/1−0) between CO’s first two emission lines and examine what information it provides about the physical properties of the cloud. To study R2−1/1−0 we perform smooth particle hydrodynamic simulations with time dependent chemistry (using GADGET-2), along with post-process radiative transfer calculations on an adaptive grid (using RADMC-3D) to create synthetic emission maps of a MC. R2−1/1−0 has a bimodal distribution that is a consequence of the excitation properties of each line, given that J = 1 reaches local thermal equilibrium (LTE) while J = 2 is still sub-thermally excited in the considered clouds. The bimodality of R2−1/1−0 serves as a tracer of the physical properties of different regions of the cloud and it helps constrain local temperatures, densities and opacities. Then to study the dependence line emission has on environment we perform a set of smoothed particle hydrodynamics (SPH) simulations with time-dependent chemistry, in which environmental conditions – including total cloud mass, density, size, velocity dispersion, metallicity, interstellar radiation field (ISRF) and the cosmic ray ionisation rate (CRIR) – were systematically varied. The simulations were then post-processed using radiative transfer to produce synthetic emission maps in the 3 transitions quoted above. We find that the cloud-averaged values of the line ratios can vary by up to ±0.3 dex, triggered by changes in the environmental conditions. Changes in the ISRF and/or in the CRIR have the largest impact on line ratios since they directly affect the abundance, temperature and distribution of CO-rich gas within the clouds. We show that the standard methods used to convert CO emission to H2 column density can underestimate the total H2 molecular gas in GMCs by factors of 2 or 3, depending on the environmental conditions in the clouds. One of the underlying assumptions in star formation is that stars are formed in long lived, bound molecular clouds. This paradigm comes from examining the virial parameter of molecular clouds. To calculate the virial parameter we rely on three quantities: velocity dispersion, size and mass, each of which have their own underlying assumptions, uncertainties and biases. It should come as no surprise that variations in these quantities can have a significant impact on our assessment of cloud dynamics and hence our overall understanding of star formation. We therefore use CO line emission from synthetic observation to study how the dynamical state of clouds changes as a function of metallicity and to test how accurately the virial parameter traces these changes. First we show how the ”observed” velocity dispersion significantly decreases with lower metallicities and how this is reflected on the virial parameter. Second we highlight the importance of understanding the intrinsic assumptions that go into calculating the virial parameter, such as how the mass and radius are derived. Finally, we show how the virial parameter of a cloud changes with metallicity and how the ’observed’ virial parameter compares to the ’true’ value in the simulation.
15
Hunt, Maria, University of Western Sydney, of Science Technology and Environment College e School of Engineering and Industrial Design. "Molecules in southern molecular clouds: a millimetre-wave study of dense cores". THESIS_CSTE_EID_Hunt_M.xml, 2001. http://handle.uws.edu.au:8081/1959.7/116.
Testo completoAbstract (sommario):
This thesis presents an observational study of molecular abundances in the dense cores of 27 prominent molecular clouds in the southern galactic plane.The molecular abundances and physical conditions in dense condensations have been derived from millimetre-wavelength observations of molecular rotational transitions.The study has produced a comprehensive data set of transition intensities and abundances for 10 different molecules in bright southern molecular clouds, and the general characteristics of emissions from these molecules such as optical depth, excitation and relative abundances are discussed. A comparison of different methods of calculating molecular hydrogen column density from observations of carbon monoxide emission is included.Both the analysis and the data collected provide an excellent starting point for further observational and theoretical studies of molecular clouds in the southern Milky Way utilising new instruments such as the millimeter-wave upgrade to the Australia Telescope Compact Array and the Attacama Large Millimetre Array (ALMA).Doctor of Philosophy (PhD)
16
Hunt, Maria. "Molecules in southern molecular clouds: a millimetre-wave study of dense cores". Thesis, View thesis View thesis, 2001. http://handle.uws.edu.au:8081/1959.7/116.
Testo completoAbstract (sommario):
This thesis presents an observational study of molecular abundances in the dense cores of 27 prominent molecular clouds in the southern galactic plane.The molecular abundances and physical conditions in dense condensations have been derived from millimetre-wavelength observations of molecular rotational transitions.The study has produced a comprehensive data set of transition intensities and abundances for 10 different molecules in bright southern molecular clouds, and the general characteristics of emissions from these molecules such as optical depth, excitation and relative abundances are discussed. A comparison of different methods of calculating molecular hydrogen column density from observations of carbon monoxide emission is included.Both the analysis and the data collected provide an excellent starting point for further observational and theoretical studies of molecular clouds in the southern Milky Way utilising new instruments such as the millimeter-wave upgrade to the Australia Telescope Compact Array and the Attacama Large Millimetre Array (ALMA).
17
Hunt, Maria. "Molecules in southern molecular clouds : a millimetre-wave study of dense cores /". View thesis View thesis, 2001. http://library.uws.edu.au/adt-NUWS/public/adt-NUWS20030416.160909/index.html.
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18
Dobbs, Clare L. "The formation of molecular clouds in spiral galaxies". Thesis, University of St Andrews, 2007. http://hdl.handle.net/10023/214.
Testo completoAbstract (sommario):
Molecular clouds are imperative to astronomy as the sites of all known star formation. The problem of how molecular clouds are formed in spiral galaxies is approached numerically, by modelling the response of a gas disk to a spiral potential. The importance of spiral shocks is highlighted as a dominant formation mechanism for molecular clouds in grand design galaxies, where a strong density wave is present. The spiral shock both increases the density of the interstellar gas significantly, and produces structure in the spiral arms. The gas evolves into discrete clumps, which are shown to contain substantial densities of molecular hydrogen, and are therefore identified as molecular clouds. The formation of these clouds requires that the interstellar medium (ISM) is cold and inhomogeneous. The passage of an inhomogeneous gas distribution through a spiral potential further shows that supersonic velocities are induced as the gas shocks. This can explain the velocity dispersion relation observed in molecular clouds. Finally, the shearing of clumps of gas in the spiral arms leads to the formation of inter-arm structures, which are commonly observed in spiral galaxies.
19
Olofsson, Sven. "Extinction in Molecular Clouds : Case of Barnard 335". Doctoral thesis, Stockholms universitet, Institutionen för astronomi, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-72523.
Testo completoAbstract (sommario):
The Bok globule B335 is a small molecular cloud in the solar neighbourhood near the galactic plane. The aim for this three-paper-study is to construct and analyze the extinction for this globule. The method we apply is to use the light from field stars behind the cloud in broadband filters ranging from UV to the mid-infrared. We have observations performed at the ESO telescopes at La Silla and Paranal as well as at the Nordic 2.5 m telescope at La Palma. Together with images and spectra from 2MASS-, ISO- and Spitzer-archives we are able to cover the wavelength range from 0.35 to 24 μm. An important tool to analyze these observations results in order to get the extinction is the grid of synthetic stellar atmospheric spectra provided by Hauschildt (2005). The extinction so received is a result in itself. From the analysis of the extinction wavelength dependence we derive properties of the dust, especially its composition and grain size distribution. By modeling the grain size distribution we are able to find the extinction from the reddening of the stars. We find that the extinction in the optical wavelength 0.35 to 2 μm range nicely follows the functional form described by Cardelli et al. (1989). Our result from the wavelength range redward of 2 μm show an extinction dependent on the part of the cloud examined. For the rim of the cloud we get an extinction similar to that reported earlier for the diffuse interstellar medium. From the central parts of the cloud, however, a higher extinction was found. Our grain size model contains a carbonaceous particle distribution and a silicate one. The result can be explained by depletion of carbon onto carbonaceous grains and also by carbon onto all grains including the silicates. Our modeling of the extinction and our classification of the background stars allow us to - determine the distance to the globule - estimate the gas column density ratio - estimate the mass of globule - get a handle on the dust conversion processes through the grain size distribution From the water- and CO-ice spectra we are able to estimate the ice column densities. We find similar ice column densities for the two ices. The estimates differ, when calculated from band strengths or from Lorenz-Mie calculations of ice mantles on the grain size distribution, by a factor of two.
20
Clark, Paul Campbell. "The onset of gravitational collapse in molecular clouds". Thesis, University of St Andrews, 2005. http://hdl.handle.net/10023/12945.
Testo completoAbstract (sommario):
We conduct an investigation into the role that turbulence plays in the formation of stars. In small clouds, with masses of ~ 30 Mʘ and where the turbulence is only injected at the start, we find that the turbulence does not trigger star formation. Instead, the dissipation of the kinetic energy allows the mean Jeans mass of the cloud to control the formation of stars. The equipartition of the kinetic and thermal energies in the final stages before star formation, allows the pre-protostellar clumps to fragment. Binary and multiple systems are thus a natural product of star formation in a turbulent environment. We find that globally unbound clouds can be the sites of star formation. Furthermore the star formation efficiency is naturally less than 100%, thus in part providing an explanation for the low efficiency in star forming regions. Globally unbound GMCs not only form stars, and naturally disperse, within a few crossing times, but also provide a mechanism for the formation of OB associations.
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McLaughlin, Dean E. "Star formation in molecular clouds and globular clusters". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp03/NQ30104.pdf.
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Parker, Nicholas David. "Studies of star formation in dark molecular clouds". Thesis, University of Cambridge, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315051.
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23
Anathpindika, Sumedh V. "Smoothed particle hydrodynamics simulations of colliding molecular clouds". Thesis, Cardiff University, 2008. http://orca.cf.ac.uk/54779/.
Testo completoAbstract (sommario):
The galactic disk is largely composed of hot, rarefied gas also called the inter cloud medium (ICM). The cooler regions of the ICM are dominated by molecular species and dust. Immersed in this neutral medium are dense agglomerations of primarily H2, called giant molecular clouds (GMCs). The GMCs have a velocity dispersion of order a few km s_1, superimposed on their orbital motion. A GMC, over a single period of rotation of the galaxy, may undergo a few tens of collisions. In the present work, we investigate this rather violent phenomenon and examine the prospects of star formation in the post collision composite gas body. The star formation code, DRAGON, employed for the present work is ill equipped to study the effects of cloud collision on the chemical composition of the ICM. We draw a distinction between the regime of high velocity (precollision Mach numbers in excess of ten) and low velocity (precollision Mach numbers of order unity) cloud collisions, on the basis of the evolution of the gas slab produced in either cases. While the former leads to the formation of a dense shock compressed gas slab, the latter results in a dense pressure compressed gas slab. We observe that strong internal shear in a shock compressed slab suppresses gravitational instability in it. In particular, we observe evidence for the non-linear thin shell instability (NTSI) in the shocked slab formed in a head-on cloud collision. The slab thus dissipates thermal energy and upon the loss of thermal support, collapses to form a thin, long filament along the collision axis. Star formation proceeds in this filament. There is however, no evidence of the NTSI in the oblique shocked slab resulting from off centre cloud collisions, although it is dominated by internal shearing motion. On the other hand, the pressure compressed slab is dominated by gravitational instability and fragments, when the fastest growing mode dominates. The slab develops a number of floccules, which merge to form larger clumps and filamentary structures. The densest regions in these large scale structures then collapse gravitationally. We suggest this as a possible mechanism for the formation of star clusters. YSOs forming in filamentary structures are fed with material streaming along the axis of respective filaments. This material also transfers angular momentum to the accreting protostellar core and the attendant accretion disk is orthogonal to the angular momentum vector of this inflowing material. In the filaments resulting from the collapse of the post-collision shocked slab in a head-on cloud collision, we observe that the accretion disks circumscribing the sinks, are orthogonal to the filament. However, the gas slab resulting from a low velocity, off centre cloud collision is wrapped around by angular momentum and gravitationally fragments to form filaments. This slab tumbles in the plane of the collision (and therefore the axis about which it tumbles, comes out of this plane), the filaments in the slab also tumble with it. In the process they become offset relative to each other and feed angular momentum to the candidate protostellar core along the direction normal to the angular momentum axis. Thus, any attendant accretion disk is expected to be parallel to the filament (also the angular momentum) axis (Whitworth et al., 1995). To test this hypothesis, we collated data for YSOs located in filamentary star forming regions, and outflows originating from them. The scope of our work was limited and restricted to only five filamentary star forming regions in the local universe. Outflows from YSOs generally have small opening angles and are approximately normal to the circumstellar disk. Under this premise, we can get an idea of the orientation of the circumstellar disks relative to their natal filaments. We concluded that 72% outflows were distributed within 45 of being orthogonal to their natal filaments and 28% were distributed within 45 of being parallel to their natal filaments. It is difficult to make a strong claim simply on the basis of this work, which therefore needs to be extended. None the same, it tends to support the mechanism elucidated by Whitworth et al. (1995).
24
Aguti, Esther D. "A study of dense and translucent molecular clouds". Thesis, University of Bristol, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.441649.
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25
Richardson, K. M. "Gamma rays, cosmic rays and local molecular clouds". Thesis, Durham University, 1988. http://etheses.dur.ac.uk/942/.
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26
Karlsson, Roland. "Studies of molecular clouds at the Galactic centre". Doctoral thesis, Stockholms universitet, Institutionen för astronomi, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-126752.
Testo completoAbstract (sommario):
Interstellar molecular clouds play an essential role in the Universe. Such clouds are invoked for the production and destruction of stars, galaxies and gas and also for energy transport in galaxies. The Galaxy, or the Milky Way, is a large spiral galaxy, with a central bar structure, that harbours a few hundred billion stars and large amounts of gas and dust. At the centre of the Galaxy, a 4 million solar mass supermassive black hole resides, surrounded by a dense core of millions of stars, as well as molecular and dust clouds. The Galactic centre (GC) is hidden by gas and dust, such that only astronomical observations of radio-, infrared-, X-rays and gamma-rays are available for a gathering of information at the centre. In this work, I have studied neutral molecular clouds in absorption at the innermost 50 light years from the centre with the Karl Jansky Very Large Array Observatory in New Mexico in the USA, and with data from observations with the Swedish-ESO Submillimetre Telescope in Chile, and also from the orbital observatory Odin. I have detected a new stream-like feature of gas that seems to link a previously known ring of gas clouds (the CND) and the GC. Moreover, the hypothesis of feeding the CND from an outside cloud is supported by this work. Contemporary discussions in the literature that the central bar structure would act as a pump of material inwards from the spiral arms towards the GC via molecular clouds are also suggested by the data. A number of maser sources have been observed and some of those are shown to reside at shock fronts or anticipated regions of collisions between molecular clouds or at star forming regions. Unusually high water abundance was detected at the south-west part of the CND, indicative of shocks and strong turbulence. Moreover, I have produced high-resolution spectral line maps of hydroxyl (OH) absorption intensity in the four main transition lines of OH at 1612, 1720, 1665 and 1667 MHz, as well as apparent opacity and position-velocity maps of the GC region.
27
Lee, Ho-Hsin. "Gas-phase chemical models of interstellar molecular clouds /". The Ohio State University, 1997. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487948440824473.
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Rice, Johnathan Scott. "The Transition From Diffuse to Dense Molecular Clouds". University of Toledo / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1534945134382193.
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Gritschneder, Matthias. "Ionization and Triggered Star Formation in Turbulent Molecular Clouds". Diss., lmu, 2009. http://nbn-resolving.de/urn:nbn:de:bvb:19-104903.
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30
Gong, Hao. "Dense core formation and collapse in giant molecular clouds". Thesis, University of Maryland, College Park, 2013. http://pqdtopen.proquest.com/#viewpdf?dispub=3587424.
Testo completoAbstract (sommario):
In this thesis we present a unified model for dense core formation and collapse within post-shock dense layers inside giant molecular clouds. Supersonic converging flows collide to compress low density gas to high density clumps, inside which gravitational collapse can happen. We consider both spherically symmetric and planar converging flows, and run models with inflow Mach number from 1.1-9 to investigate the relation between core properties and the bulk velocity dispersion of the mother cloud. Four stages of protostar formation are identified: core building, core collapse, envelope infall, and late accretion. The core building stage takes 10 times as long as core collapse, which lasts a few × 105 yr, consistent with observed prestellar core lifetimes. We find that the density profiles of cores during collapse can be fitted by Bonnor-Ebert sphere profiles, and that the density and velocity profiles approach the Larson-Penston solution at the core collapse instant. Core shapes change from oblate to prolate as they evolve. Cores with masses varying by three orders of magnitude (~ 0.05 - 50 solar mass) are identified in our high Mach number simulations, and a much smaller mass range for models having low Mach number. The median core mass versus Mach number lies between the minimum mass that can collapse in late times Ma-1 and the most evolved core mass Ma-1/2. We implement sink particles to the grid code Athena to track the collapse of other dense regions of a large scale simulation after the most evolved core collapses, We demonstrate use of our code for applications with a simulation of planar converging supersonic turbulent flows, in which multiple cores form and collapse to create sinks; these sinks continue to interact and accrete from their surroundings over several Myr.
31
Bell, G. S. "HARP-B and wide-field imaging of molecular clouds". Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596542.
Testo completoAbstract (sommario):
Part I of this thesis describe HARP-B – a new heterodyne array receiver operating at 345 GHz on the James Clerk Maxwell Telescope. The work described focuses on the optics, interferometer and commissioning of HARP-B. Careful testing and alignment of the optics was necessary to ensure that HARP-B would perform as required. Its Mach-Zehnder interferometer was put under computer control and characterised to allow it to be used as an effective sideband filter. This part concludes with the integration and commissioning of the receiver, leading up to first light. Part II then presents wide-field observations of the IC5146 and L977 molecular clouds, made with HARP-B and other instruments at the JCMT and IRAM 30 m telescopes. For IC5146, SCUBA continuum images and a dust extinction map were already available. These were complemented by spectral observations of C18O 1-0, C18O 2-1 and the 3-2 transition of 12CO, 13CO and C18O. The data were used to study the cloud structure, excitation conditions and dust properties. SCUBA continuum and C18O 2-1 observations were made of L977 and analysed along with the existing dust extinction map in order to study the dust and gas properties, and the structure of the molecular cloud.
32
Rigby, A. J. "Molecular clouds and star formation in the Inner Galaxy". Thesis, Liverpool John Moores University, 2016. http://researchonline.ljmu.ac.uk/4172/.
Testo completoAbstract (sommario):
A detailed understanding of the process of star formation is crucial for modern astrophysics. Stars form from the gravitational collapse of molecular gas clouds; it is the process by which cold molecular gas is transformed into the stars and planets that make up the many billions of galaxies in the observable Universe. However, there are a number of open questions that have yet to be answered and a comprehensive theory that explains and predicts how, where and why stars and their clusters form proves elusive. One such open question is how does the environment, on both local scales and galactic scales, influence star formation? The enormous radiative and mechanical outputs of high-mass stars (M > 8M_sol ) are known to have a strong impact on their surroundings and are able to erode their natal molecular clouds via their stellar winds, ionizing radiation and supernovae. It has been proposed that the shock fronts at the edges of expanding HII regions might trigger subsequent generations of star formation (e.g. Elmegreen & Lada, 1977; Bertoldi, 1989), and there are observational studies to support this (e.g. Thompson et al., 2012). It has also been proposed that large-scale effects such as the spiral structure of galaxies like the Milky Way might trigger the formation of stars in otherwise quiescent gas (e.g. Dobbs et al., 2008), though observations within the Galaxy appear to suggest that spiral arms are playing only a minor role, if any, in the triggering of star formation (e.g. Moore et al., 2012; Eden et al., 2015). To answer this question, and others concerning star formation, large samples of imminently and currently star-forming regions are required, and surveys of the plane of the Milky Way in various tracers are providing the data to acquire these. Molecular clouds are the initial conditions for star formation, and a complete theory of star formation must necessarily involve a detailed understanding of molecular clouds. In this thesis a survey of molecular gas in the Inner Galaxy known as CHIMPS is presented; these data provide measurements of denser and more optically thin molecular gas at a higher angular resolution than preceding surveys and over a significant area of the first quadrant of the Galactic plane. The combination of CHIMPS data with data from other surveys, such as Hi-GAL, allows the star-forming content of clumps of dense molecular gas to be studied. The clumps of molecular emission identified within CHIMPS appear to be highly turbulent in nature, and are over-pressurized with respect to the encompassing neutral gas. This would appear to suggest that they are transient features in a highly dynamic interstellar medium. The efficiency of star formation within the CHIMPS clumps is not found to vary significantly on kiloparsec scales between the spiral arms and their inter-arm regions, with the exception of the Scutum-Centaurus arm, within which the current level of star formation per unit gas mass appears to be somewhat suppressed. On a clump-to-clump basis, the distribution of star formation efficiency is log-normal, indicating that the efficiency is determined by many random processes, with no single dominant agent. The conclusion is that it is turbulence that controls the star formation efficiency, which is powered on a wide range of scales from the feedback of high-mass stars to the shear induced by the rotation of the entire Galaxy.
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van, Dishoeck E. F. "Interstellar C2, CH, and CN in Translucent Molecular Clouds". Steward Observatory, The University of Arizona (Tucson, Arizona), 1988. http://hdl.handle.net/10150/623918.
Testo completoAbstract (sommario):
Optical absorption line techniques have been applied to the study of a number of translucent
molecular clouds in which the total column densities are large enough that substantial
molecular abundances can be maintained. Results are presented for a survey of absorption
lines of interstellar C2, CH, and CN. Detections of CN through the A2II -X2E+ (1,0) and
(2,0) bands of the red system are reported, and are compared with observations of the
blue system for one line of sight. The population distributions in C2 provide diagnostic
information on temperature and density. The measured column densities of the three
species can be used to test details of the theory of molecule formation in clouds where
photo -processes still play a significant role. The C2 and CH column densities are strongly
correlated with each other and probably also with the H2 column density. In contrast, the
CN column densities are found to vary greatly from cloud to cloud. The observations are
discussed with reference to detailed theoretical models.
34
Chapman, Nicholas. "Dust structure and composition within molecular clouds and cores". College Park, Md.: University of Maryland, 2007. http://hdl.handle.net/1903/7613.
Testo completoAbstract (sommario):
Thesis (Ph. D.) -- University of Maryland, College Park, 2007.Thesis research directed by: Dept. of Astronomy. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
35
Yan, Qingzeng. "Molecular clouds and star formation in the Milky Way". Thesis, Curtin University, 2018. http://hdl.handle.net/20.500.11937/75787.
Testo completoAbstract (sommario):
Using molecular line observations, this thesis studied the molecular clouds of three typical regions of the Milky Way, which are the Galactic centre, high Galactic latitudes, and the Galactic plane. The properties of molecular clouds show remarkable variations among these three regions, which explains why the star formation activities are so different.
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Murphy, Brian Timothy. "Isotopomeric carbon compounds in star formation regions". Thesis, University of Kent, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.270812.
Testo completo
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Park, In Hee. "Dark cloud modeling for the ortho-to-para abundance ratio of the cyclic C3H2". Connect to this title online, 2005. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1117125089.
Testo completoAbstract (sommario):
Thesis (M.S.)--Ohio State University, 2005.Title from first page of PDF file. Document formatted into pages; contains xiii, 107 p.; also includes graphics Includes bibliographical references (p. 103-107). Available online via OhioLINK's ETD Center
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Vichetti, Rafael Mário. "Síntese dos isótopos do monóxido de carbono no meio interestelar /". Rio Claro : [s.n.], 2009. http://hdl.handle.net/11449/91889.
Testo completoAbstract (sommario):
Orientador: Carmen Maria AndreazzaBanca: Edson Denis Leonel
Banca: José Williams dos Santos Vilas Boas
Resumo: De acordo com os resultados observacionais de condensações de nuvens moleculares escuras, grandes variações na razão 13CO/C18O são observadas quando se comparam os resultados obtidos nas condensações situadas dentro da mesma nuvem, bem como de nuvem para nuvem. O valor médio dessa razão na condensação principal de Ophiuchus é inferior a 5. Por outro lado, o valor encontrado nas condensações que estão situadas ao norte de Oph é maior que 10. Grandes diferenças também são encontradas quando se comparam os resultados observacionais de diferentes nuvens escuras, tais como Ophiuchus e Taurus, onde são observados também um decréscimo da razão C18O/C17O com o aumento da densidade. Os processos químicos e físicos que governam essas variações ainda não estão claros. Nesse sentido, o objetivo da presente proposta é analisar a influência do colapso gravitacional de condensações de nuvens moleculares escuras na síntese das moléculas CO, C17O, C18O, 13CO, 13C17O e 13C18O. Tal análise é feita com base em comparações entre modelos que consideram diferentes condições entre si, tais como, tamanho da cadeia química, velocidade de colapso, densidade inicial e processos de congelamento de espécies químicas na superfície de grãos de poeira. Os resultados obtidos mostram que o tamanho da cadeia química tem influência nas razões 13CO/C18O e C18O/C17O, mas não tanto quanto a densidade inicial e a velocidade do colapso. Além disso, o congelamento das espécies químicas nos grãos é mais significativo nos estágios mais avançados da evolução da condensação. Os modelos de condensações escuras que sofrem colapso gravitacional lento e em queda livre reproduzem satisfatoriamente as razões 13CO/C18O e C18O/C17O observadas, o que permite concluir que o colapso gravitacional pode ter um importante efeito nas referidas razões.
Abstract: According to the observational results of dark molecular clouds condensations, large variations in the ratio 13CO/C18O are observed when comparing the results obtained in the condensations located within the same cloud and cloud to cloud. The average value of this ratio in the main condensation of Ophiuchus is below 5. On the other hand, the value found in the condensations that are located north of Oph is larger than 10. Large differences are also found when comparing the observational results of different dark clouds such as Ophiuchus and Taurus, in which are also found a decrease of the C18O/C17O ratio with increasing density. The chemical and physical processes that govern these variations are still unclear. In this sense, the objective of this proposal is to analyze the influence of the gravitational collapse of centrally condensed clumps of dense molecular gas in the synthesis of the CO, C17O, C18O, 13CO, 13C17O and 13C18O molecules. This analysis is based on comparisons among models that consider different condition, such as, chemical chain, initial density, speed of collapse and freezing processes of the chemical species on the surface of dust grains. The results show that the size of the chemical chain has influence on the 13CO/C18O and C18O/C17O ratios, but they are not as important as the initial density and the speed of the collapse. Furthermore, the freezing of chemical species on the grains occurs at later times of the collapse. The models of a gravitational free-fall collapsing core and of slowly contracting core with higher initial density are consistent with observations. These results indicate that the gravitational collapse of molecular cores can have an important effect in the 13CO/C18O and C18O/C17O ratios.
Mestre
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Finn, Susanna C. "Molecular line observations of Infrared Dark Clouds in the Galaxy". Thesis, Boston University, 2012. https://hdl.handle.net/2144/32014.
Testo completoAbstract (sommario):
Thesis (Ph.D.)--Boston UniversityPLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.
Although massive stars play many important roles in the universe, their formation is poorly understood. Recently, a class of interstellar clouds known as Infrared Dark Clouds (IRDCs) has been identified as likely progenitors of massive stars and clusters. These clouds are dense (nH2 > 10^5 cm-3), cold (T < 20 K), have very high column densities (N ~ 10^23 10^25 cm-2), and contain dense clumps and cores. In this dissertation, I present radio observations of a large sample of IRDCs in order to examine their properties and explore the hypothesis that high-mass stars and clusters form in these dense, cold molecular clouds. I determine kinematic distances to a large sample of IRDCs in the inner Galaxy based on CS (2-1) radial velocities. IRDCs are concentrated at specific Galactocentric radii and their distribution appears to trace Milky 'vVay spiral structure. To identify IRDC clumps and determine properties such as mass, size, and chemical evolution, I map a sample of IRDCs in various high density-tracing molecular transitions. The size and mass estimates show that IRDC clumps are comparable in size to more evolved regions of massive star formation. I compare the integrated intensities and linewidths of the molecular emission with a proposed evolutionary sequence of the clumps. The ratio of N2H+ with HNC, HCN, and HCQ+ is a function of evolutionary stage. The linewidths and virial parameters of the clumps show no clear trend with the evolutionary sequence. Finally, I explore the filamentary shape of IRDCs. The "sausage instability," which describes clumps forming in a gas cylinder, is explored as a mechanism for star-forming clumps to collapse in filaments. First, I compare observations of the "Nessie Nebula," an extreme case of a filamentary IRDC, with predictions from the theory of the fluid instability and then expand the sample to other filamentary IRDCs. The observations are consistent with theoretical predictions of clump spacing, clump masses, and linear mass density. Fragmentation of filaments due to the sausage instability might be the dominant mode of star formation in the Universe.
2031-01-02
40
Greene, T. P., e E. T. Young. "IRAS Observations of Dust Heating and Energy Balance in the FHO Ophiuchi Dark Cloud". Steward Observatory, The University of Arizona (Tucson, Arizona), 1989. http://hdl.handle.net/10150/623887.
Testo completoAbstract (sommario):
The total luminosity of the Rho Ophiuchi molecular cloud is derived from IRAS
data and is found to match the luminosity of known embedded sources very closely.
High resolution 60 and 100 micron band IRAS images have been reduced to yield
equilibrium color temperature maps and 60 micron band dust optical depth maps
for the region. These data along with optically thin C18O column density data
are used to evaluate dust grain sizes and compositions via competing grain models.
Radiative modeling shows that a standard power law distribution of graphite and
silicate grains is responsible for IRAS 60 and 100 micron band emissions. These
grains are heated to about one tenth of the cloud's depth in the core region. Their
optical depths closely follow molecular column density structure, but these grains
are considerably colder than the molecular gas. We also find that a 10 nm minimum
particle radius cutoff is appropriate for the 60 and 100 micron band emissions while
very small grains or PAH molecules dominate the cloud's 12 and 25 micron band
emissions.
41
Matthews, Brenda Christine. "A polarimetric study of magnetic fields in star-forming molecular clouds /". *McMaster only, 2001.
Cerca il testo completo
42
Walker, Christopher Kidd. "An observational study of the dynamics of molecular cloud cores". Diss., The University of Arizona, 1988. http://hdl.handle.net/10150/184585.
Testo completoAbstract (sommario):
How are stars formed? This is one of the most fundamental questions in astronomy. It is therefore ironic that to date, no object has been unambiguously identified as a true protostar; an object which derives the bulk of its luminosity from accretion. While this may be ironic, it is not surprising. Stars are believed to form as a result of the gravitational collapse of a portion of a molecular cloud. Theory predicts that the cloud core in which the star is formed will be cold, dense and possess hundreds of magnitudes of extinction, rendering it opaque at visible and near-infrared wavelengths. Continuum observations at far-infrared, submillimeter, and millimeter wavelengths can be used to identify candidate protostars, but spectroscopic observations are needed to detect infall. The difficulties arise when there are systematic velocity fields present in the cloud core which are not the result of infall, such as would be produced by either a molecular outflow or rotation. In this dissertation we use both observations and theoretical models to sort through these problems and develop a strategy which could be used to identify and study protostars.
43
Rey, Raposo Ramon. "The interplay between stellar feedback and galactic environment in molecular clouds". Thesis, University of Exeter, 2015. http://hdl.handle.net/10871/21022.
Testo completoAbstract (sommario):
In this thesis we address the problem of understanding the star formation process in giant molecular clouds in a galactic context. Most simulations of molecular clouds to date use an oversimplified set of initial conditions (turbulent spheres/boxes or colliding flows). Full galactic scale models are able to generate molecular clouds with complex morphologies and velocity fields but they fail to reproduce in detail the effects that occur at sub-pc scales (e.g. stellar feedback). Our goal is to build the bridge between these two scenarios, and to model the star formation process in molecular clouds produced in a galactic context. We extract our molecular clouds from full-scale galactic simulations, hence we need to increase the resolution by two orders of magnitude. We introduce the details of the program used to simulate molecular clouds in Chapter 2, and describe in detail the method we follow to increase the resolution of the galactic clouds. In Chapter 3 we compare our simulated galactic clouds with the more conventional approach of using turbulent spheres. We create turbulent spheres to match the virial state of three galactic clouds. We perform isothermal simulations and find that the velocity field inherited from the full-scale galactic simulations plays an important role in the star formation process. Clouds affected by strong galactic shear produce less stars compared with clouds that are compressed. We define (and test) a set of parameters to characterise the dynamical state of our clouds. To include stellar feedback in our simulations we need to introduce a cooling/heating algorithm. In Chapter 4 we analyse how the different velocity fields of our clouds change the temperature distribution even in the absence of feedback. To study the formation of molecules we need to model the chemistry of H2 in our clouds. We also add CO chemistry, and produce synthetic observations of our clouds. The effect of feedback from winds and supernovae in galactic clouds is studied in Chapter 5. We analyse the effect of winds in clouds with very different velocity fields. We find that the effect of winds is stronger in highly virialised, high star forming clouds, with clouds with weak galactic shear, compared to unbound shear-dominated clouds. The steady and continuous action of the winds appears to have a greater effect than the supernovae. In summary, the inherited properties from the galaxy have an impact on many relevant processes in star formation, influencing gravitational collapse, the formation of filamentary structures, the temperature field of the cloud, and have a considerable effect on the impact of feedback in the clouds.
44
Lang, William Jonathan. "Molecular clouds in the #lambda#-orionis ring : a new CO survey". Thesis, University of Bristol, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.389119.
Testo completo
45
Zakri, Wafa. "A Search for Large Amplitude Variability in the Orion Molecular Clouds". University of Toledo / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1596736274108871.
Testo completo
46
Richter, Philipp. "FUV absorption spectroscopy of interstellar molecular hydrogen towards the Magellanic clouds /". Aachen : Shaker, 1999. http://catalogue.bnf.fr/ark:/12148/cb37739235j.
Testo completoAbstract (sommario):
Dissertation--Mathematisch-Naturwissenschaftliche Fakultät--Bonn--Rheinische Friedrich-Wilhelms-Universität, 1999. Titre de soutenance : FUV spectroscopy of interstellar molecular hydrogen towards the Magellanic clouds.FUV = far ultraviolet. Bibliogr. p. 83-85.
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Maloney, Philip Richard. "Global properties of molecular clouds and the interstellar medium in galaxies". Diss., The University of Arizona, 1987. http://hdl.handle.net/10150/184271.
Testo completoAbstract (sommario):
Molecular gas in other galaxies is generally studied by observations of CO emission; a conversion from CO integrated intensity to H₂ column density must be made. Modelling of the emission from an ensemble of molecular clouds shows that these conversion factors are sensitive to temperature, so that molecular gas masses in galaxies with high star formation rates have probably been overestimated. Conversely, models of molecular clouds in low metallicity systems (such as irregular galaxies) demonstrate that the use of CO as a tracer can severely underestimate the molecular gas abundance. The observed properties of dark clouds and high latitude clouds are consistent with clouds in equilibrium with an intercloud pressure of P/k ≈ 10⁴. Detailed comparison of the CO and 170μm emission from the disks of NGC 6946 and M51 shows that the far-infrared flux must arise from dust in molecular clouds, not atomic clouds; this emission may be powered by embedded young stars or by the interstellar radiation field. The interpretation of the ratio of infrared to CO luminosities as a star formation efficiency is of dubious validity. Modelling of the observed CO and far-infrared emission from a sample of galactic nuclei shows that roughly half of the CO flux is produced by very active star-forming clouds with warm CO. The constraints placed on star formation models by abundance gradients in galaxies suggests that radial gradients in star forming efficiency generally exist in galaxies. The actual distribution of molecular gas in galaxies may be closely tied to the radial mass distribution.
48
Moorhouse, Alan. "Molecular hydrogen line ratios as probes of shocks in dense clouds". Thesis, University of Edinburgh, 1990. http://hdl.handle.net/1842/28656.
Testo completoAbstract (sommario):
This thesis is concerned with the structure of shocks occurring in dense regions of molec?ular clouds. These shocks are associated with the outflows from young stars, Herbig-Haro objects, expanding HII regions and the interaction of supernovae remnants with molecu?lar clouds. Momentum, mass and energy are imparted to the cloud. A full understanding of the shock process is thus needed if we are to understand the structure of molecular clouds and the impact on star formation. Emission from the near-infrared transitions of molecular hydrogen is commonly excited in these shocks. A major puzzle is that emis?sion is seen at velocities that would collisionally dissociate molecular hydrogen, and this is a central question that this thesis seeks to answer. This is approached observationally by trying to relate the observed emission to shock models. Fairly accurate semi-analytic derivations of the emission spectrum expected from hydrodynamic and magnetohydrodynamic molecular shocks are used to fully explore the parameter space of the initial conditions, without resort to expensive numerical calculations. The emission spectrum is then related to that observed. Most of this work is based on a spectroscopic multi-line study of the near-infrared H2 emission in two sources, the Orion outflow and the supernova remnant IC443. These observations are then compared with those expected from the models. In both sources it is found that planar hydrodynamic jump-type shocks (J) are consistent with the new observations. Whplanar magnetically moderated continuous shocks (C), which have been invoked to explain the emission from the shock in Orion, are not. Neither shock types can explain the intensities of CO rotational lines and the H2 line ratios simultaneously. The high velocities that are observed still present a problem. In IC443 the conclusion is the same but, in addition, the pressure needed to explain the observations is higher than that observed in the supernova remnant. It is suggested that this discrepancy may naturally occur when radiative shocks are driven through a clumpy medium. This approach of using line ratios as shock discriminators is extended by velocity resolved spectroscopy of three highly excited emission lines from Orion. These observa?tions demonstrate that there are no discernible differences in the line ratios with velocity despite the large change in the energies of the upper energy levels involved. It is discussed how this further constrains the shock type and limits the contribution from non-thermal excitation (such as fluorescence). The possible physical processes that could lead to high velocity, shocked molecular hydrogen are then discussed. Models proposed in the past are, it is argued, inadequate. It is then shown that the line ratios observed can be closely matched with non-planar continuous type shocks which occur in a bow shock. The densities and pressures needed are still high. The general conclusions are that previous plane parallel C-shock models invoked to explain the molecular shocks are inconsistent with the observations. The line ratios imply that either J-type shocks, in which the cooling takes a long time compared to the initial heating, or C-type bow shocks which produce a range of temperatures are responsible for the emission. It is finally suggested that C-shocks in gas with a very high magnetic field can produce the high velocity H2 emission observed without dissociating the molecules.
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Valdivia, Valeska. "Impact of radiative transfer and chemistry on the formation of molecular clouds". Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066709/document.
Testo completoAbstract (sommario):
Le milieu interstellaire (MIS) est un système extrêmement complexe. Il correspond à une échelle intermédiaire entre les étoiles et les galaxies. Le gaz interstellaire est présent dans toute la galaxie, remplissant l’espace entre les étoiles. Une grande diversité de processus couplés, comme la gravité, le champs magnétiques, la turbulence et la chimie, participe à son évolution, faisant de la modélisation du MIS un problème ardu. Une description correcte du MIS nécessite un bon traitement des équations de la magnetohydrodynamique (MHD), de la gravité, du bilan thermique et de l’évolution chimique à l’intérieur du nuage moléculaire.L’objectif de ce travail de thèse est une meilleure compréhension de la formation et de l’évolution des nuages moléculaires, et plus particulièrement de la transition du gaz atomique en gaz moléculaire. Nous avons réalisé des simulations numériques de la formation des nuages moléculaires et de la formation de l’hydrogène moléculaire sous l’influence de la gravité et de la turbulence MHD, en utilisant des estimations précises de l’écrantage par les poussières et de l’auto-écrantage par la molécule H2. Ceci a été calculé grâce à une méthode en arbre, à même de fournir une rapide estimation des densités de colonne.Nous avons trouvé que l’hydrogène moléculaire se forme plus rapidement que prévu par les estimations classiques du fait de l’augmentation de densité locale provoquée par les fluctuations turbulentes du gaz. L’hydrogène moléculaire, formé à des densités plus élevées, peut alors migrer vers les régions plus chaudes et moins denses.Les densités de colonne totale d’hydrogène moléculaire montrent que la transition HI-H2 se produit à des densités de colonne de quelques 10^20 cm−2. Nous avons calculé les populations des niveaux rotationnels de H2 à l’équilibre thermique et intégré le long de plusieurs lignes de visée. Ces résultats reproduisent bien les valeurs observées par Copernicus et FUSE, suggérant que la transition observée et les populations excitées pourraient être une conséquence de la structure multi-phasique des nuages moléculaires. Comme la formation de H2 précède la formation des autres molécules, le H2 chaud pourrait permettre le développement d’espèces endothermiques et éventuellement expliquer certains aspects de la richesse moléculaire observée dans l’ISMThe interstellar medium (ISM) is a highly complex system. It corresponds to an intermediate scale between stars and galaxies. The interstellar gas is present throughout the galaxy, filling the volume between stars. A wide variety of coupled processes, such as gravity, magnetic fields, turbulence and chemistry, participate in its evolution, making the modeling of the ISM a challenging problem. A correct description of the ISM requires a good treatment of the magnetohydrodynamics (MHD) equations, gravity, thermal balance, and chemical evolution within the molecular clouds.This thesis work aims at a better understanding of the formation and evolution of molecular clouds, specially how they become "molecular", paying particular attention to the transition HI-to-H2. We have performed ideal MHD simulations of the formation of molecular clouds and the formation of molecular hydrogen under the influence of gravity and turbulence, using accurate estimates for the shielding effects from dust and the self-shielding for H2, calculated with a Tree-based method, able to provide fast estimates of column densities.We find that H2 is formed faster than predicted by the usual estimates due to local density enhancements created by the gas turbulent motions. Molecular hydrogen, formed at higher densities, could then migrate toward low density warmer regions.Total H2 column densities show that the HI-to-H2 transition occurs at total column densities of a few 10^20 cm−2. We have calculated the populations of rotational levels of H2 at thermal equilibrium, and integrated along several lines of sight. These two results reproduce quite well the values observed by Copernicus and FUSE, suggesting that the observed transition and the excited populations could arise as a consequence of the multi-phase structure of molecular clouds. As H2 formation is prior to further molecule formation, warm H2 could possibly allow the development of a warm chemistry, and eventually explain some aspects of the molecular richness observed in the ISM
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Smith, Rowan Johnston. "The earliest fragmentation in molecular clouds : and its connection to star formation". Thesis, University of St Andrews, 2010. http://hdl.handle.net/10023/929.
Testo completoAbstract (sommario):
Stars are born from dense cores of gas within molecular clouds. The exact nature of the connection between these gas cores and the stars they form is an important issue in the field of star formation. In this thesis I use numerical simulations of molecular clouds to trace the evolution of cores into stars. The CLUMPFIND method, commonly used to identify gas structures is tested. I find that the core boundaries it yields are unreliable, but in spite of this, the same profile is universally found for the mass function. To facilitate a more robust definition of a core, a modified clumpfind algorithm which uses gravitational potential instead of density is introduced. This allows the earliest fragmentation in a simulated molecular cloud to be identified. The first bound cores have a mass function that closely resembles the stellar IMF, but there is a poor correspondence between individual core masses and the stellar masses formed from them. From this, it is postulated that environmental factors play a significant part in a core’s evolution. This is particularly true for massive stars, as massive cores are prone to further fragmentation. In these simulations, massive stars are formed simultaneously with stellar clusters, and thus the evolution of one can affect the other. In particular, the global collapse of the forming cluster aids accretion by the precursors of the massive stars. By tracing the evolution of the massive stars, I find that most of the material accreted by them comes from diffuse gas, rather than from a well-defined stellar core.