Dissertations / Theses on the topic 'Star formation'
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1
Moeckel, Nickolas Barry. "Massive stars, disks, and clustered star formation." Connect to online resource, 2008. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3303877.
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Kraus, Adam L. Brown Michael E. Hillenbrand Lynne A. "Multiple star formation." Diss., Pasadena, Calif. : California Institute of Technology, 2010. http://resolver.caltech.edu/CaltechETD:etd-08252009-233632.
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Ferreira, Carolina Gribel de Vasconcelos. "Connecting the cosmic star formation rate with the local star formation rate." Instituto Nacional de Pesquisas Espaciais (INPE), 2018. http://urlib.net/sid.inpe.br/mtc-m21b/2018/02.05.17.02.
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We present a model that unifies the cosmic star formation rate (CSFR), obtained through the hierarchical structure formation scenario, with the (Galactic) local star formation rate (SFR). It is possible to use the SFR to generate a CSFR mapping through the density probability distribution functions (PDFs) commonly used to study the role of turbulence in the star-forming regions of the Galaxy. We obtain a consistent mapping from redshift z 20 up to the present (z = 0). Our results show that the turbulence exhibits a dual character, providing high values for the star formation efficiency (h"i 0.32) in the redshift interval z 3.5−20 and reducing its value to h"i = 0.021 at z = 0. The value of the Mach number (Mcrit), from which h"i rapidly decreases, is dependent on both the polytropic index () and the minimum density contrast of the gas. We also derive Larsons first law associated with the velocity dispersion (hVrmsi) in the local star formation regions. Our model shows good agreement with Larsons law in the 0.1 − 30pc range (when our model is compared to the observational data), providing typical temperatures T0 2−50K for the gas associated with star formation. As a consequence, dark matter halos of great mass could contain a number of halos of much smaller mass, and be able to form structures similar to globular clusters. Thus, Larsons law emerges as a result of the very formation of large-scale structures, which in turn would allow the formation of galactic systems, including our Galaxy.Apresentamos um modelo que unifica a Taxa Cósmica de Formação Estelar (CSFR em inglês), obtida atravez do cenário de formação de estruturas, com a taxa de formação estelar local (Galáctica) (SFR em inglês). É possível utilizar a SFR para gerar um mapa da CSFR através da função de distribuição de probabilidade (PDFs) da densidade comumente utilizada no estudo do papel da turbulência nas regiões de formação estelar na Galáxia. Obtemos um mapa consistente a partir de redshift z 20 até o presente (z = 0). Nossos resultados mostram que a turbulência exibe um caráter dual, resultando em altos valores para a eficiência de formação estelar (h"i 0.32) no intervalo de redshift z 3.5 − 20 e reduzindo seu valor para h"i = 0.021 em z = 0. O valor do número de Mach (Mcrit), para o qual h"i decresce rapidamente, é dependente em ambos do índice politrópico () e do contraste de densidade do gás (scrit). Derivamos a primeira Lei de Larson associada a disperção de velocidade (hVrmsi) nas regiões de formação de estelar local. Nosso modelo mostra boa concordância com a Lei de Larson no intervalo 0.1 − 30pc (quando nosso modelo é comparado com dados observacionais), com temperaturas típicas T0 2 − 50K para o gás associado a formação estelar. Como consequência, os halos de matéria escura com maior massa poderiam conter halos de menor massa, formando estruturas semelhantes aos aglomerados globulares. Sendo assim, a Lei de Larson emerge como um resultado da formação estelar cosmológica e vinculada com a formação das estruturas em grande escala do universo, da qual possibilitaria a formação de sistemas galacticos, incluindo a nossa Galáxia.
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Belles, Pierre-Emmanuel Aime Marcel. "Formation of stars and star clusters in colliding galaxies." Thesis, University of Hertfordshire, 2013. http://hdl.handle.net/2299/10312.
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Mergers are known to be essential in the formation of large scale structures and to have a significant role in the history of galaxy formation and evolution. Besides a morphological transformation, mergers induce important bursts of star formation. These starburst are characterised by high Star Formation Efficiencies (SFEs) and Specific Star Formation Rates, i.e., high Star Formation Rates (SFR) per unit of gas mass and high SFR per unit of stellar mass, respectively, compared to spiral galaxies. At all redshifts, starburst galaxies are outliers of the sequence of star-forming galaxies defined by spiral galaxies. We have investigated the origin of the starburst-mode of star formation, in three local interacting systems: Arp 245, Arp 105 and NGC7252. We combined high-resolution JVLA observations of the 21-cm line, tracing the Hi diffuse gas, with UV GALEX observations, tracing the young star-forming regions. We probe the local physical conditions of the Inter- Stellar Medium (ISM) for independent star-forming regions and explore the atomic-to-dense gas transformation in different environments. The SFR/H i ratio is found to be much higher in central regions, compared to outer regions, showing a higher dense gas fraction (or lower Hi gas fraction) in these regions. In the outer regions of the systems, i.e., the tidal tails, where the gas phase is mostly atomic, we find SFR/H i ratios higher than in standard Hi-dominated environments, i.e., outer discs of spiral galaxies and dwarf galaxies. Thus, our analysis reveals that the outer regions of mergers are characterised by high SFEs, compared to the standard mode of star formation. The observation of high dense gas fractions in interacting systems is consistent with the predictions of numerical simulations; it results from the increase of the gas turbulence during a merger. The merger is likely to affect the star-forming properties of the system at all spatial scales, from large scales, with a globally enhanced turbulence, to small scales, with possible modifications of the initial mass function. From a high-resolution numerical simulation of the major merger of two spiral galaxies, we analyse the effects of the galaxy interaction on the star forming properties of the ISM at the scale of star clusters. The increase of the gas turbulence is likely able to explain the formation of Super Star Clusters in the system. Our investigation of the SFR–H i relation in galaxy mergers will be complemented by highresolution Hi data for additional systems, and pushed to yet smaller spatial scales.
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Crawford, John W. "Star formation in galaxies." Thesis, Queen Mary, University of London, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.437102.
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Glenn, Jason 1968. "Millimeter-wave polarimetry of star formation regions and evolved stars." Diss., The University of Arizona, 1997. http://hdl.handle.net/10150/282440.
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A new λ = 1.3 mm polarimeter, Cyclops, was constructed to make observations of dust continuum emission from star formation regions. The polarization of the inner arcminute of DR 21 was mapped with Cyclops. The polarization percentage and position angle are remarkably constant, indicating a uniform magnetic field throughout the cloud. Turbulent gas motions are a more significant source of support against self gravity in the cloud core than thermal pressure or magnetic fields. The polarization toward the cloud core increases slightly from λ = 100 μm to λ = 2 mm and is consistent with the standard dust composition of silicates and graphite. A small continuum polarization survey of cloud cores with embedded protostars was made with Cyclops and combined with observations from the literature. There is no clear tendency for any preferred alignment of cloud core elongations with respect to magnetic field lines, especially for the bright, high mass star forming regions. This confirms that the massive cloud cores are magnetically supercritical. The magnetic field lines appear randomly oriented with respect to the local Galactic plane position angles, implying that the random component of the Galactic magnetic field dominates the spiral component in this sample. Three-σ upper limits of 0.4%, 1.2%, and 1.2% were placed on the polarization of the HCO⁺ J = 1-0 emission line from the DR 21 and Mon R2 molecular outflows, and the CS J = 2-1 line from the IRAS 16293-2422 molecular outflow, respectively. These polarizations are an order of magnitude lower than predicted by theoretical models. In the case of DR 21, the lack of polarization is probably due to a disordered magnetic field in clumpy, turbulent gas, although multiple scattering may also diminish the polarization. CS J = 2-1 polarizations of 0.9% ± 0.1% and 5.1% ± 1.5% were observed from the envelopes of the evolved stars IRC+10216 and CRL 2688, respectively. An anisotropic optical depth to escape of infrared photons from the central star, perhaps caused by a toroidal dust distribution, could generate the IRC+10216 polarization.
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Cernohorsky, Jan. "Neutrino driven neutron star formation." Amsterdam : Amsterdam : Rodopi ; Universiteit van Amsterdam [Host], 1990. http://dare.uva.nl/document/91884.
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Dale, J. E. "Feedback in star cluster formation." Thesis, University of Cambridge, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598249.
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Massive stars emit strong fluxes of ionising radiation and their dynamical impact on their natal clusters is expected to be severe. The outflows generated expel residual gas from the cluster and can potentially gravitationally disrupt it. The loss of its reserves of molecular gas also prevents the cluster forming more stars. Star-formation and star cluster evolution cannot be fully understood without a proper treatment of feedback. I present a novel technique I have developed to allow the inclusion of the effects of ionising radiation in smoothed particle hydrodynamics (SPH) simulations of star clusters. The new algorithm is able to reproduce the results of simple analytical models and also gives results in good agreement with a more sophisticated Monte Carlo radiative transfer code when tested under highly anisotropic conditions. I simulate the effects of ionising radiation in globular clusters and compare my results with one-dimensional calculations with which I find good agreement. I investigate three cases in which different quantities of gas are distributed in my model cluster such that the as becomes fully ionised either during the HII region’s formation phase, or during its expansion phase, or such that the HII region is trapped inside the cluster core. I find gas expulsion to be quite efficient in the calculations in which the HII region escapes the core. I observe an instability in the second calculation which causes the shocked shell driven by the ionisation front to fragment as the HII region exits the core. The instability produces new structure from the smooth gas in the system, but this structure is rapidly destroyed by the radiation field and the effect of the instability on the evolution of the system is minimal. I also simulate feedback in the context of young embedded clusters, a highly inhomogeneous and anisotropic environment. I find that, again, photoionisation is able to produce novel structure in the ambient gas, causing it to fragment into filaments and beads. This fragmentation of the neutral gas, together with compression by hot ionised gas, which decreases the Jeans mass, lead me to conclude that feedback promotes star formation.
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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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Blain, Andrew William. "Star formation in distant galaxies." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.360569.
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Ford, George Philip. "Star formation in nearby galaxies." Thesis, Cardiff University, 2014. http://orca.cf.ac.uk/63670/.
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This work uses multiwavelength observations of nearby galaxies to explore the relationship between star formation and the interstellar medium in galaxies of various sizes and morphology. Galaxies in the Herschel Reference Survey are divided into barred and unbarred spirals to test for differences in dust temperature, dust mass, star formation rate, farinfrared luminosity, NUV-r colour and stellar mass between the two populations. The only significant observed difference is with stellar mass, where barred spirals are generally less massive. I suggest this is due to the speed of bar creation depending on galaxy mass, although this is counter to some previous observations. Trends with Hubble-type and environment are consistent with previous work. The resolved star formation law is studied in the two largest extragalactic sources in the local group, Andromeda (M31) and the Triangulum (M33). The two are measured to have global star formation rates (SFR) of 0.25M⊙ yr−1 and 0.16M⊙ yr−1 respectively using far-ultraviolet and 24 mm emission as star formation tracers. M33 has a higher mean surface density of star formation, as expected as it is later type than M31, and a higher star formation efficiency. Both galaxies appear consistent with the globally averaged SFR and gas surface density of normal spirals studied in previous work, with M31 at the low end in terms of SFR. When looking at smaller scales, both galaxies show evidence of saturation of neutral monatomic hydrogen at §Gas = 10M⊙ pc−2 when looking at the star formation law with total gas. They also appear to follow close to linear star formation laws with molecular gas only, consistent with previous work on resolved galaxies. M31 shows evidence of a sub-linear star formation law with molecular gas, indicating that star formation efficiency is lower in the highest density regions. Testing the relationship in M31 on different pixel scales does not effect the measured Kennicutt-Schmidt index, as has been suggested in previous work. M33 shows a significant portion of the galaxy has a relatively high SFR surface density, but little molecular gas as traced by CO. I suggest this could be evidence of CO-free molecular hydrogen in these regions.
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Miah, Junad Alam. "Star formation in merging galaxies." Thesis, Durham University, 2014. http://etheses.dur.ac.uk/10859/.
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Star formation is detected in any galaxy with an appreciable amount of gas and the vast majority of stars form in embedded clusters. However, very few bound star clusters are detected in the Milky Way, which has led to the hypothesis that as many as 90% of these clusters are disrupted during the early stages of their evolution. Many of those that do survive are likely to be progenitors of globular clusters that are observed in elliptical galaxies, and in the bulge and halo regions of spiral galaxies. In order to understand how star clusters form and evolve, and the disruption mechanisms they encounter, it is necessary to observe star clusters during their earliest evolutionary stages. This is difficult to observe in quiescent galaxies like the Milky Way where only a few newly formed star clusters have currently been detected. Gas-rich interacting and merging galaxies however, host thousands of newly formed star clusters and are ideal targets to observe the evolution of star clusters. In this thesis star clusters are observed in galaxy mergers for a range of evolutionary states using both photometric and spectroscopic data. We find evidence that tidal interactions have produced new cluster populations in two separate galaxy mergers. Analysis of the cluster populations in these mergers also suggest that tidal interactions do not destroy more clusters than they produce, in disagreement with simulations in the literature. Furthermore, we observe several star clusters that may be the product of merging between multiple clusters. These star clusters show spectral features consistent with multiple episodes of star formation. If these clusters remain bound for the next few Gyrs, they could explain the multiple stellar population feature observed in globular clusters in the Galaxy. We also explore the possibility of star clusters evolving to form the halo cluster population of their host merger before the progenitor disks coalesce. Star clusters are generally found in the halo population of a galaxy merger once the progenitor nuclei coalesce. However, some galaxy mergers like the Antennae harbour star clusters that may be forming their halo cluster population before their progenitor nuclei merge. This suggests that star clusters begin to form the halo cluster population of a galaxy merger before the progenitor nuclei coalesce. We recommend future surveys of gas-rich major galaxy mergers to study young ( ∼ few hundred Myr) star clusters in dense and quiescent regions to further support our findings.
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Boily, Christian M. "Homological flows & star formation." Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.321079.
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Dutta, Jayanta. "Fragmentation during primordial star formation." Doctoral thesis, Universidade de Évora, 2016. http://hdl.handle.net/10174/20823.
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Compreender os mecanismos físicos que são responsáveis pela formação
e evolução das primeiras estrelas do universo, conhecidas como estrelas
de população III (ou Pop II) é fundamental para compreendermos como
evoluiu o universo até hoje. No modelo padrão da formação de estrelas
de Pop III, a matéria bariónica é constituída principalmente por hidrogénio
atómico na forma gasosa, e colapsa gravitacionalmente em mini-halos
(pequenos halos) de matéria escura, dando origem à formação das
estrelas. No entanto, muito pouco se sabe como a evolução dinâmica e
química do gás primordial são afetadas pelas condições iniciais dos minihalos,
em particular no que diz respeito ao efeito da rotação nos
aglomerados estelares instáveis que se formam dentro dos mini-halos, ao
impacto da turbulência, à formação de hidrogénio molecular, e ao impacto
das variações cósmicas entre mini-halos.
Neste trabalho usamos uma versão modificada do código Gadget-2, um
programa de simulação hidrodinâmica baseado num algoritmo numérico
conhecido por SPH (“smoothed particle hydrodynamics”), que permite
seguir a evolução do gás durante o colapso, tanto no caso de mini-halos
idealizados como em casos de mino-halos mais realistas. Em contraste
com algumas simulações numéricas mais antigas, a implementação das
partículas coletoras (“sink particles”) permite seguir a evolução do disco
de acreção que se forma no centro dos fragmentos e dos mini-halos.
Descobrimos que o processo de fragmentação depende do valor adotado
para a taxa de formação (“three-body H2 formation rate”) de hidrogénio
molecular (H2). Verificamos que o aumento da taxa de arrefecimento
durante o período em que o hidrogénio atómico é convertido em
hidrogénio molecular é compensado pelo aquecimento causado pela
contração do gás. Propomos que o arrefecimento de H2, o aquecimento
devido a formação de H2, e o aquecimento devido à compressão do gás,
juntamente com a densidade e temperatura determinam a estrutura do
disco que favorece a fragmentação. Também descobrimos que a rotação
inicial da nuvem de gás tem um impacto muito relevante na evolução
térmica e dinâmica do gás em colapso Nuvens com uma elevada rotação apresentam filamentos idênticos a
braços espirais que se tornam gravitacionalmente instáveis, dando origem
à fragmentação em várias escalas. Estes tipos de nuvens têm mais
tendência a fragmentar e têm menores taxas de acreção em relação a
nuvens com menor rotação. Adicionalmente, verificamos que a
distribuição especifica de momento angular (L) do gás é descrita por uma
relação de potência com a massa do gás capturado (M) dada pela
expressão L = M1.125. Por sua vez o momento angular é controlado pelo
torque gravitacional e pela pressão, embora não dependa dos valores de
rotação e turbulência da nuvem inicial; Abstract:
Understanding the physics of the very first stars in the universe, the so-called Population
III (or Pop III) stars, is crucial in determining how the universe evolved
into what we observe today. In the standard model of Pop III star formation, the
baryonic matter, mainly atomic hydrogen, collapses gravitationally into small Dark
Matter (DM) minihalos. However, so far there is little understanding on how the thermal,
dynamical and chemical evolution of the primordial gas depend on the initial
configuration of the minihalos (for example, rotation of the unstable clumps inside
minihalos, turbulence, formation of molecular hydrogen and cosmic variance of the
minihalos). We use the modified version of the Gadget-2 code, a three-dimensional
smoothed particle hydrodynamics (SPH) simulations, to follow the evolution of the
collapsing gas in both idealized as well as more realistic minihalos. Unlike some earlier
cosmological calculations, the implementation of sink particles allows us to follow
the evolution of the accretion disk that builds up in the centre of each minihalo and
fragments. We find that the fragmentation behavior depends on the adopted choice
of three-body H2 formation rate coefficient. The increasing cooling rate during rapid
conversion of the atomic to molecular hydrogen is offset by the heating due to gas
contraction. We propose that the H2 cooling, the heating due to H2 formation and
compressional heating together set a density and temperature structure in the disk
that favors fragmentation. We also find that the cloud’s initial degree of rotation has a
significant effect on the thermal and dynamical evolution of the collapsing gas. Clouds
with higher rotation exhibit spiral-arm-like structures that become gravitationally unstable
to fragmentation on several scales. These type of clouds tend to fragment more
and have lower accretion rates compared to their slowly rotating counterparts. In addition, we find that the distribution of specific angular momentum (L) of the gas
follows a power-law relation with the enclosed gas mass (M), L ∝ M1.125, which
is controlled by the gravitational and pressure torque, and does not depend on the
cloud’s initial degree of rotation and turbulence.
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Olmsted, Susan C. "Star Formation in Ring Galaxies." Digital Commons @ East Tennessee State University, 2016. https://dc.etsu.edu/honors/322.
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Ring galaxies are specific types of interacting galaxies in which a smaller galaxy has passed through the center of the disk of another larger galaxy. The intrusion of the smaller galaxy causes the structure of the larger galaxy to compress as the smaller galaxy falls through, and to recoil back after the smaller galaxy passes through, hence the ring-like shape. In our research, we studied the star-forming regions of a sample of ring galaxies and compared to those of other interacting galaxies and normal galaxies. Using UV, optical, and IR archived images in twelve wavelengths from three telescopes, we analyzed samples of star-forming regions in ring and normal spiral galaxies using photometry. To measure the star formation rates of the star forming regions, we used computer software that picked out the regions and measured their luminosities in all twelve wavelengths, before comparing the luminosities in these wavelengths to determine the rate of star formation. We have determined that ring galaxies have proportionally more clumps with higher star formation rates than spirals, and a similar trend was suggested when comparing ring galaxies to other interacting galaxies (though more data is required for that comparison). These findings can help us understand galaxy evolution, including the evolution of our own galaxy.
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Adelberger, Kurt L. Steidel Charles C. "Star formation and structure formation at redshifts 1 ." Diss., Pasadena, Calif. : California Institute of Technology, 2002. http://resolver.caltech.edu/CaltechETD:etd-09252008-090426.
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Pozzo, Monica. "The effect of high-mass stars on low-mass star formation." Thesis, Keele University, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.366445.
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Sheehan, Patrick Duffy, and Patrick Duffy Sheehan. "Planet Formation In the Early Stages of Star Formation." Diss., The University of Arizona, 2017. http://hdl.handle.net/10150/625877.
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Recent studies suggest that many protoplanetary disks around pre-main sequence stars with inferred ages of 1-5 Myr (known as Class II protostars) may contain insufficient mass to form giant planets. This may be because by this stage much of the material in the disk has already grown into larger bodies, hiding the material from sight. If this is the case, then these older disks may not be an accurate representation of the initial mass budget in disks for forming planets.
To test this hypothesis, I have observed a sample of protostars in the Taurus star forming regions identified as Class I in multiple independent surveys, whose young (<1 Myr old) disks are more likely to represent the initial mass budget of protoplanetary disks. For my dissertation I have used detailed radiative transfer modeling of a multi-wavelength dataset to determine the geometry of the circumstellar material and measure the mass of the disks around these protostars. I discuss how the inferred disk mass distribution for this sample compares with results for the existing 1-5 Myr old disk samples, and what these results imply for giant planet formation.
Next, I discuss the cases of three separate, individual Class I protostars discovered through my ongoing survey of Class I protostars whose disks are all of particular interest, each for its own reasons. Each of these disks may provide clues that even at the young ages of Class I protostars, planet formation may already be well underway in their disks.
Finally, large disk mass surveys of large star forming regions like the Orion Nebula Cluster may be contaminated by free-free emission from disks that are being photoevaporated by nearby massive stars. I discuss my work with the VLA to constrain the free-free emission spectra for these sources so that current and future millimeter surveys can accurately measure disk masses in the ONC.
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Jessop, Nicholas Edgar. "The initial conditions of star formation." Thesis, University of Edinburgh, 1999. http://hdl.handle.net/1842/28303.
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This thesis is a study of the physical conditions typical in regions of the Inter Stellar Medium which are likely to go on to form stars. In recent years considerable progress has been made in the understanding of low mass star formation following various studies of opaque regions in near by parts of the Galaxy. The regions' high densities and high opacities make them favourable environments for star formation to occur. Using the IRAS Sky Survey Atlas a number of high latitude clouds are selected, and optical depths maps of these clouds are constructed. The most opaque regions in these clouds are identified and catalogued as a set of cloud cores. The column density and mass of each core are calculated. The majority of the cores are found to be gravitationally bound, and a small fraction are found to have signs of protostellar content; they contain IRAS point sources within them with spectral characteristics typical of Protostars or embedded Young Stellar Objects. An analysis of the typical properties of this set of cores and the typical properties found in previous studies of more opaque and more dense regions in the Inter Stellar Medium reveals that the timescale in which a cloud core forms a protostar decreases as both the opacity and density of the core increases. The results compare well with physical models of star formation in which the prestellar evolution is regulation by ambipolar diffusion, and the strong influence of various sources of ionizing on the star formation timescale is discussed. The very earliest protostars typically have a massive infalling envelope (visible in submillimetre continuum and molecular line observations), vigorous outflows, and radio emission due to shocks. The latter two processes are thought to be powered by the dynamically infalling envelope. Cores without any sign of protostellar content often contain dense regions similar in mass, but less dense, than the protostars' envelopes. Dense and massive enough to go on to form stars, these objects are precursors to the protostars; prestellar cores. A high resolution submillimetre study of a subset of these cores carried out with the JCMT in Hawaii is presented. Various isotopes of CO were detected towards the cores. One core in particular, L1689B, proved bright enough to map over a considerable region in both C18O(J=2→1) and C18O(J=3→2). All these observations are presented. In order to fully interpret the observations of L1689B, and to compare its properties with theoretical models of the early stages of star formation, a parameterised representation of L1689B is modelled with a radiative transfer code in order to produce predicted maps which can be compared with the observations. It is found that simple models of L1689B, assuming an isothermal gas temperature and constant abundance account for L1689B's appearance. Either a temperature drop or a drop in CO abundance towards its centre are needed to explain the observations. Neither however are uniquely implied by the C18O observations alone. A reanalysis of a submillimetre continuum maps of L1689B is made, and using these results the degeneracy is partly lifted. L1689B appears to have both a drop in temperature and a fall in abundance, both of which could be causally linked one to the other, or more likely, both of which are dependent on an external factor like the lack of ultra violet penetration towards the centre. Either way it is shown that the central region of L1689B is unsupported by pressure gradients, and susceptible to rapid free fall unless the existence of magnetic fields is assumed. This core therefore seems to be being supported by magnetic fields and evolving by ambipolar diffusion.
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Puxley, Philip John. "Vigorous star formation in galactic nuclei." Thesis, University of Edinburgh, 1989. http://hdl.handle.net/1842/27222.
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Balfour, Scott K. "Numerical simulations of triggered star formation." Thesis, Cardiff University, 2016. http://orca.cf.ac.uk/94927/.
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Feedback from massive stars is thought to be very important in regulating star formation on a range of scales. However, it is not clear if this feedback acts in a positive way by triggering star formation, or negatively by terminating it. In this thesis we investigate what role feedback plays in determining both the structure of molecular clouds and the rate of star formation, using Smoothed Particle Hydrodynamics. We begin by looking at how the evolution of an HII region is dependent on the amount of ionising radiation the exciting star produces. We then go on to explore the stellar populations created by cloud-cloud collisions and assess their ability to form high mass stars capable of producing large amounts of feedback. We then model the HII regions of these stars and determine what impact these have on star and structure formation. We find that there is a minimum stellar mass required to produce enough feedback to maintain an HII region. Below this value an HII region will either not form, or form and then implode. Above this value the HII region will act as a traditional HII region, and expand. When two clouds collide we �nd that they produce a shock compressed layer which forms �lamentary structures. The arrangement of these �laments is highly dependent on the collision velocity. Low velocity collisions produce a hub and spoke system in which competitive accretion dominates and produces a few very massive stars and a plethora of low mass stars. High velocity collisions produce lamentary networks that resemble a spider's web. In these spider's webs the stars form at nodes where multiple �laments meet. These nodes act as small local sites for star formation and form either a single, or small collection of stars. As a result stars formed in these systems tend to have a characteristic mass and there is less low mass or high mass star formation. However, we do find that eventually stars capable of producing signi�cant feedback form in all simulations. We model the HII regions of these stars and �find that they very quickly terminate star formation. They also produce very interesting bi-polar HII regions that are diffi�cult to interpret when viewed from some directions.
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Sawicki, Marcin. "Star formation history of the universe." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/nq41042.pdf.
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Lyder, David A. "Star formation in camelopardalis, Cam OB1." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp03/NQ32717.pdf.
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Edgar, R. G. "Radiative feedback and massive star formation." Thesis, University of Cambridge, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598750.
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In this thesis, I develop a new method of treating the problem of radiative transfer around a forming massive star. This new method addresses the failings of grey radiative transfer through dusty gas, while avoiding the computational cost of frequency dependent transfer algorithms. My new algorithm is validated by comparison with previous frequency dependent calculations. This method is then applied to collapses of spherically symmetry gas clouds. I use this to show that there is no absolute radiative limit to stellar masses. Radiative feedback on the dusty inflow will cause a star to stop accreting, but this does not occur at any particular fixed mass. However, applying my new method to the Bondi-Hoyle geometry (likely to be found in a competitively accreting stellar cluster), I show that radiative feedback can still cause great difficulties in forming massive stars. The decreased central concentration, and presence of centrifugal support mean that radiative feedback can disrupt the Bondi-Hoyle flow. Although lack of time prevented me from exploring much parameter space, I am able to suggest the regions which might be fruitfully explored in the future.
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Lomax, Oliver David. "Simulations of star formation in Ophiuchus." Thesis, Cardiff University, 2013. http://orca.cf.ac.uk/50030/.
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The way in which stars form from the interstellar medium is poorly understood. In this thesis we investiage the process star formation in molecular clouds via core fragmentation using Smoothed Particle Hydrodynamics (SPH). The initial conditions of the simulations are informed as closely as possible by observations of Ophiuchus. We run large ensebles of individual core simulations and compare the collective results with observations of stars and brown dwarfs. We use observations of Ophiuchus by Motte et al. (1998) and Andre et al. (2007) to calibrate a lognormal distribution from which we draw correlated masses, sizes and velocity dispersions. We assume that the cores are intrinsically triaxial. The distribution of core shapes is then inferred by fitting a single parameter family of ellipsoidal shapes to the observed core aspect ratios. Each core is given the density profile of a critical Bonnor-Ebert sphere and a turbulent velocity field which is modified to include ordered rotation and radial excursions. We evolve one hundred of the model prestellar cores using the Seren SPH code (Hubber et al., 2011). The simulations are repeated with continuous accretion heating, no accretion heating and episodic accretion heating (Stamatellos et al., 2012). We find that simulations with episodic accretion heating can reproduce the general features of the Chabrier (2005) initial mass function. This includs the ratio of stars to brown dwarfs and the turn-over at 0:2M�. We demonstrate that the mass of a star is not related to the mass of the prestellar core in which it formed. Low mass cores with Mcore � 0:1M� tend to collapse into single objects whereas higher mass cores with Mcore & 1M� can fragment into tens of objects. We finally show that the multiplicy statistics of the protostars formed in these simulations are well matched by observations. Multiplicity frequencies are higher than those of field stars and we note the presence of long-lived quadruple, quintuple and sextuple systems.
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Hutchings, Roger M. "The formation of primordial star clusters." Thesis, University of Sussex, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.323041.
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Mahajan, Smriti. "Star formation and environment of galaxies." Thesis, University of Birmingham, 2011. http://etheses.bham.ac.uk//id/eprint/1633/.
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Across cosmic time, stars have contributed most of the radiant energy found in the Universe and have created nearly all the chemical elements heavier than helium. Understanding the evolution of the Universe requires understanding the history of star formation. Stars form in galaxies, which are in essence huge aggregations of stars, dust and gas. Understanding star formation requires, among other things, measuring the rate at which interstellar gas is being converted into stars in a given galaxy. Star formation, a critical driver of galaxy evolution, responds both to external influences (local and global environment) and internal factors (e.g. dust).For several decades, the properties of star formation have been studied in galaxies residing in the dense environments of galaxy clusters and compared to those in the sparsely populated field. In this thesis we aim to bridge this gap in the study of the star formation-density relation by studying the evolution of galaxy properties, particularly their star formation rate (SFR) in the critical intermediate region on the periphery of rich galaxy clusters. Using photometric and spectroscopic data from the Sloan Digital Sky survey (SDSS) we show how common observables, such as colour, SFR, and SFR/M* are influenced when galaxies are assembled into clusters via large-scale filaments. We discover that complex galaxy populations (e.g. blue passive galaxies and [optically] red star-forming galaxies) are commonly found in and around rich clusters in the nearby (z ~ 0.1) universe. While the blue passive galaxies are the progenitors of passive red cluster galaxies, the (optically) red star-forming galaxies are a mix of at least two different populations. One set of red star-forming galaxies are dust obscured star-forming x galaxies, while the rest of them are normal star-forming galaxies whose colour is a result of metal-rich dominant stellar populations.
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Vidal, Thomas. "Revisiting the chemistry of star formation." Thesis, Bordeaux, 2018. http://www.theses.fr/2018BORD0151/document.
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Les études astrochimiques de la formation stellaire sont particulièrement importantes pour la compréhension de l'évolution de l'Univers, du milieu interstellaire diffus à la formation des systèmes stellaires. Les récentes avancées en matière de modélisation chimique permettent d'apporter de nouveaux résultats sur le processus de formation stellaire et les structures mises en jeu. L'objectif de ma thèse était donc d'apporter un regard neuf sur la chimie de la formation stellaire en utilisant les récentes avancées sur le modèle chimique Nautilus. J'ai pour cela étudié l'évolution de la chimie du soufre durant la formation stellaire pour tenter d'apporter de nouvelles réponses au problème de déplétion du soufre. J'ai d'abord effectué une révision du réseau chimique soufré et étudié son effet sur la modélisation du soufre dans les nuages denses. En comparant aux observations, j'ai montré que le modèle textsc{Nautilus} était capable de reproduire les abondances des espèces soufrées dans les nuages denses en utilisant comme abondance élémentaire de soufre son abondance cosmique. Ce résultat m'a permis d'apporter de nouveaux indices sur les reservoirs de soufre dans ces objets. Puis j'ai effectué une étude complète de la chimie du souffre dans les coeurs chauds en me concentrant sur les effets sur la chimie de la composition pre-effondrement. J'ai également étudié les conséquences des différentes simplifications couramment faites pour la modélisation des coeurs chauds. Mes résultats montrent que la composition pre-effondrement est un paramètre majeur de l'évolution chimique des coeurs chauds, fournissant de nouveaux indices pour expliquer la variété de compositions en espèces soufrées observée dans ces objets. De plus, ma recherche a mis en évidence la nécessité d'uniformiser les modèles de chimie utilisés pour les coeurs chauds. Enfin, j'ai développé une méthode efficace pour inverser les paramètres initiaux d'effondrement de nuages denses en me basant sur une base de données de modèles physico-chimiques d'effondrement, ainsi que sur l'observation d'enveloppes de protoétoiles de Classe 0. A partir d'un échantillon de 12 sources, j'ai pu en déduire des probabilités concernant les possibles paramètres initiaux d'effondrement de la formation d'étoiles de faible masseAstrochemical studies of star formation are of particular interest because they provide a better understanding of how the chemical composition of the Universe has evolved, from the diffuse interstellar medium to the formation of stellar systems and the life they can shelter. Recent advances in chemical modeling, and particularly a better understanding of grains chemistry, now allow to bring new hints on the chemistry of the star formation process, as well as the structures it involves. In that context, the objective of my thesis was to give a new look at the chemistry of star formation using the recent enhancements of the Nautilus chemical model. To that aim, I focused on the sulphur chemistry throughout star formation, from its evolution in dark clouds to hot cores and corinos, attempting to tackle the sulphur depletion problem. I first carried out a review of the sulphur chemical network before studying its effects on the modeling of sulphur in dark clouds. By comparison with observations, I showed that the textsc{Nautilus} chemical model was the first able to reproduce the abundances of S-bearing species in dark clouds using as elemental abundance of sulphur its cosmic one. This result allowed me to bring new insights on the reservoirs of sulphur in dark clouds. I then conducted an extensive study of sulphur chemistry in hot cores and corinos, focusing on the effects of their pre-collapse compositions on the evolution of their chemistries. I also studied the consequences of the use of the common simplifications made on hot core models. My results show that the pre-collapse composition is a key parameter for the evolution of hot cores which could explain the variety of sulphur composition observed in such objects. Moreover, I highlighted the importance of standardizing the chemical modeling of hot cores in astrochemical studies. For my last study, I developed an efficient method for the derivation of the initial parameters of collapse of dark clouds via the use of a physico-chemical database of collapse models, and comparison with observations of Class 0 protostars. From this method, and based on a sample of 12 sources, I was able to derive probabilities on the possible initial parameters of collapse of low-mass star formation
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Hubber, David Anthony. "Numerical simulations of binary star formation." Thesis, Cardiff University, 2006. http://orca.cf.ac.uk/56092/.
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Binary star formation is the dominant mode of star formation, in contrast to the traditional picture of single star formation. The work in this thesis investigates the properties of binary stars with the aid of numerical simulations, using N-body and Smoothed Particle Hydrodynamics codes. First, we develop a simple model of isolated binary star formation assuming prestellar cores fragment due to rotational instabilities into a ring of J f (< 6) stars. We follow the decay of this small-N cluster into singles and multiple systems using the N-body code NBODY3. We can reproduce most of the observed stellar and binary properties of young stars, including the high multiplicity and wide separation distribution, in low-mass star forming regions like Taurus. We extend this further into a model of clustered binary star formation assuming 100 small-N clusters form in fractal clusters of radius 1 pc, similar to many young embedded clusters. We follow the dynamical interactions of these clusters using the N-body code NBODY6. We find that disruptive binary-binary encounters in dense clusters can explain the differences between binary properties in low-density and high-density star forming regions. We develop a new test of Smoothed Particle Hydrodynamics (SPH) called the Jeans Test. We demonstrate that SPH correctly models fragmentation and that under-resolved SPH simulations supress real fragmentation rather than promote artificial fragmentation. Thus binary and multiple systems produced in SPH simulations are real and not the result of numerical effects. Finally, we perform simulations of turbulent prestellar cores in the context of binary star formation. We extend the work of Goodwin, Whitworth & Ward-Thompson (2004) by investigating 2.17 M0 and 4.34 M0 cores.
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White, Sarah Virginia. "Accretion and star formation in quasars." Thesis, University of Oxford, 2015. https://ora.ox.ac.uk/objects/uuid:94fa7a0c-83be-4283-9bf5-558b9354044d.
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Studying the interplay of accretion and star formation is crucial to our understanding of galaxy evolution. The new generation of radio surveys are able to play a key role in this area, since both processes produce radio emission. As we probe low radio-luminosities, the two main populations are star-forming galaxies and radio-quiet quasars (RQQs). How they contribute to the total radio emission is under debate, with previous authors arguing that star formation dominates. In this thesis I investigate the relative levels of radio emission due to accretion and star formation in RQQs. Firstly, I select a sample of 74 quasars from the VISTA Deep Extragalactic Observations (VIDEO) survey, whose depth allows me to study very low accretion rates and/or lower-mass black holes. By comparing radio emission from the quasars with that from a control sample of galaxies, and calculating two independent estimates of star-formation rate, I show that this emission is predominantly related to the accretion process. A second sample of 72 RQQs is obtained from the Spitzer-Herschel Active Galaxy Survey, spanning a factor of ~100 in optical luminosity over a narrow redshift range at z~1. This enables evolutionary effects to be decoupled when comparisons are made with the VIDEO sample. I reduce radio data from the Karl G. Jansky Very Large Array (JVLA) for these objects, and find further support that accretion makes a significant contribution to the radio emission in RQQs. In addition, the levels of accretion and star formation appear to be weakly correlated with each other, and with optical luminosity. These findings offer an insight into how the mechanisms behind galaxy evolution may interact differently in RQQs compared to their radio-loud counterparts. They also have important implications for modelling radio populations below 1mJy, which is necessary for the development of the Square Kilometre Array.
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Merilan, Michael Preston. "Supermassive star formation : the early phases /." The Ohio State University, 1985. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487260531958504.
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Prescott, Matthew. "An investigation of cosmic star formation." Thesis, Liverpool John Moores University, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.549426.
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Liu, Charles Tsun-Chu. "The star formation history of galaxies." Diss., The University of Arizona, 1996. http://hdl.handle.net/10150/290658.
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The star formation history of galaxies is the primary influence on galaxy evolution, and hence the evolution of almost all the visible matter in the universe. In this dissertation, I present studies of the star formation history of galaxies which have come from two distinct perspectives: the study of galaxies that have unusual star formation histories, and the search within the general galaxy population for galaxies with unusual star formation histories. A spectrophotometric atlas of 40 merging and strongly interacting galaxies is obtained and analyzed in order to examine their stellar populations and star formation histories. Within the sample, the subsample of 10 ultraluminous IRAS systems is compared with the optically selected subsample. The population of objects in the sample with anomalously strong Balmer absorption lines, a spectral signature indicative of post-starburst evolution, is examined and compared with distant "E+A galaxies" which have similar spectrophotometric properties. Spectrophotometry across the entire optical wavelength range is obtained and analyzed for a sample of 8 E+A galaxies, ranging in redshift from 0.09 ≤ z ≤ 0.54. The method of stellar population modeling, widely used with only minor variations in the astronomical community, is examined and its strengths and limitations are discussed.
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Randriamanakoto, Rojovola Zara-Nomena. "Formation of young massive star clusters: a high-resolution multi-wavelength study of intensely star-formation galaxies." Doctoral thesis, University of Cape Town, 2015. http://hdl.handle.net/11427/15766.
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Super star clusters (SSCs) represent the youngest and most massive form of known gravitationally bound star clusters in the Universe. They are born abundantly in environments that trigger strong and violent star formation (SF) such as in galaxy mergers and interacting systems. SSCs are thus used as fundamental tools to understand the context of massive SF and galaxy evolution in general. This thesis investigates properties of these young, massive and dense star clusters in a sample of 42 nearby starbursts and luminous infrared galaxies (LIRGs) ...
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Johnston, Katharine G. "Observational signatures of massive star formation : an investigation of the environments in which they form, and the applicability of the paradigm of low-mass star formation." Thesis, University of St Andrews, 2011. http://hdl.handle.net/10023/1895.
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This thesis presents both a study of the cluster-scale environments in which massive stars form, investigating in particular how the ionized gas in these regions relates to the molecular star-forming material, as well as detailed studies of two luminous forming stars, AFGL 2591 and IRAS 20126+4104, to determine whether they are forming similarly to their low-mass counterparts. The results of this work include the identification of 35 HII regions (20 newly discovered) via a radio continuum survey of ionized gas towards 31 molecular cluster-forming clumps. The observed ionized gas was found to be preferentially associated with the clumps, which were shown to have a range of evolutionary stages. The massive star formation efficiency was determined for the clumps with associated ionized gas, and a relationship was found between the mass of the clumps and the mass of their embedded massive stars. By modelling the SEDs and images of AFGL 2591 and IRAS 20126+4104, it was found that the geometry of their circumstellar material was generally consistent with an envelope plus disk, similar to that expected for low-mass protostars. However, within the central ~1800 AU, the mid-IR images of IRAS 20126+4104 were better described by only a flattened envelope, suggesting that the radiation from IRAS 20126+4104 may be affecting the regions closest to the star. Observations of the ionized and molecular gas towards AFGL 2591 were carried out, and a photoionization code was developed to interpret these observations. The results showed that the observed 3.6 cm emission is likely to be produced by both a shock-ionized jet and a hypercompact HII region that does not appear to have disrupted the jet or the large-scale circumstellar environment. In addition, the C¹⁸O(1-0) emission observed towards AFGL2591 traces the densest parts of the outflow, with the blue-shifted emission exhibiting many of the properties of the outflows from low-mass protostars.
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Schael, Anita M. "The star-formation history of massive galaxies." Thesis, University of Edinburgh, 2009. http://hdl.handle.net/1842/3126.
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This thesis presents multi-frequency data, galaxy identifications, estimated redshifts, and derived physical properties for the sub-millimetre source sample produced by the SCUBA HAlf Degree Extragalactic Survey (SHADES). SHADES is the largest, complete, sub-millimetre survey undertaken to date, and the aim of this work is to exploit this survey to study the evolution of sub-mm selected galaxies at high redshift, explore their possible connectionwith localmassive ellipticals, and to test current models of galaxy formation. The SHADES sample was selected from 850 micron images made with the submillimetre camera SCUBA at the James Clerk Maxwell Telescope. These submillimetre maps cover a total area of 720 arcmin2 split between two well-studied extra-galactic survey fields, the Lockman Hole East and the Subaru/XMMNewton Deep Field (SXDF). The resulting sample of 120 sub-millimetre sources is the focus of this thesis. Here the wealth of information provided by deep radio, optical, near-infrared and mid-infrared imaging of the two SHADES fields is exploited to complete the identification of the SHADES sample, and then to derive a robust redshift estimate for every sub-millimetre source. Where possible this is achieved from the optical+ infrared photometry using a new two-component redshift estimation code developed specifically to deal with starbursting galaxies with potentially highly stochastic star-formation histories. The effectiveness of this code is demonstrated via comparison with the small subset of SHADES source which possess robust spectroscopic redshifts. For those galaxies which are too faint for effective redshift constraints to be provided by the existing optical+infrared photometry, the information on the long-wavelength spectral energy distribution provided by the radio+submm photometry is utilised to provide cruder constraints or limits on redshift. The result is the first complete and unbiased estimate of the redshift distribution of the bright extragalactic sub-millimetre galaxy population. It is found that the brightest sub-mm sources are confined to the redshift range 2 < z < 4, while more moderate luminosity sources span the full range of redshift out to z ∼ 5. The fits to themulti-frequency photometry provided by the redshift estimation technique are also used to derive estimates of the stellar mass, and star-formation history of each SHADES galaxy. The average derived stellar mass is ∼ 3 × 1011 M⊙ and it is found that the violent starburst powering the sub-millimetre emission typically contributes less than 10% of the stellar mass of the galaxy which has been assembled prior to the “current” starburst event. The distributions of redshift, stellar mass, and star-burst ages are compared with the predictions of a range of galaxy models, including the suite of models originally used to motivate the SHADES survey in van Kampen et al. (2005), and themost recent incarnation of the Durhamsemi-analytic galaxy formationmodels described by Swinbank et al. (2008). It is found that the redshift distribution and sub-mmflux versus redshift for bright sub-mmgalaxies can be reproduced best by one of the van Kampen models, which is based on semi-analytic modelling with a Chabrier IMF. We can rule out the non-semi-analytic prediction models and the Durham semi-analytic model with a top-heavy IMF. However the stellar masses are systematically underpredicted by all of the models. Either the stellar masses derived from the SHADES data have been systematically over-estimated, or the models need to be modified (perhaps by the inclusion of AGN feedback) to allow larger galaxy masses to assembled prior to z ∼ 2. Finally, it is demonstrated that themass in place prior to the observed starburst cannot have been produced by an analogous super-burst at higher redshift, but rather requires to have been assembledmore gradually over a timescale of ∼ 1−2 Gyr. It is thus concluded thatmassive galaxies undergo theirmost violent phase of star formation at redshifts 2 < z < 4, but that the enormous starbursts which lead to detection in current sub-millimetre surveys can only take place in the potential well provided by an already massive galaxy. This supports a scenario in which bright sub-millimetre galaxies are indeed the progenitors of the massive elliptical galaxies found in the local Universe.
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Mitchell, Peter Daniel. "Star formation and stellar mass assembly in galaxy formation models." Thesis, Durham University, 2015. http://etheses.dur.ac.uk/11125/.
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We use the semi-analytic galaxy formation model, GALFORM, to explore the implications of results from multi-wavelength galaxy surveys within the context of the hierarchical structure formation paradigm. Specific topics which we investigate include (i): the biases that can be introduced by using spectral energy distribution fitting to infer stellar masses from broad-band photometry, (ii) the reasons why galaxy formation models struggle to reproduce the exponential drop with time in star formation rates of star-forming galaxies inferred from a wide range of observations, (iii) the physical processes that control the evolution in the median relationship between stellar mass and halo mass predicted by galaxy formation models. We show that stellar masses of compact dusty star-forming galaxies could be underestimated by SED fitting as a result of assuming a uniform foreground dust screen geometry. We explain how the standard implementation of supernova feedback and gas reincorporation within galaxy formation models results in flat predicted star formation histories for star forming galaxies. We show that this is inconsistent with observational data which imply that these star formation histories should instead be peaked at intermediate redshift. We also show how the supernova feedback and gas reincorporation implementations within standard galaxy formation models result in a baryon conversion efficiency within haloes that is roughly independent of cosmic time at fixed halo mass. Consequently, the median stellar mass versus halo mass relationship is predicted by these models to not evolve significantly.
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Maschberger, Thomas [Verfasser]. "Star formation in galaxies and star clusters / Thomas Maschberger. Mathematisch-Naturwissenschaftliche Fakultät." Bonn : Universitäts- und Landesbibliothek Bonn, 2011. http://d-nb.info/1019547219/34.
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Isaacs, Narusha. "A multi-scale study of the star formation law in nearby galaxies." University of the Western Cape, 2020. http://hdl.handle.net/11394/7912.
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>Magister Scientiae - MScThis research aimed to evaluate the effects of changing length-scales on the star formation laws and star formation efficiencies for our selected sample of galaxies. We have combined high-resolution Hi data from The Hi Nearby Galaxy Survey, CO data from HERA CO–Line Extragalactic Survey and the Nobeyama CO Atlas of Nearby Spiral Galaxies and 12 𝜇m data from the Wide-field Infrared Survey Explorer of a sample of five nearby galaxies to study the relationship between star formation rate surface density, ΣSFR, and gas surface density, Σgas, at various length-scales. In order to probe the star formation law of each galaxy, all image sets were placed on common astrometric grids and evaluated on a pixel-by-pixel basis over a range of sub-kpc length-scales. We investigated whether the star formation law changes with length-scales and found that as resolution becomes coarser, the Kennicutt Schmidt power-law index decreases for the correlation between ΣSFR and ΣH2 . Our results show that the index values are close to unity but are not consistent with it.
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Howell, Justin H. "Star formation histories of nearby elliptical galaxies /." Diss., Digital Dissertations Database. Restricted to UC campuses, 2004. http://uclibs.org/PID/11984.
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Leurini, Silvia. "Methanol: a diagnostic tool for star formation." [S.l.] : [s.n.], 2005. http://deposit.ddb.de/cgi-bin/dokserv?idn=973948639.
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Gurkan, Uygun Gulay. "Accretion modes, AGN feedback and star formation." Thesis, University of Hertfordshire, 2016. http://hdl.handle.net/2299/17239.
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I study mid-infrared and star formation properties of AGN samples using infrared observations, and star-forming galaxies using radio observations in order to investigate the link between star formation, AGN activity and radio luminosity. I present the results of these investigations in this thesis. I carried out an analysis of four complete samples of radio-loud AGN (3CRR, 2Jy, 6CE and 7CE) using near- and mid-IR data taken by the Wide-Field Infrared Survey Explorer (WISE). The combined sample consists of quasars and radio galaxies, and covers a redshift range 0:003 < z < 3:395. The dichotomy in the mid-IR properties of low- and high-excitation radio galaxies (LERGs - HERGs) is analysed using large complete samples. The results show that a division in the accretion modes of powerful LERGs and HERGs clearly stands out in the mid-IR radio plane. Evaluation of the positions of the sample objects in WISE colour-colour diagrams shows that widely used WISE colour cuts are not completely reliable in selecting AGN. I examined the link between AGN activity and star formation by constructing matched samples of local (0 < z < 0:6) radio-loud and radio-quiet AGN in the Herschel-ATLAS fields. AGN accretion and jet powers in these active galaxies are traced by [OIII] emission-line and radio luminosity, respectively. Star formation properties were derived using Herschel 250-_m and stellar mass measurements are taken from the SDSS-MPA-JHU catalogue. The stacking analyses show that star formation rates (SFRs) and specific star formation rate (SSFRs) of both radio-loud and radio-quiet AGN increase with increasing AGN power but that radio-loud AGN tend to have lower SFR. Additionally, radio-quiet AGN are found to have approximately an order of magnitude higher SSFRs than radio-loud AGN for a given level of AGN power. The difference between the star formation properties of radio-loud and -quiet AGN is also seen in samples matched in stellar mass. I also investigated the relationship between SFR and low-frequency radio luminosity observed in star-forming galaxies. I used a sample of star-forming galaxies in the 19 local Universe selected from the SDSS-MPA-JHU catalogue. LOFAR observations of the Herschel-ATLAS North Galactic Pole field (NGP) were carried out as part of the LOFAR surveys Key Science Project at an effective frequency of 150 MHz, which provided low-frequency radio luminosity of sample galaxies. SFRs of galaxies in the sample were derived using MAGPHYS spectral energy distribution (SED) fitting. The results of this study show that the slope of L150/SFR is less than unity and not universal for all star-forming galaxies (SFGs) in the local Universe (0 < z < 0:3). The slope of the L150/SFR relation is also found to be steeper than the L1:4/SFR relation, probably due to the contribution from thermal radio emission at 1.4 GHz. If the L150=SFR relation for strongly star-forming objects is explained naively by electron calorimetry, I conclude that low luminosity sources are not ideal calorimeters and differ from the main locus of SFGs at low redshifts. The different gradients we obtain for the far- IR/radio correlation using samples selected at different frequencies reveal the selection effects on relations derived in this thesis.
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Lee, Aaron Thomas. "Star and Planet Formation through Cosmic Time." Thesis, University of California, Berkeley, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10619929.
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The computational advances of the past several decades have allowed theoretical astrophysics to proceed at a dramatic pace. Numerical simulations can now simulate the formation of individual molecules all the way up to the evolution of the entire universe. Observational astrophysics is producing data at a prodigious rate, and sophisticated analysis techniques of large data sets continue to be developed. It is now possible for terabytes of data to be effectively turned into stunning astrophysical results. This is especially true for the field of star and planet formation. Theorists are now simulating the formation of individual planets and stars, and observing facilities are finally capturing snapshots of these processes within the Milky Way galaxy and other galaxies. While a coherent theory remains incomplete, great strides have been made toward this goal.
This dissertation discusses several projects that develop models of star and planet forma- tion. This work spans large spatial and temporal scales: from the AU-scale of protoplanetary disks all the way up to the parsec-scale of star-forming clouds, and taking place in both contemporary environments like the Milky Way galaxy and primordial environments at redshifts of z ~ 20.
Particularly, I show that planet formation need not proceed in incremental stages, where planets grow from millimeter-sized dust grains all the way up to planets, but instead can proceed directly from small dust grains to large kilometer-sized boulders. The requirements for this model to operate effectively are supported by observations. Additionally, I draw suspicion toward one model for how you form high mass stars (stars with masses exceeding ~ 8 Msun), which postulates that high-mass stars are built up from the gradual accretion of mass from the cloud onto low-mass stars. I show that magnetic fields in star forming clouds thwart this transfer of mass, and instead it is likely that high mass stars are created from the gravitational collapse of large clouds. This work also provides a sub-grid model for computational codes that employ sink particles accreting from magnetized gas. Finally, I analyze the role that radiation plays in determining the final masses of the first stars to ever form in the universe. These stars formed in starkly different environments than stars form in today, and the role of the direct radiation from these stars turns out to be a crucial component of primordial star formation theory.
These projects use a variety of computational tools, including the use of spectral hydrodynamics codes, magneto-hydrodynamics grid codes that employ adaptive mesh refinement techniques, and long characteristic ray tracing methods. I develop and describe a long characteristic ray tracing method for modeling hydrogen-ionizing radiation from stars. Additionally, I have developed Monte Carlo routines that convert hydrodynamic data used in smoothed particle hydrodynamics codes for use in grid-based codes. Both of these advances will find use beyond simulations of star and planet formation and benefit the astronomical community at large.
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Nolan, Louisa A. "The star formation history of elliptical galaxies." Thesis, University of Edinburgh, 2002. http://hdl.handle.net/1842/27129.
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Three different groups of elliptical galaxies, covering a range of redshift and nuclear activity are investigated. The rest-frame optical spectra of a population of 24 low-redshift (z ? 0.2) AGN host galaxies, together with the high-quality rest-frame ultraviolet spectra of two high-redshift (z ? 1.5) mJy radio galaxies and ultraviolet-to-optical spectra of two low-redshift (z < 0.1) inactive galaxies are compared with a range of simple, near-instantaneous starburst stellar population model spectra from various authors. With the benefit of non-solar metallicity models, the attempt is made to reliably determine the ages and metallicities of the stellar populations of the observed galaxies via continuum fitting. A simultaneous test of the ability of these models to accurately reproduce the spectra of real stellar populations is also presented. This is carried out both by testing the models' ability to recover the correct ages and metallicities of two well-studied F stars and the Sun, in the regimes where these stars dominate the integrated model flux, and by investigating the quality of the continuum fits to the observed galaxy spectra. With high-quality data, and reliable model spectra, it is possible to lift age-metallicity degeneracy, and robustly constrain the ages and metallicities of the galaxies studied. Models with stellar populations of more than one age and/or metallicity have been constructed, and this has allowed composite galaxy populations to be disentangled. The results are consistent with the existence of two classes of elliptical galaxies, one formed at high redshift, with predominantly passive evolution, and the other formed at lower redshift, from galaxy-galaxy merging, with associated star formation. The spectroscopic determination of the stellar content of the galaxies studied has enabled their epochs of star formation to be determined, and hence their formation routes. The constraints imposed by the reliable identification of old stellar populations at known redshift favour a L-dominated cosmological model, and strongly reject an Einstein-de Sitter universe.
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Scott, Susan. "Dust-enshrouded star formation at high redshift." Thesis, University of Edinburgh, 2003. http://hdl.handle.net/1842/27360.
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Since the advent of SCUBA, deep submillimetre surveys have succeeded in resolving the bulk of the far-infrared extragalactic background into discrete sources, revealing a population of high-redshift (z > 1) heavily dust-enshrouded massive-starforming galaxies. Here, the nature of the most luminous 850μm sources (S850 > 5 mJy) are considered, in particular their link with the formation and evolution of the most massive elliptical galaxies visible in the present-day Universe. The “SCUBA 8 mJy Survey” is the largest of the blank field submillimetre surveys completed to date, designed specifically with the aim of identifying the brightest 850μm sources. It covers ~ 260 square arcminutes of sky to a depth of σrms ~ 2.5mJy/beam, evenly split between two areas of low galactic cirrus emission; the Lockman Hole East and ELAIS N2. The data have in part been reduced by the standard JCMT SURF procedures, but the primary reduction method was an alternative IDL-based pipeline which has the advantage of producing uncorrelated noise images. This later approach has enabled me to develop a maximum-likelihood source extraction algorithm which simultaneously measures the statistical significance of every peak in a SCUBA map, leading to properly quantified errors on the flux densities of all potential sources. Applying the source extraction algorithm to these two fields has revealed 19 sources with S/N > 4.00, 40 sources with S/N > 3.50, and 85 sources with S/N > 3.00. Completeness, mean output vs. input flux density, and contamination from spurious/confused sources were quantified using extensive Monte Carlo simulations. Using deep 1.4 GHz imaging of the survey fields to determine the radio-to-submillimetre spectral indices for every 850μm detection, all sources were constrained to lie at z > 1, with a median redshift zmed ~ 2.4. This being the case, the inferred star formation rates are ~ 1000 M? yr-1, sufficient to form the most massive elliptical galaxies on timescales of ~ 1 Gyr, but heavily obscured by 108 – 109 M? of dust. The commoving number density of high redshift galaxies forming stars at > 1000 M? yr-1 is ~ 10-5 Mpc-3, with only a weak dependence on the precise redshift distribution, also corresponding to the number density of massive ellipticals with L > 3 – 4L* in the present-day Universe, as well as the co-moving number density of comparably massive, passively-evolving objects in the redshift band 1 < z < 2 inferred from recent surveys of extremely red objects. This suggests that the bright submillimetre sources uncovered by this survey can plausibly account for the formation of all present-day massive spheroids.
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Attwood, Rhianne. "Simulating star formation in molecular cloud cores." Thesis, Cardiff University, 2008. http://orca.cf.ac.uk/54778/.
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In this thesis we investigate the influence of certain physical effects on the collapse and fragmentation of isolated, low-mass, low-turbulence cores, in particular on the mass distribution, binary statistics and kinematics of the resulting stars. We perform numerical simulations using a Smoothed Particle Hydrodynamics code to model this mode of star formation. Firstly we model acoustic oscillations of a self-gravitating isentropic monatomic gas sphere using our SPH code and find that if the smoothing lengths are adjusted so as to keep the number of neighbours in the range AAu, NNE1B should be set to zero, to reduce the level of numerical dissipation and diffusion. We suggest that this should become a standard test for codes simulating star formation, since pressure waves generated by the switch from approximate isothermality to approximate adiabaticity play a crucial role in the fragmentation of collapsing cores. We perform a large ensemble of SPH simulations of cores having different levels of turbulence, using a new, more realistic treatment of thermodynamics, developed by Stamatellos et al. (2007), which takes into account the thermal history of protostellar gas and captures the thermal inertia effects. We compare the results with simulations using a standard barotropic equation of state. We find that increasing the level of turbulence generally tends to reduce the fraction of the core mass which is converted into stars, and increase the number of stars formed by a single core. Using the new treatment results in more protostellar objects being formed, and a higher proportion of brown dwarfs. Of the multiple systems that form, they tend to have shorter periods, higher eccentricities and higher mass ratios. We also note that in our simulations the process of fragmentation is often bimodal, in the following sense. The first protostar to form is usually, at the end, the most massive, i.e. the primary. However, frequently a disc-like structure subsequently forms round this primary, and then, once it has accumulated sufficient mass, quickly fragments to produce several secondaries. We believe that this delayed fragmentation of a disc-like structure is likely to be an important source of very low-mass stars in nature (both low-mass hydrogen-burning stars and brown dwarf stars). We also model the evolution of an ensemble of prestellar cores in the Ophiuchus Main Cloud using initial conditions for the sizes and levels of turbulence constrained by the observations of Motte et al. (1998) and Andre' et al. (2007), and the recently revised core masses of Stamatellos et al. (2007). We find that star formation in these core is extremely efficient with typically the formation of a single star, but we also see the formation of multiple systems in a number of cores. We find that the number of stars formed by a core is highest if the core has high mass, and/or if it has a high initial level of turbulence, and/or if it starts from a low initial density. We explain why. Finally we explore the effect metallicity has on the mass distribution and binary statistics of stars formed from low-mass low-turbulence cores. We find that reducing the metallicity decreases the number of stars formed from a single core and reduces the number of brown dwarfs formed. It also reduces the binary frequency.
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Gusdorf, Antoine. "Molecular emission in regions of star formation." Thesis, Durham University, 2008. http://etheses.dur.ac.uk/2304/.
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Recent observations show that young stars being formed eject matter at several tens of kilometers per second, in the form of jets and outflows that impact the matter whose collapse is at the origin of the formation of the star. The supersonic impact between this jet and the parent interstellar cloud of the star generates a shock front, in the form of a bow-shock, which propagates in the collapsing interstellar gas, and also an inverse shock that propagates along the jet itself. The structure of these shocks depends on their velocity as well as on the physical properties of the gas in which they propagate. Numerical MagnetoHydroDynamİGal (MHD) simulations of the propagation of such shocks are a way to model the molecular emission arising from these regions, and thus to constrain the physical and chemical properties of the gas in which these molecular lines are emitted. A large grid of shock models is ran, for different values of key parameters such as the shock velocity, the pre-shock density, the magnetic field, ad the shock age. The emission of molecular hydrogen (whose treatment is included inside the shock code) is studied first. Pure rotational and rovibrational excitation diagrams are built for each model, and compared to the available observations of the bipolar outflow L1157. These comparisons confirm the necessity to use non stationary models to be able to interpret the observed column densities of H(_2). The emission of other characteristic molecules in the shocked region is then studied. The radiation transfer is computed thanks to a program based on the LVG (Large Velocity Gradient) approximation. In the case of SiO, comparisons with observed integrated intensities in L1157 are done, independently from the molecular hydrogen results, with a good agreement for stationary shock models and under diverse assumptions regarding the initial repartition of silicon in the dust grains, and oxygen in the gas phase. An attempt to simultaneous fitting of SiO and H(_2) observational data is then done, that is their fit by a very same (non stationary) shock model, with encouraging results. To complete this study, CO emission is treated similarly as SiO, and studied over the whole models grid. CO is then added to the list of molecules whose production and emission can be modelled by the same shock model as H(_2) and SiO with a satisfying agreement, even if this addition does not yield further constrain on the shock and medium properties.
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Delgado, Donate Eduardo Juan. "Multiple star formation in molecular cloud cores." Thesis, University of Cambridge, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.615675.
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Bell, Eric Findlay. "Exploring the star formation histories of galaxies." Thesis, Durham University, 1999. http://etheses.dur.ac.uk/4796/.
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In this thesis, I explore the star formation histories of both spiral and elliptical galaxies. In Part 1,1 present an in-depth study of the star formation histories of spiral galaxies with a wide range of properties. Optical and near-infrared colours are used in conjunction with up-to-date stellar population synthesis models to constrain the ages and metallicities of my sample galaxies. I find that age and metallicity gradients are common in spiral galaxies of all types. The age of a spiral galaxy correlates mainly with its surface brightness, and its metallicity correlates strongly with both its surface brightness and absolute magnitude. Using simple models, I demonstrate that the correlations observed in this thesis show that the star formation history of a region within a galaxy depends primarily on its surface density, and possibly on the dynamical time. Metal- enriched outflow from low mass galaxies seems to be required to reproduce a reasonably strong metallicity-magnitude correlation. These variations in star formation history are a continuous function of the physical parameters: in particular, I find no evidence for a bimodal spiral galaxy surface brightness distribution. In Part 2, I present a short study on the formation epoch of early-type galaxies. I developed a photometric redshift estimator optimised for redshifts z ~ 1. The redshift estimator provides redshifts accurate to ~ 10 per cent. This redshift estimator is then applied to a sample of morphologically-selected early-type galaxies in the northern Hubble Deep Field. Comparison of their colour-magnitude relation with a passively evolved Coma cluster colour-magnitude relation indicates that over half of the sample must form at redshifts greater than two.
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Phillips, Robin R. "Radiative transfer modelling of star formation regions." Thesis, University of Kent, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.267438.
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