Observations with powerful X-ray telescopes, such as XMM-Newton and Chandra, significantly advance our understanding of massive stars. Nearly all early-type stars are X-ray sources. Studies of their X-ray emission provide important diagnostics of stellar winds. High-resolution X-ray spectra of O-type stars are well explained when stellar wind clumping is taking into account, providing further support to a modern picture of stellar winds as non-stationary, inhomogeneous outflows. X-ray variability is detected from such winds, on time scales likely associated with stellar rotation. High-resolution X-ray spectroscopy indicates that the winds of late O-type stars are predominantly in a hot phase. Consequently, X-rays provide the best observational window to study these winds. X-ray spectroscopy of evolved, Wolf-Rayet type, stars allows to probe their powerful metal enhanced winds, while the mechanisms responsible for the X-ray emission of these stars are not yet understood.

Recombination line profile shapes are derived for ionized spherical stellar winds at radio wavelengths. It is assumed that the wind is optically thick owing to free-free opacity. Emission lines of arbitrary optical depth are obtained assuming that the free-free photosphere forms in the outer, constant expansion portion of the wind. Previous works have derived analytic results for isothermal winds when the line and continuum source functions are equal. Here, semi-analytic results are derived for unequal source functions to reveal that line shapes can be asymmetric about line center. A parameter study is presented and applications discussed.

We examine over 160 archival Hα spectra from the Ritter Observatory for the interacting binary β Lyr obtained between 1996 and 2000. The emission is characteristically double-peaked, but asymmetric, and with an absorption feature that is persistently blueshifted. Using a set of simplifying assumptions, phase varying emission line profiles are calculated for Hα formed entirely in a Keplerian disk, and separately for the line formed entirely from an off-center bipolar flow. However, a dynamic spectrum of the data indicates that the blueshifted feature is not always present, and the data are even suggestive of a drift of the feature in velocity shift. We explore whether a circumbinary envelope, hot spot on the accretion disk, or accretion stream could explain the observations. While none are satisfactory, an accretion stream explanation is somewhat promising.

Oxygen sequence Wolf-Rayet stars (WO) are thought to be the final evolution phase of some high-mass stars, as such they may be the progenitors of Type Ic SNe as well as potential progenitors of broad-lined Ic and long gamma-ray bursts. We present the first spectropolarimetric observations of the Galactic WO stars WR93b and WR102 obtained with FORS1 on the Very Large Telescope. We find no sign of a line effect, which could be expected if these stars were rapid rotators. We also place constraints on the amplitude of a potentially undetected line effect. This allows us to derive upper limits on the possible intrinsic continuum polarization and find Pcont < 0.077 per cent and Pcont < 0.057 per cent for WR93b and WR102, respectively. Furthermore, we derive upper limits on the rotation of our WO stars by considering our results in the context of the wind compression effect. We estimate that for an edge-on case the rotational velocity of WR93b is vrot < 324 km s−1 while for WR102 vrot < 234 km s−1. These correspond to values of vrot/vcrit < 19 per cent and j) < 18.0 cm2 s−1 for WR93b and 2 s−1 for WR102. The upper limits found on vrot/vcrit and log(j) for our WO stars are therefore similar to the estimates calculated for Galactic Wolf-Rayet (WR) stars that do show a line effect. Therefore, although the presence of a line effect in a single WR star is indicative of fast rotation, the absence of a line effect does not rule out significant rotation, even when considering the edge-on scenario.

We present new, contemporaneous Hubble Space Telescope STIS and XMM-Newton observations of the O7 III(n)((f)) star ξ Per. We supplement the new data with archival IUE spectra, to analyze the variability of the wind lines and X-ray flux of ξ Per. The variable wind of this star is known to have a 2.086-day periodicity. We use a simple, heuristic spot model that fits the low-velocity (near-surface) IUE wind line variability very well, to demonstrate that the low-velocity absorption in the new STIS spectra of N iv λ1718 and Si iv λ1402 vary with the same 2.086-day period. It is remarkable that the period and amplitude of the STIS data agree with those of the IUE spectra obtained 22 yr earlier. We also show that the time variability of the new XMM-Newton fluxes is also consistent with the 2.086-day period. Thus, our new, multiwavelength coordinated observations demonstrate that the mechanism that causes the UV wind line variability is also responsible for a significant fraction of the X-rays in single O stars. The sequence of events for the multiwavelength light-curve minima is Si iv λ1402, N iv λ1718, and X-ray flux, each separated by a phase of about 0.06 relative to the 2.086-day period. Analysis of the X-ray fluxes shows that they become softer as they weaken. This is contrary to expectations if the variability is caused by periodic excess absorption. Furthermore, the high-resolution X-ray spectra suggest that the individual emission lines at maximum are more strongly blueshifted. If we interpret the low-velocity wind line light curves in terms of our model, it implies that there are two bright regions, i.e., regions with less absorption, separated by 180°, on the surface of the star. We note that the presence and persistence of two spots separated by 180° suggest that a weak dipole magnetic field is responsible for the variability of the UV wind line absorption and X-ray flux in ξ Per.

Recent large-scale mapping of dust continuum emission from star-forming clouds has revealed their hierarchical nature, which includes web-like filamentary structures that often harbor clumpy over-densities where new stars form. Understanding the motions of these structures and how they interact to form stars, however, can only be learned through observations of emission from their molecular gas. Observations of tracers such as ammonia (NH3), in particular, reveal the stability of dense gas structures against forces such as the inward pull of gravity and the outward push of their internal pressure, thus providing insights into whether or not those structures are likely to form stars in the future. Due to recent large-scale ammonia surveys that have mapped both nearby and distant clouds in the Galaxy, it is finally possible to investigate and compare the stability of star-forming structures in different environments. In this dissertation, we utilize ammonia survey data to provide one of the largest investigations to date into the stability of structures in star-forming regions. Dense gas structures have been identified in a self-consistent manner across a variety of star-forming regions and the environmental factors (e.g., the presence or lack of local filaments and heating by local massive stars) most influential to their stability were investigated. The analysis has revealed that dense gas structures identified by ammonia observations in nearby star-forming clouds tend to be gravitationally bound. In high-mass star-forming clouds, however, bound and unbound ammonia structures are equally likely. This result suggests that either gravity is more important to structure stability at the small scales probed in nearby clouds or ammonia is more widespread in high-mass star-forming regions. In addition, a new method to detect and measure emission with multiple velocity components along the line of sight has been developed. Based on convolutional neural networks and named Convnet Line-fitting Of Emission-line Regions (CLOVER), the method is markedly faster than traditional analysis techniques, requires no input assumptions about the emission, and has demonstrated high classification accuracy. Since high-mass star-forming regions are often plagued by multiple velocity components along the line of sight, CLOVER will improve the accuracy of stability measurements for many clouds of interest to the star formation community. / Graduate

Gas phase molecular clusters present an ideal medium for observing factors that drive chemical reactions without outside interferences from excessive solvent molecules. Introducing an ion into the cluster promotes ion-molecule interactions that may manifest in a variety of non-covalent or even covalent binding motifs and are of significant importance in many fields including atmospheric and astronomical sciences. For instance, in outer space, molecules are subject to ionizing radiation where ion-molecule reactions become increasingly competitive to molecule-molecule interactions. To elucidate individual ion-molecule interaction information, mass spectrometry was used in conjunction with appropriate theoretical calculations. Three main categories of experiment were conducted in this dissertation. The first of which were thermochemical equilibrium measurements where an ion was introduced to an ion mobility drift cell wherein thermalizing collisions occur with helium buffer gas facilitating a reversible reaction with a neutral molecule allowing the standard changes in enthalpy and entropy to be determined. The second type of experiment was an ion mobility experiment where an ionized homo- or hetero-cluster was injected into the drift cell at specific conditions allowing the reduced mobility and collisional cross-section to be evaluated. Thirdly, kinetics measurements were taken following injection of an ion into the drift cell were an irreversible reaction ensued with the neutral species hindering equilibrium, but prompting rate constant assessment. Previous research has laid the groundwork for this dissertation as the results and discussion contained herein will build upon existing data while maintaining originality. For example, past work has given support for ion-molecule reactions involving precursor species such as acetylene and hydrogen cyanide to form more complex organics, perhaps leading to biologically relevant species. The chemical systems studied for this research are either ionized substituted benzenes like phenylacetylene and benzonitrile or polycyclic aromatic nitrogen-containing hydrocarbons like quinoline and quinoxaline interacting with a variety of neutral species. Hydrogen bonding and its many sub-sections are of the utmost importance to the kinds of reactions studied here. Past work has shown the tendency of organic radical cations to form conventional and unconventional ionic hydrogen bonds with gas phase solvents. Other non-covalent modes of interaction have also been detected in addition to the formation of covalently bound species. Gas phase reactions studied here will explore, via mass-selected ion mobility, reversible and irreversible reactions leading to binding enthalpy and entropy and rate constant determination, respectively, in addition to collisional cross-section determination.

L’évolution chimique des phases les plus diffuses aux plus denses du milieu interstellaireest un processus continu : la composition chimique du milieu interstellairedans une phase dépend de sa composition dans sa phase antérieure.Les études, qui s’intéressent à la chimie du milieu dense et froid ainsi qu’à l’évolutionde sa composition au cours du temps, font de fortes hypothèses sur son évolutiondepuis le milieu diffus.L’objectif de ma thèse a donc été de suivre l’évolution de la chimie de la matièreinterstellaire du milieu diffus jusqu’à la formation des nuages denses.J’ai pour cela utilisé un modèle de chimie gaz-grain dépendant du temps que j’aisignificativement contribué à améliorer pour la partie chimie de surface. J’ai dansun premier temps suivi une approche "classique" (c.-à-d. : semblable aux études préexistantes)de la modélisation des régions froides. Cela m’a permis d’étudier en détailles processus physiques et chimiques à l’origine de la complexité moléculaire dans lesnuages denses et froids et de comparer les prédictions du modèle avec les études existantes.Dans une deuxième partie, j’ai appliqué ce modèle pour suivre l’évolution de lacomposition chimique du milieu interstellaire au cours du processus de formation desnuages moléculaires à partir du milieu diffus. Pour cette étude, j’ai utilisé les résultatsd’une simulation hydrodynamique à l’échelle galactique. Cela m’a permis de montrerque l’histoire de l’évolution des conditions physiques dans les phases antérieures à laformation des nuages moléculaires peut avoir un impact significatif sur la compositionchimique de ces derniers. / The chemical evolution from the most diffuse parts of the interstellar medium tothe formation of dense clouds is a continuous process : the chemical composition inone phase depends on the chemical composition in the previous one.However, most studies of the time dependent chemistry in the cold and dense interstellarmedium make strong assumptions on the transition between diffuse and densemedium.The goal of my thesis was to study the chemical evolution of the interstellar mediumfrom the most diffuse parts to the formation of dense clouds in a continuousway.To do so, I used a time dependent gas-grain model that I significantly contributedto improve for the treatment of the surface chemistry. In a first part, I followed a "classical"approach (i.e. : similar to most of the pre-existing studies) to model cold denseclouds. This allowed me to study in details the physical and the chemical mechanismsresponsible for the chemical complexity of dense clouds and to compare the modelpredictions with the existing literature. In a second part, I applied this model to followthe evolution of the chemical composition during the formation process of denseclouds from the diffuse medium. I used results from an hydrodynamical simulation ofthe interstellar medium at galactic scales. This study allowed me to show that the pastphysical history of each particles that form the dense clouds have a significant impacton their chemical composition.

This dissertation describes the current version of GenASiS and reports recent progress in its development. GenASiS is a new computational astrophysics code built for large-scale and multi-dimensional computer simulations of astrophysical phenomena, with primary emphasis on the simulations of neutron star mergers and core-collapse supernovae. Neutron star mergers are of high interest to the astrophysics community because they should be the prodigious source of gravitation waves and the most promising candidates for gravitational wave detection. Neutron star mergers are also thought to be associated with the production of short-duration, hard-spectral gamma-ray bursts, though the mechanism is not well understood. In contrast, core-collapse supernovae with massive progenitors are associated with long-duration, soft-spectral gamma-ray bursts, with the `collapsar' hypothesis as the favored mechanism. Of equal interest is the mechanism of core-collapse supernovae themselves, which has been in the forefront of many research efforts for the better half of a century but remains a partially-solved mystery. In addition supernovae, and possibly neutron star mergers, are thought to be sites for the emph{r}-process nucleosynthesis responsible for producing many of the heavy elements. Until we have a proper understanding of these events, we will have only a limited understanding of the origin of the elements. These questions provide some of the scientific motivations and guidelines for the development of GenASiS. In this document the equations and numerical scheme for Newtonian and relativistic magnetohydrodynamics are presented. A new FFT-based parallel solver for Poisson's equation in GenASiS are described. Adaptive mesh refinement in GenASiS, and a novel way to solve Poisson's equation on a mesh with refinement based on a multigrid algorithm, are also presented. Following these descriptions, results of simulations of neutron star mergers with GenASiS such as their evolution and the gravitational wave signals and spectra that they generate are shown. In the context of core-collapse supernovae, we explore the capacity of the stationary shock instability to generate magnetic fields starting from a weak, stationary, and radial magnetic field in an initially spherically symmetric fluid configuration that models the stalled shock in the post-bounce supernova environment. Our results show that the magnetic energy can be amplified by almost 4 orders of magnitude. The amplification mechanisms for the magnetic fields are then explained.

<p> Stars are born in turbulent molecular clouds that fragment and collapse under the influence of their own gravity, forming a cluster of hundred or more stars. The star formation process is controlled by the interplay between supersonic turbulence and gravity. In this work, the properties of stellar clusters created by numerical simulations of gravoturbulent fragmentation are compared to those from observations. This includes the analysis of properties of individual protostars as well as statistical properties of the entire cluster. </p> <p> It is demonstrated that protostellar mass accretion is a highly dynamical and time-variant process. The peak accretion rate is reached shortly after the formation of the protostellar core. It is about one order of magnitude higher than the constant accretion rate predicted by the collapse of a classical singular isothermal sphere, in agreement with the observations. </p> <p> For a more reasonable comparison, the model accretion rates are converted to the observables bolometric temperature, bolometric luminosity, and envelope mass. The accretion rates from the simulations are used as input for an evolutionary scheme. The resulting distribution in the T_bol-L_bol-M_env parameter space is then compared to observational data by means of a 3D Kolmogorov-Smirnov test. The highest probability found that the distributions of model tracks and observational data points are drawn from the same population is 70%. </p> <p> The ratios of objects belonging to different evolutionary classes in observed star-forming clusters are compared to the temporal evolution of the gravoturbulent models in order to estimate the evolutionary stage of a cluster. While it is difficult to estimate absolute ages, the realtive numbers of young stars reveal the evolutionary status of a cluster with respect to other clusters. The sequence shows Serpens as the youngest and IC 348 as the most evolved of the investigated clusters. </p> <p> Finally the structures of young star clusters are investigated by applying different statistical methods like the normalised mean correlation length and the minimum spanning tree technique and by a newly defined measure for the cluster elongation. The clustering parameters of the model clusters correspond in many cases well to those from observed ones. The temporal evolution of the clustering parameters shows that the star cluster builds up from several subclusters and evolves to a more centrally concentrated cluster, while the cluster expands slower than new stars are formed. </p> / <p>Sterne entstehen im Inneren von turbulenten Molekülwolken, die unter dem Einfluss ihrer eigenen Gravitation fragmentieren und kollabieren. So entsteht ein Sternhaufen aus hundert oder mehr Objekten. Der Sternentstehungsprozess wird durch das Wechselspiel von Überschallturbulenz und Gravitation reguliert. In dieser Arbeit werden verschiedene Eigenschaften solcher Sternhaufen, die mit Hilfe von numerischen Simulationen modelliert wurden, untersucht und mit Beobachtungsdaten verglichen. Dabei handelt es sich sowohl um Eigenschaften einzelner Protosterne, als auch um statistische Parameter des Sternhaufens als Ganzes.</p> <p>Es wird gezeigt, dass die Massenakkretion von Protosternen ein höchst dynamischer und zeitabhängiger Prozess ist. Die maximale Akkretionsrate wird kurz nach der Bildung des Protosterns erreicht, bevor sie annähernd exponentiell abfällt. Sie ist, in Übereinstimmung mit Beobachtungen, etwa um eine Größenordnung höher als die konstante Rate in den klassischen Modellen.</p> <p>Um die Akkretionsraten der Modelle zuverlässiger vergleichen zu können, werden sie mit Hilfe eines Evolutionsschemas in besser beobachtbare Parameter wie bolometrische Temperatur und Leuchtkraft sowie Hüllenmasse umgewandelt. Die dreidimensionale Verteilung dieser Parameter wird anschließend mittels eines Kolmogorov-Smirnov-Tests mit Beobachtungsdaten verglichen.</p> <p>Die relative Anzahl junger Sterne in verschiedenen Entwicklungsstadien wird mit der zeitlichen Entwicklung der Modelle verglichen, um so den Entwicklungsstand des Sternhaufens abschätzen zu können. Während eine genaue Altersbestimmung schwierig ist, kann der Entwicklungsstand eines Haufens relativ zu anderen gut ermittelt werden. Von den untersuchten Objekten stellt sich Serpens als der jüngste und IC 348 als der am weitesten entwickelte Sternhaufen heraus.</p> <p>Zuletzt werden die Strukturen von jungen Sternhaufen an Hand verschiedener statistischer Methoden und eines neuen Maßes für die Elongation eines Haufens untersucht. Auch hier zeigen die Parameter der Modelle eine gute Übereinstimmung mit solchen von beobachteten Objekten, insbesondere, wenn beide eine ähnliche Elongation aufweisen. Die zeitliche Entwicklung der Parameter zeigt, dass sich ein Sternhaufen aus mehreren kleineren Gruppen bildet, die zusammenwachsen und einen zum Zentrum hin konzentrierten Haufen bilden. Dabei werden neue Sterne schneller gebildet als sich der Sternhaufen ausdehnt.</p>