In this thesis, I focus on the physical and chemical evolution at the earliest stages of low-mass star formation. I report results from the Spitzer Space Telescope and molecular line observations of 9 species toward the dark cloud L43, a survey of 10 Class 0 and 6 Class I protostars with 8 molecular lines, and a survey of 9 Very Low Luminosity Objects (VeLLOs) with 11 molecular lines. From the observational results, CO depletion is extensively observed with C¹⁸O(2-1) maps. A general evolutionary trend is also seen toward the Class 0 and I samples: higher deuterium fractionation at higher CO depletion. For the VeLLO candidates and starless cores with N₂D⁺(3-2) detection, we found the deuterium ratio of N₂D⁺/N₂H⁺ is higher comparing with the Class 0 and I samples. We use DCO⁺(3-2) maps to trace the velocity structures. Also, HCO⁺(3-2) blue profiles are seen toward the VeLLO candidate L328, indicating possible infall. To test theoretical models and to interpret the observations, we adopt a modeling sequence with self-consistent calculations of dust radiative transfer, gas energetics, chemistry, and line radiative transfer. In the L43 region described in Chapter 2, a starless core and a Class I protostar are evolving in the same environment. We modeled both sources with the same initial conditions to test the chemical characteristics with and without protostellar heating. The physical model consists of a series of Bonner-Ebert spheres describing the pre-protostellar (PPC) stages following by standard inside-out collapse (Shu 1977). The model best matches the observed lines suggests a longer total timescale at the PPC stage, with faster evolution at the later steps with higher densities. In Chapter 3, we modeled the entire group of Class 0 and I protostars. The trend of decreasing deuterium ratio can be seen after the temperature is high enough for CO to evaporate. After the evaporation, the history of heavy depletion (e.g, from longer PPC timescales or different grain surface properties) no longer affects the line intensities of gas-phase CO. The HCO⁺ blue profiles, which are used as infall indicators, are predicted to be observed when infall is beyond the CO evaporation front. The low luminosity of VeLLOs cannot be explained by standard models with steady accretion, and we tested an evolutionary model incorporating episodic accretion to investigate the thermal history and chemical behaviors. We tested a few chemical parameters to compare with the observations and the results from Chapter 2 and 3. The modeling results from episodic accretion models show that CO and N₂ evaporate from grain mantle surfaces at the accretion bursts and can freeze back onto grain surfaces during the long periods of quiescent phases. Deuterated species, such as N₂D⁺ and H₂D⁺, are most sensitive to the temperature. Possible good tracers for the thermal history include the line intensities of gas-phase N₂H+ relative to CO, as well as CO₂ and CO ice features. / text

Comprendre le processus de formation stellaire est un objectif majeur en astronomie. Sur ce sujet les observations ne donnent que très peu d'information, et les modèles numériques sont donc naturellement privilégiés. De tels modèles s'attachent à suivre la dynamique du gaz, sous l'effet de processus physique variés, ce qui nécessite un temps de calcul très important et ne permet pas de modéliser l'évolution au delà de 0.2 Myr environ. Or les résultats observationnels sont essentiellement issus du champ galactique proche, des amas évolués, voire des regions jeunes ou associations d'étoiles, dont l'âge peut varier de 1 Myr à quelques Gyr. Par conséquent, il est nécessaire pour comparer les résultats des modèles aux observations de comprendre ce qu'il se passe durant cet intervalle de temps. La formation stellaire tend à produire des étoiles en groupes, à partir de l'effondrement gravitationnel d'un nuage moléculaire turbulent. A mesure que les étoiles se forment, le gaz est éjecté et l'évolution est dominée par les interactions gravitationnelles. Suivre l'évolution sous l'effet de ces interactions est couramment utilisé afin de contraindre les modèles et de mieux comprendre l'origine des populations stellaires observées. Les étoiles se forment en sous-groupes ou structures hiérarchisées, qui peuvent ensuite fusionner pour donner des amas stellaires proche des amas ouverts, ou au contraire finir en associations distinctes. Dans ma thèse, je me suis intéressé à l'évolution dynamique de petits groupes d'étoiles, jusqu'alors peu étudiés par rapport aux groupes à 1000 ou 10^4 étoiles. J'ai simulé l'évolution de groupes à N < 100, dans le but d'en étudier la dynamique d'un point de vue statistique, grâce notamment au grand nombre de simulations effectuées, et afin d'identifier les signatures observationnelles propres à une situation initiale donnée. A partir d'un grand nombre de configurations initiales (avec N=20, 50, 100, un rayon typique de 0.025 pc à 1 pc) et 500 simulations par configurations, j'ai étudié l'évolution dynamique de groupes composés d'étoiles de même masse ou comprenant un spectre de masse, et sans population de binaire initiale. L'évolution de tels groupes s'est révélée similaire à celle de groupes plus grands, mais avec une phase d'effondrement plus rapide et surtout moins prononcée. Je décris le comportement moyen menant à une lente expansion de l'amas, ainsi qu'une voie d'évolution très différente, apparaissant dans 17% des cas étudiés, où l'amas est complètement dispersé suite à l'éjection d'une binaire centrale serrée. J'ai également recherché dans quelle mesure les données en densité et en vitesse 3D pouvaient permettre d'identifier l'état dynamique initial d'un groupe. L'utilisation de ces seules données suffisait dans certain cas à déterminer la densité initiale, mais elles devraient être complétées par des données concernant la population de binaire. Ce travail pourra être mis en application pour étudier l'origine dynamique d'association ou de groupes stellaires connus. Enfin, j'ai effectué un grand nombre de simulations numériques dans le but de reproduire l'état observé de l'amas eta Chamaeleontis par pure évolution dynamique à partir de conditions initiales standards. Cette association présente des caractéristiques d'amas évolué, telle que son spectre de masse pauvre en objets de faible masse et l'absence de binaires larges. Je montre que ces propriétés ne peuvent pas être reproduites uniquement par la dynamique, et sont donc les traces d'un processus de formation non standard. / Understanding the star formation process is a key issue in astronomy. Since direct observation provide only very limited information, this issue is investigated by models. Such models need to take into account complex physical processes while following the gas dynamics, so that simulations need a lot of time to run and do not follow the star formation process for longer than 0.2 Myr. The best known observational results concerns the field population, evolved open clusters or younger clusters or associations, which are between 1 Myr and a few Gyr old. Therefore in order to compare the results from models to known observations, we need to bridge the gap between the two. Star formation appears to produce groups of stars from the collapse of turbulent molecular clouds. As stars form, the gas is progressively ejected from the cluster, and the evolution is dominated by gravitational interactions. Following the dynamical evolution of a group of star using N-Body codes is a standard way used to constraint the models and understand the origin of the different populations. Star formation may produce sub-structure or small groups that merge to form bigger entities, or end up as loose association. In my thesis I focused on the dynamics of small groups, that have not been investigated as thoroughly as 1000 or 10^4 star groups. I performed N-Body simulations of small stellar groups, with N<100, in order to study their dynamics using a statistical approach, made possible by running a large number of simulations, and to find some observational signatures of given initial conditions. This approach enable to take full account of stochastic effects due to dynamical interactions. Using a large number of initial configurations (with N=20, 50, 100, a typical radius from 0.025 pc to 1 pc) and a sample of 500 simulations per configuration, I looked at equal mass groups as well as groups having a mass spectrum, without any binary initially. Such small groups show similar evolution to bigger groups, but with faster and less pronounced collapse phase. I described the average behaviour of slow expansion of the cluster, and an alternative evolution, occurring with 17% probability, that ended in the complete dissolution of the group due to ejection of a central binary. Searching for a way to identify the initial configuration from observational measure, I looked at the complementarity of density and 3D velocity and was able to show that it could be sufficient in some cases to determine the initial density. Further investigations are needed to take into account the information on the binary population and will be used to investigate the formation of known associations or young regions. Finally, I ran a large number of simulations, aiming at reproducing the observed state of the eta Chamaeleontis from standard initial conditions and pure dynamical evolution. This association properties are consistent with a dynamical evolved cluster, namely low-mass object poor and having only tight binaries. I showed that these properties cannot be reproduced with pure dynamical evolution from standard initial mass function and binary population, meaning that its particular features must have been pristine.

<p> In this thesis we investigate the dynamical effect of a second generation of stellar formation in globular clusters in the context of the anomalous horizontal branch of NGC 2808. The horizontal branch of NGC 2808 is bifurcated in colour and exhibits an extended blue tail. This morphology can be explained if the blue tail stars have an enhanced helium content due to cluster self-enrichment. Specifically it has been proposed that NGC 2808 has experienced two distinct generations of star formation. The first generation has a top-heavy IMF, enhanced in 3 - 5Mo stars, and would produce many AGB stars within the first 200 Myrs of its life. The second generation then forms out of the helium-rich ejecta of the AGB stars and goes on to produce the blue tail in the horizontal branch that is currently observed in NGC 2808.</p> <p> We use three types of simulations to investigate this scenario. For a control model we run a simulation with a Salpeter IMF and a single generation. We then run models with a top-heavy IMF and a single generation and models with a top-heavy IMF and two generations. In the two generation models we also investigate the effect of concentration by examining simulations with two different length scales.</p> <p> We find that the models with the top-heavy IMF and a single generation are subject to extensive mass-loss in their early phases due to the large number of intermediate-mass stars and are less strongly affected by two-body relaxation than simulations with a Salpeter IMF. The models with two generations appear to be dynamically stable and long-lived objects, at least in their early stages. They seem to be observationally indistinguishable from single-generation clusters with Salpeter IMFs on the basis of their dynamics. The stellar populations of the two-generation clusters are found to have a much higher fraction of C-O white dwarfs than clusters with a Salpeter IMF. We find no evidence that these bodies will be preferentially scattered out of the system and they should remain part of the cluster until it dissolves after core collapse. The abundance of white dwarfs would provide an observational method of identifying two generation cluster candidates.</p> <p> Overall we find the two-generation scenario to be plausible on the basis of dynamics but due to the overabundance of white dwarfs produced by the top-heavy IMF and based on other studies of the chemistry of AGB stars we conclude that this scenario is unlikely to be the sole explanation for globular cluster self-enrichment.</p> / Thesis / Master of Science (MSc)

La formation des étoiles massives reste, à ce jour, encore mal connue à cause de l’extrême quantité d’énergie que ces étoiles dégagent, limitant en conséquence leurs masses théoriques et contredisant les observations de ce type d’étoile. Les observatoires du futur (en particulier l’observatoire spatial Herschel) vont tenter de répondre à cette problématique grâce notamment aux émissions moléculaires de l’eau. L’analyse précise et correcte de ces données, dans l’avenir, nécessite donc dès aujourd’hui un travail associant des observations et des modélisations des objets concernés. C’est dans ce but que cette thèse a consisté en l’élaboration d’une méthode de modélisation dite « globale » d’objets protostellaires massifs (proto-amas ou cœurs denses massifs). Celle-ci a permis une description physique et une étude chimique des multiples cœurs denses massifs étudiées, et a ouvert de nombreuses voies vers des aspects évolutifs. Elle a également donné des indices pour a?ner le programme d’observation en temps garanti WISH des raies moléculaires de l’eau et con?rmé le rôle clef de cette molécule pour la compréhension de la formation des étoiles massives. / Today the formation of massive stars is still not well understood due to the huge interac- tion of these objects with their environment, leading to a theoretical limit in the ?nal mass that observations contradict. The future observatories, like the Herschel Space Observatory, will try to answer some of the questions linked to this topic, particularly through the water line emissions. The correct and precise analysis of the future data is then necessary and needs a full work linking the observations and the modelling of the objects that will be studied. Hence the main goal of this PhD Thesis was to elaborate a robust and global modeling method of the massive dense cores in which high-mass stars are forming. The method leaded to a physical description and a chemical study of multiple massive dense cores, opening new views on evolution aspects. In addition it gave some tweaks on the guaranteed-time key program WISH for the water line emissions and con?rmed the key role of this molecule for a better understanding of the high-mass star formation.