During the initial phase of a program to search for sub -stellar candidates at high galactic latitudes in the IRAS Serendipitous Survey Catalog, one source, SSC 08546 +1732, was found to have no optical counterpart on the Palomar Observatory Sky Survey plates. Ground based positional, photometric and spectroscopic observations have identified this source as a heavily dust - enshrouded carbon star, similar to those found in the Galactic plane. The high latitude and relative faintness of this source imply that it lies at a distance of 20 - 50 kpc, and is 10 to 30 kpc out of the plane of the Galaxy. SSC 08546 +1732, along with one other distant obscured carbon star found in the IRAS Point Source Catalog (Low 1987; Beichman e1: al. 1988), represent the first examples of such objects found in the Galactic halo. These stars may either have evolved from population I precursors ejected from the disk, or they may be the first obscured Population II halo carbon stars to be observed. A survey for other distant enshrouded carbon stars in the IRAS Faint Source Catalog should help to elucidate the nature of this new population.

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In this study we present photometric predictions for C-type Asymptotic Giant Branch Stars (AGB) stars from Eriksson et al. (2014) for the James Webb Space Telescope (JWST) and the Wide-field Infrared Survey Explorer (WISE) instruments. The photometric predictions we have done are for JWST’s general purpose wide-band filters on NIRCam and MIRI covering wavelengths of 0.7 — 21 microns. AGB stars contribute substantially to the integrated light of intermediate-age stellar popula- tions and is a substantial source of the metals (especially carbon) in galaxies. Studies of AGB stars are (among other reasons) important for the understanding of the chemical evolution and dust cycle of galaxies. Since the JWST is scheduled for launch in 2018 it should be a high priority to prepare observing strategies. With these predictions we hope it will be possible to optimize observing strategies of AGB stars and maximize the science return of JWST. By testing our method on Whitelock et al. (2006) objects from the WISE catalog and comparing them with our photometric results based on Eriksson et al. (2014) we have been able to fit 20 objects with models. The photometric data set can be accessed at: http://www.astro.uu.se/AGBmodels/ / I den här studien har jag gjort fotometriska förutsägelser för asymptotis- ka jättegrensstjärnor (AGB-stjärnor) av C typ från Eriksson et al. (2014) modifierade för instrument ombord på James Webb Space Telescope (JWST) och Wide-field Infrared Survey Explorer (WISE). AGB-stjärnor bidrar kraftigt till det totala ljuset av stjärnor av intermediär ålder och är också en stor källa till metaller (speciellt kol) i galaxer. Studier av AGB stjärnor är viktiga av flera anledningar, däribland för att förstå den kemiska evolutionen och stoftcykler i galaxer. JWST är planerad att skjutas upp 2018 och fram till dess bör det vara en hög prioritet att förbereda observeringsstrategier. Med den fotometriska datan i den här studien hoppas vi att användare av JWST kommer kunna optimera sina observeringsstrategier av AGB-stjärnor och få ut så mycket som möjligt av sin obseravtionstid med teleskopet. Vi har testat metoden genom att titta på objekt från Whitelock et al. (2006) i WISE-katalogen och jämföra dem med de fotometriska resultaten baserade på modellerna från Eriksson et al. (2014). På detta sett har vi lyckats matcha 20 objekt med modeller. Den fotometriska datan går att ladda ner ifrån: http://www.astro.uu.se/AGBmodels/

Cette thèse nous a permis de présenter la bibliothèque spectrale x-shooter (xsl) et quelques aspects de la réduction de données. Nous avons concentré notre analyse sur un type spectral particulier, à savoir les étoiles carbonées. Jusqu'à présent, le faible nombre de bibliothèques stellaires contenant des étoiles carbonées ne permettait pas de reproduire leur diversité dans les modèles de populations stellaires. XSL nous permet de faire un pas en avant car cette collection accroit les précédentes et présente même une diversité. Une première comparaison avec des modèles hydrostatiques d’étoiles carbonées a été réalisée. La prochaine étape est de se tourner vers les modèles dynamiques qui prennent en compte les propriétés de pulsation. Pour le moment, il est mieux de moyenner nos spectres de carbonées plutôt que de les utiliser individuellement pour les applications liées a la synthèse de population. XSL regorge d’étoiles de type spectraux varies, et des analyses du même type que celle menée dans cette thèse devront être faites avant de cerner la formation et l'évolution des galaxies. / In this thesis, we have developed the x-shooter spectral library and some backstage process. We have focused our analysis on one spectral type, the carbon stars. Until now, the small number of carbon stars in stellar libraries prevented the reproduction of their diversity in stellar population synthesis. With XSL, we go a step further: this collection extends the previous ones and even shows diversity. A comparison with hydrostatic carbon-rich models was done as a first pass, and the next step is now to turn to dynamical models, which take into account the pulsation properties. For now, we advise users to average our c-star spectra instead of using individual ones for stellar population synthesis applications. The X-shooter spectral library is full of stars from various spectral types, and more analysis like that in this thesis need to be done, before reaching for the galaxies.

Dwarf carbon (dC) stars are main-sequence stars with carbon molecular bands (C_2, CN, CH) in their optical spectra. They are an important class of post-mass transfer binaries since, as main-sequence stars, dCs cannot have produced carbon themselves. Rather, the excess carbon originated in an evolved companion, now a white dwarf, and was transferred to the dC. Because of their complex histories, dCs are an excellent sample for testing stellar physics, including common-envelope evolution, wind accretion, mass transfer efficiencies, and accretion spin-up. However, their fundamental properties remain a mystery, and this impedes efforts to use dCs to constrain the evolution of binary systems. Here, I have investigated the observed properties of dCs, both as a population and as individual objects. Using multi-epoch spectroscopy, I constrained the dC binary fraction to be consistent with 100% binarity. The best-fit orbital separation distribution agrees with the few known dC orbital periods, and suggests a bimodal distribution (one sample with mean periods of hundreds of days, the other thousands of days). I also built a set of optical templates to find and classify additional dCs in spectroscopic surveys. Further, I discovered periodic variability in photometry of 34 dCs, dramatically increasing the number of measured periods. This allowed me to investigate mass transfer mechanisms that are likely to be important in the formation of dCs. Interestingly, some of these objects have short periods (P < 2d), indicating they have gone through a common-envelope phase. I explored the implications of these short-period dCs and how they will allow for constraints to be placed on the physics of common-envelope evolution. Finally, I searched for signs of spin-up and activity in dCs using X-ray emission. From this, I found that dCs are consistent with being rapid rotators, similar to what is observed in samples of normal young dwarfs. In summary, this dissertation presents the most extensive set of dC observational properties that has been compiled to date. I have confirmed the binary origin of dCs and linked some to post-common-envelope binaries. My work has provided a firmer foundation for the use of dCs to explore many essential astrophysical phenomena.

Carbon is the basis for life, as we know it, but its origin is still largely unclear. Carbon-rich Asymptotic Giant Branch (AGB) stars (carbon stars) play an important rôle in the cosmic matter cycle and may contribute most of the carbon in the Galaxy. In this thesis it is explored how the dust-driven mass loss of these stars depends on the basic stellar parameters by computing a large grid of wind models. The existence of a critical wind regime and mass-loss thresholds for dust-driven winds are confirmed. Furthermore, a steep dependence of mass loss on carbon excess is found. Exploratory work on the effects of different stellar metallicities and the sizes of dust grains shows that strong dust-driven winds develop also at moderately low metallicities, and that typical sizes of dust grains affect the wind properties near a mass-loss threshold. It is demonstrated that the mass-loss rates obtained with the wind models have dramatic consequences when used in models of carbon-star evolution. A pronounced superwind develops soon after the star becomes carbon rich, and it therefore experiences only a few thermal pulses as a carbon star before the envelope is lost. The number of dredge-up events and the thermal pulses is limited by a self-regulating mechanism: each thermal pulse dredges up carbon, which increases the carbon excess and hence also the mass-loss rate. In turn, this limits the number of thermal pulses. The mass-loss evolution during a thermal pulse (He-shell flash) is considered as an explanation of the observations of so-called detached shells around carbon stars. By combining models of dust-driven winds with a stellar evolution model, and a simple hydrodynamic model of the circumstellar envelope, it is shown that wind properties change character during a He-shell flash such that a thin detached gas shell can form by wind-wind interaction. Finally, it is suggested that carbon stars are responsible for much of the carbon in the interstellar medium, but a scenario where high-mass stars are major carbon producers cannot be excluded. In either case, however, the carbon abundances of the outer Galactic disc are relatively low, and most of the carbon has been released quite recently. Thus, there may neither be enough carbon, nor enough time, for more advanced carbon-based life to emerge in the outer Galaxy. This lends some support to the idea that only the mid-part of the Galactic disc can be a “Galactic habitable zone”, since the inner parts of the Galaxy are plagued by frequent supernova events that are presumably harmful to all forms of life.