Monte Carlo simulations of the population of single and binary white dwarfs of our galaxy

Abstract

Since white dwarfs are the final stage of the evolution of the vast majority of stars, they carry important information about the chemical evolution of our Galaxy, its star formation rate, and its structure and dynamics. This thesis pays attention to two related but distinct astrophysical problems involving white dwarfs. The first of these problems concern the nature and the location of the microlensing events towards the Large Magellanic Clouds (LMC), which still remains a mystery. The main observational groups, MACHO and EROS, are in dispute each, yet agreement has now been reached in some of the most important points. The second of the problems we address in this thesis is an open problem as well. Close compact binaries are at the heart of several interesting phenomena in our Galaxy as well. Close compact binaries are formed through at least one common envelope episode. Even though the basics concepts of the evolution during a common envelope phase are rather simple, the details are still far from being well understood. To shed light on these problems, we used an existing Monte Carlo simulator. We expanded this simulator integrating the most up-to-date white dwarfs cooling models as well as detailed modeling of our Galaxy and the LMC in order to mimic both the MACHO and EROS experiments. Additionally, we included the red dwarf population and performed a joint analysis of the contributions of both populations to the dark matter content of our Galaxy. Moreover, we studied the contribution of the subpopulation of white dwarfs with hydrogen-deficient atmospheres. On the other hand, our Monte Carlo code has been expanded to deal with those systems composed by a white dwarf and a main sequence star, which have evolved through a common episode. A detailed implementation of several different physical processes, including a full description of the mass transfer episode, a complete treatment of the Roche lobe overflow episode, gravitational tides and orbital evolution of the binary system, was performed. Furthermore, in our treatment we carefully included all the different selection criteria and observational biases. This allowed us to make a meaningful comparison with the available data, besides examining the role played by the binding energy parameter and by the common envelope parameter, not to mention the role played by the distribution of secondary masses of the binary systems. The results of our Monte Carlo simulations of the microlensing experiments show agreement with the findings of the EROS and MACHO survey. Our findings show that neither white dwarfs nor red dwarfs can be major contributors to the microlensing depth towards the LMC. These facts reinforce the idea, previously pointed by others studies, that the optical depth found by the MACHO survey is highly likely an overestimate, probably due to contamination of self-lensing objects, amid other possible explanations. Concerning the second point of this thesis, our Monte Carlo simulations correctly reproduce the properties of the observed population of post-common envelope white dwarf plus main sequence binaries, once biases are taken into account. The best-fit models are obtained with fractions less than ~20% of the internal energy contributing to the ejection of the star progenitor¿s envelope, and values for the common-envelope efficiency parameter less than ~0.3. To conclude, the work presented in this thesis poses an important step forward not only in constraining the microlensing discoveries, but also in validating models for the observed white-dwarf populations of our Galaxy.