Title: White dwarfs with hydrogen-deficient atmospheres and the dark matter content of the Galaxy Show Chinese title

Title: 缺乏氢的大气白矮星和银河系的暗物质含量

Abstract: The nature of the several microlensing events observed by the MACHO team towards the Large Magellanic Cloud (LMC) is still a subject of debate. Low-mass substellar objects and stars with masses larger than ~M_{sun} have been ruled out as major components of a Massive Astrophysical Compact Halo Object (MACHO) Galactic halo, while stars of half a solar mass seem to be viable candidates. Main sequence stars have been already discarded, and there are tight restrictions on the role played by white dwarfs with hydrogen-dominated atmospheres. In this paper we evaluate the contribution to the dark matter content of the Galaxy of white dwarfs with hydrogen-deficient atmospheres. For this purpose we use a Monte Carlo simulator which incorporates up-to-date evolutionary sequences of white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres. We also take into account detailed descriptions of the thick disk and the halo of our Galaxy as well as of a reliable model of the LMC. We find that the contribution of white dwarfs with hydrogen-deficient atmospheres moderately increases the theoretical estimate of the optical depth with respect to the value obtained when only hydrogen-rich white dwarfs are considered. We also find that the contribuiton of the thick disk population of white dwarfs is comparable to the halo contribution. However, the contributions of both the halo and the thick disk white-dwarf populations are still insufficient to explain the number of events observed by the MACHO team. Finally, we find that the contribution to the halo dark matter of the entire population under study is less than 10% at the 95% conficence level. Show Chinese abstract

Abstract: 几种由MACHO团队观测到的微透镜事件的性质仍然是争论的焦点。 已经排除了低质量的亚恒星天体和质量大于~M_{太阳}的恒星作为银河系晕中大质量天体（MACHO）的主要组成部分，而质量为太阳一半的恒星似乎成为可行的候选者。 主序星已经被排除，而氢主导大气层的白矮星在其中的作用受到严格限制。 在本文中，我们评估了氢缺乏大气层的白矮星对银河系暗物质含量的贡献。 为此，我们使用了一个蒙特卡罗模拟器，该模拟器结合了最新版本的氢丰富和氢缺乏大气层的白矮星演化序列。 我们还考虑了银河系厚盘和晕的详细描述以及一个可靠的LMC模型。 我们发现，与仅考虑氢丰富白矮星时得到的值相比，氢缺乏大气层的白矮星的贡献适度增加了理论上的光学深度。 我们还发现，厚盘白矮星群体的贡献与晕的贡献相当。 然而，晕和厚盘白矮星群体的贡献仍然不足以解释MACHO团队观测到的事件数量。 最后，我们发现，在95%的置信水平下，所研究整个群体对晕暗物质的贡献小于10%。