标题： 银河分叉中具有大气碳痕迹的隐藏白矮星种群 显示英文标题

标题： A hidden population of white dwarfs with atmospheric carbon traces in the Gaia bifurcation

摘要： ESA盖亚空间任务在颜色-星等图上揭示了白矮星（WD）序列的分支现象，分为A分支和B分支。 虽然A分支主要由具有氢丰富大气的WD组成，但B分支尚未完全理解。 尽管被认为主要由氦丰富WD组成，但B分支与一个纯氦包层的$\sim 0.8M_\odot$演化轨迹重叠，这一事实意味着WD质量分布中出现了意想不到的峰值。 在冷氦丰富WD中，预计外层对流区会渗透到深层碳丰富层，从而在$\sim 10,000$K时导致其表面出现轻微的碳污染。在这里，我们旨在研究盖亚分支作为冷氦主导WD中对流引起的碳上涌的自然结果。 依靠精确的大气模型，我们提供了用于氦丰富WD的新一组演化模型，采用了不同的碳富集方案。 基于这些模型，我们对盖亚100pc WD样本进行了种群合成研究，以确定最符合分支的模型。 我们的研究表明，具有低于光学检测限的轻微碳污染的氦丰富WD模型可以准确再现盖亚分支。 我们将这些恒星称为隐匿DQ WD，因为它们在光学光谱中不表现出可检测的碳特征，但其大气中存在碳会产生连续吸收，有利于蓝波长的发射，从而形成分支的B分支。 此外，我们还表明，当考虑隐匿碳污染时获得的氦丰富WD质量分布与氢丰富WD的质量分布以及它们形成的典型演化路径是一致的。 我们得出结论，隐匿DQ WD可以解释盖亚分支的下分支。 这些恒星的碳特征可能在紫外光谱中可被检测到。 显示英文摘要

摘要： The ESA Gaia space mission has revealed a bifurcation of the white dwarf (WD) sequence on the color magnitude diagram in two branches: A and B. While the A branch consists mostly of WDs with H-rich atmospheres, the B branch is not completely understood. Although invoked to be populated mainly by He-rich WDs, the B branch overlaps a $\sim 0.8M_\odot$ evolutionary track with a pure He envelope, fact that would imply an unexpected peak in the WD mass distribution. In cold He-rich WDs, it is expected that the outer convective zone penetrates into deep C-rich layers, thus leading to a slight C contamination in their surfaces at $\sim 10,000$K. Here we aim at studying the Gaia bifurcation as the natural consequence of C dredge-up by convection in cold He-dominated WDs. Relying on accurate atmosphere models, we provide a new set of evolutionary models for He-rich WDs employing different prescriptions for the C enrichment. On the basis of these models, we made a population synthesis study of the Gaia 100pc WD sample to constrain the models that best fit the bifurcation. Our study shows that He-rich WD models with a slight C contamination below the optical detection limit can accurately reproduce the Gaia bifurcation. We refer to these stars as stealth DQ WDs because they do not exhibit detectable C signatures in their optical spectra, but the presence of C in their atmosphere produces a continuum absorption favouring the emission in bluer wavelengths, thereby creating the B branch of the bifurcation. Also, we show that the mass distribution for He-rich WDs obtained when a stealth C contamination is considered is consistent with the mass distribution for H-rich WDs and with the standard evolutionary channels for their formation. We conclude that stealth DQ WDs can account for the lower branch in the Gaia bifurcation. The C signatures of these stars could be detectable in Ultra-Violet spectra.