The Chemical Evolution of Cool White Dwarfs and the Age of the Local Galactic Disk
The Chemical Evolution of Cool White Dwarfs and the Age of the Local Galactic Disk
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冷白矮星的化学演化和当地银盘的年龄
DOI：
10.1086/312955
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发表时间：
1997
期刊：
影响因子：
--
通讯作者：
Sandy Leggett
中科院分区：
文献类型：
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作者：
Pierre Bergeron;M. Ruiz;Sandy Leggett
本文介绍了110颗冷白色矮星的光度和光谱观测结果，并利用适合于纯氢和纯氦以及混合氢/氦组成的冷白色矮星的最新模型大气计算进行了分析。高信噪比光谱揭示了Hα存在于20个以前被归类为DC星的白色矮星中，其中四个是磁性的。冷白色矮星在颜色-颜色和颜色-星等图中都显示出形成一个狭窄的序列，在这些图中几乎没有证据表明富氢和富氦成分之间的分离。所观察到的能量分布是从光学BVRI和红外JHK光度数据的组合，并用于获得有效温度和大气组成的每个星星。恒星的质量也得到了60个已知三角视差的白色矮星。描述了观测到的能量分布与模型大气预测的能量分布之间的一些差异。特别是，在最冷的富氢白色矮星的紫外线不透明度源的存在的证据被解释在伪连续不透明度起源于莱曼边缘。对光度和光谱观测的同时分析提供了冷白色矮星化学成分的详细图像。大多数冷白色矮星的能量分布都可以用纯氢或纯氦模型很好地再现，几乎没有证据表明物体具有混合的大气成分。我们发现了一类特殊的非DA星星，其Teff > 6000 K，其能量分布可以用纯氢模型很好地再现，但其光谱不显示Hα。我们的结果揭示了富氢和氦的白色矮星的不均匀温度分布，特别是在5000 ~ 6000 K范围内存在非DA能隙。冷白色矮星的化学演化与我们的研究结果进行了详细讨论。我们表明，没有已知的物理机制（例如，对流混合、对流疏浚、来自星际介质的吸积）可以解释富含氢和氦的白色矮星的温度分布。可能的新机制，可以解释我们的结果。我们提出了一种机制，通过这种机制，氢被吸积到富氦的白色矮星的表面，同时保持光谱不可见。支持我们的假设的观察证据进行了讨论。通过测定我们样本中最古老的白色矮星的年龄，得到了本地银河系盘年龄的下限。年龄在6.5-10 Gyr的范围内来自各种核心组成和氦包层质量的演化模型。
Photometric and spectroscopic observations of 110 cool white dwarf stars are presented and analyzed with state-of-the-art model atmosphere calculations appropriate for cool white dwarfs with pure hydrogen and pure helium, as well as mixed H/He compositions. High signal-to-noise spectroscopy reveals the presence of Hα in 20 white dwarfs previously classified as DC stars, four of which are magnetic. Cool white dwarfs are shown to form a narrow sequence in both color-color and color-magnitude diagrams, with little evidence for separation between hydrogen- and helium-rich compositions in these diagrams. The observed energy distributions are obtained from a combination of both optical BVRI and infrared JHK photometric data and used to derive both the effective temperature and the atmospheric composition of each star. Stellar masses are also obtained for 60 white dwarfs with known trigonometric parallaxes. Some discrepancies between the observed energy distributions and those predicted by the model atmospheres are described. In particular, evidence for the presence of a UV opacity source in the coolest hydrogen-rich white dwarfs is interpreted in terms of a pseudo continuum opacity originating from the Lyman edge. The simultaneous analysis of the photometric and spectroscopic observations provides a detailed picture of the chemical composition of cool white dwarfs. Most cool white dwarfs have energy distributions that are well reproduced by either pure hydrogen or pure helium models, with little evidence for objects with mixed atmospheric compositions. We identify a peculiar class of non-DA star with Teff > 6000 K whose energy distributions are well reproduced by pure hydrogen models but whose spectra do not show Hα. Our results reveal an inhomogeneous temperature distribution of hydrogen- and helium-rich white dwarfs, and in particular the presence of a non-DA gap in the range 5000 ≲ Teff ≲ 6000 K. The chemical evolution of cool white dwarfs is discussed at length with respect to our findings. We show that no known physical mechanisms (e.g., convective mixing, convective dredge-up, accretion from the interstellar medium) can account for the temperature distribution of hydrogen- and helium-rich white dwarfs. Possible new mechanisms that could explain our results are presented. We propose a mechanism by which hydrogen is accreted onto the surface of helium-rich white dwarfs while remaining spectroscopically invisible. Observational evidence that supports our hypothesis is discussed. Lower limits for the age of the local Galactic disk are obtained by determining the age of the oldest white dwarfs in our sample. Ages in the range 6.5-10 Gyr are derived from evolutionary models with various core compositions and helium envelope masses.