斯隆数字巡天中冷白矮星的详细模式大气分析
A Detailed Model Atmosphere Analysis of Cool White Dwarfs in the Sloan Digital Sky Survey
摘要：
本文介绍了斯隆数字巡天（SDSS）中126颗冷白矮星的光谱学和近红外测光。我们的样本包括使用SDSS和USNO-B天体测量选择的高自行目标和十几个以前已知的超冷白矮星候选者。我们的光谱观测表明，在SDSS（以及SkyMapper、Pan-STARRS和大型综合巡天望远镜数据集）中，使用简化的自行图和30 km/s的切向速度截止值（取决于自行精度），可以干净地选择大样本的冷白矮星。我们的近红外观测揭示了八颗具有显著吸收的新星。我们使用光学和近红外测光进行详细的模式大气分析。在我们的样本中，超过80%的恒星都与纯氢或纯氦大气相一致。然而，八颗具有显著红外吸收的恒星和大多数先前已知的超冷白矮星候选者最好用氢和氦的混合大气模型来解释。我们样本的年龄分布与银河系盘年龄8 Gyr一致。一些超冷的白矮星可能有12-13 Gyr那么老，但我们的模型在匹配这些天体的光谱能量分布方面存在问题。在我们的样本中只有两个晕白矮星候选者。然而，三角视差观测需要准确的质量和年龄的测定，并确认其成员的光环。
英文摘要：
We present optical spectroscopy and near-infrared photometry of 126 cool white dwarfs in the Sloan Digital Sky Survey (SDSS). Our sample includes high proper motion targets selected using the SDSS and USNO-B astrometry and a dozen previously known ultracool white dwarf candidates. Our optical spectroscopic observations demonstrate that a clean selection of large samples of cool white dwarfs in the SDSS (and the SkyMapper, Pan-STARRS, and the Large Synoptic Survey Telescope datasets) is possible using a reduced proper motion diagram and a tangential velocity cut-off (depending on the proper motion accuracy) of 30 km/s. Our near-infrared observations reveal eight new stars with significant absorption. We use the optical and near-infrared photometry to perform a detailed model atmosphere analysis. More than 80% of the stars in our sample are consistent with either pure hydrogen or pure helium atmospheres. However, the eight stars with significant infrared absorption and the majority of the previously known ultracool white dwarf candidates are best explained with mixed hydrogen and helium atmosphere models. The age distribution of our sample is consistent with a Galactic disk age of 8 Gyr. A few ultracool white dwarfs may be as old as 12-13 Gyr, but our models have problems matching the spectral energy distributions of these objects. There are only two halo white dwarf candidates in our sample. However, trigonometric parallax observations are required for accurate mass and age determinations and to confirm their membership in the halo.
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