It is uncertain whether or not low-mass Population III stars ever existed. While limits on the number density of Population III stars with M _* ≈ 0.8 M _⊙ have been derived, using Sloan Digital Sky Survey (SDSS) data, little is known about the occurrence of Population III stars at lower masses. In the absence of reliable parallaxes, the spectra of metal-poor main-sequence (MPMS) stars with M _* ≲ 0.8 M _⊙ can easily be confused with those of cool white dwarfs. To resolve this ambiguity, in this paper we present a classifier that differentiates between MPMS stars and white dwarfs, based on photometry and/or spectroscopy without the use of parallax information. We build and train our classifier using state-of-the-art theoretical spectra, and evaluate it on existing SDSS-based classifications for objects with reliable Gaia DR2 parallaxes. We then apply our classifier to a large catalog of objects with SDSS photometry and spectroscopy to search for MPMS candidates. We discover several previously unknown, extremely metal-poor (EMP) candidate stars, and recover numerous confirmed EMP stars already noted in the literature. We conclude that archival SDSS spectroscopy has already been exhaustively searched for EMP stars. We predict that the lowest-mass stars of primordial composition will have redder optical-to-infrared colors than cool white dwarfs at constant effective temperature, due to surface gravity-dependent collision-induced absorption from molecular hydrogen. We suggest that the application of our classifier to data produced by next-generation spectroscopic surveys will set stronger constraints on the number density of low-mass Population III stars in the Milky Way.

尚不确定低质量的第三族群恒星是否曾经存在过。虽然已经得出了M _* ≈ 0.8 M _⊙星族 III 星数量密度的限制，但使用斯隆数字巡天 (SDSS) 数据，人们对低质量星族 III 星的出现知之甚少。在缺乏可靠视差的情况下，M _* ≲ 0.8 M _⊙贫金属主序 (MPMS) 恒星的光谱很容易与冷白矮星混淆。为了解决这种歧义，在本文中，我们提出了一种分类器，该分类器基于光度测量和/或光谱学而不使用视差信息来区分 MPMS 恒星和白矮星。我们使用最先进的理论光谱构建和训练我们的分类器，并根据现有的基于 SDSS 的分类对具有可靠 Gaia DR2 视差的对象进行评估。然后，我们将分类器应用于具有 SDSS 光度测量和光谱学的大型对象目录，以搜索 MPMS 候选对象。我们发现了几颗以前未知的、极其贫金属 (EMP) 的候选恒星，并恢复了许多文献中已经提到的已确认的 EMP 恒星。我们得出结论，档案 SDSS 光谱已经被详尽地搜索了 EMP 恒星。我们预测，由于表面重力依赖的碰撞诱导分子氢吸收，原始组成的最低质量恒星在恒定有效温度下将比冷白矮星具有更红的光学到红外颜色。我们建议将我们的分类器应用于下一代光谱调查产生的数据，将对银河系中低质量第三族恒星的数量密度设置更强的限制。