Searching for Low-mass Population III Stars Disguised as White Dwarfs

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