Probing the formation of the first low-mass stars with stellar archaeology

Abstract

We investigate the conditions under which the first low-mass stars formed in the Universe by confronting theoretical predictions governing the transition from massive Population III to low-mass Population II stars with recent observational C and/or O abundance data of metal-poor Galactic stars. We introduce a new 'observer-friendly' function, the transition discriminant Dtrans, which provides empirical constraints as well as a powerful comparison between the currently available data of metal-poor halo stars and theoretical predictions of the formation of the first low-mass stars (≤ 1 M⊙). Specifically, we compare the empirical stellar results with the theory that fine-structure lines of C and O dominate the transition from Population III to Population II in the early Universe. We find the currently available data for halo stars as well as for dwarf spheroidal (dSph) galaxies and globular clusters to be consistent with this theory. An explanation for the observed lack of metal-poor stars in dSph galaxies and globular clusters is also suggested. Finally, we predict that any star to be found with [Fe/H] ≲ -4 should have enhanced C and/or O abundances. The high C and O abundances of the two most iron-poor stars are in line with our prediction. © 2007 The Authors. Journal compilation © 2007 RAS.