The chemical signature of surviving Population III stars in the Milky Way

Jarrett L. Johnson

Published 2014-11-15Version 1

Cosmological simulations of Population (Pop) III star formation suggest that the primordial initial mass function may have extended to sub-solar masses. If Pop III stars with masses < 0.8 M_Sun did form, then they should still be present in the Galaxy today as either main sequence or red giant stars. To date, however, despite searches for metal-poor stars in both the halo and the bulge of the Milky Way, no primordial stars have been identified. It has long been recognized that the initial metal-free nature of primordial stars could be masked due to accretion of metal-enriched material from the interstellar medium (ISM) over the course of their long lifetimes. Here we point out that while gas accretion from the ISM may readily occur, the accretion of dust from the ISM can be prevented due to the pressure of the radiation emitted from low-mass stars. This implies a possible unique chemical signature for stars polluted only via accretion, namely an enhancement in gas phase elements relative to those in the dust phase. Using Pop III stellar models, we outline the conditions in which this signature could be exhibited, and we derive the expected signature for the case of accretion from the local ISM. Intriguingly, due to the large fraction of iron depleted into dust relative to that of carbon and other elements, this signature is similar to that observed in many of the so-called carbon-enhanced metal-poor (CEMP) stars. We therefore suggest that some fraction of the observed CEMP stars may, in fact, be accretion-polluted Pop III stars. We find, more broadly, that this effect could also be at play in accretion flows onto protostars, implying that it may also impact the chemical signatures of second generation (Pop II) stars.