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

TL;DR

This paper proposes a unique chemical signature for surviving Population III stars polluted only by gas accretion, which could explain the observed properties of some carbon-enhanced metal-poor stars and aid in their identification.

Contribution

It introduces a novel chemical signature based on dust and gas phase element ratios to identify primordial stars that have only been polluted by accretion.

Findings

The chemical signature involves enhanced gas phase elements relative to dust phase.

This signature resembles the composition of some observed CEMP stars.

Some CEMP stars may be accretion-polluted Population III stars.

Abstract

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. Despite broad searches, however, no primordial stars have yet 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). 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.