The extremely low metallicity star SDSS J102915+172927: a subgiant scenario

TL;DR

This paper analyzes the extremely low metallicity star SDSS J102915+172927, proposing it is a subgiant formed with higher initial metallicity, with low observed metals resulting from stellar evolution processes, challenging previous assumptions about such stars.

Contribution

It introduces a subgiant formation scenario for the star, explaining its low metallicity and lithium abundance through stellar evolution and supernova enrichment.

Findings

Star likely a subgiant with initial higher metallicity

Low observed metals due to gravitational settling and dredge-up

Formation possibly linked to a nearby type Ia supernova

Abstract

Spectroscopic analysis of the Galactic halo star SDSS J102915+172927 has shown it to have a very low heavy element abundance, Z < 7.4 10-7, with [Fe/H] = -4.89 plus/minus 0.10 and an upper limit on the C abundance of [C/H] < -4.5. The low C/Fe ratio distinguishes this object from most other extremely metal poor stars. The effective temperature and surface gravity have been determined to be Teff = 5811 plus/minus 150 K and log g = 4.0 plus/minus 0.5. The surface gravity estimate is problematical in that it places the star between the main sequence and the subgiants in the Hertzsprung-Russell diagram. If it is assumed that the star is on the main sequence, its mass and are estimated to be M = 0.72 plus/minus 0.06 Msun and L = 0.45 plus/minus 0.10 Lsun, placing it at a distance of 1.35 plus/minus 0.16 kpc. The upper limit on the lithium abundance, A(Li) < 0.9, is inconsistent with the star being a dwarf, assuming that mixing is due only to convection. In this paper, we propose that SJ102915 is a sub-giant that formed with significantly higher Z than currently observed, in agreement with theoretical predictions for the minimum C and/or O abundances needed for low mass star formation. In this scenario, extremely low Z and low Li abundance result from gravitational settling on the main sequence followed by incomplete convective dredge-up during subgiant evolution. The observed Fe abundance requires the initial Fe abundance to be enhanced compared to C and O, which we interpret as formation of SJ102915 occurring in the vicinity of a type Ia supernova.