The extremely low-metallicity star sdss j102915+172927: A subgiant scenario

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 x 10-7, with [Fe/H] = -4.89 ± 0.10 and an upper limit on the C abundance of [C/H] < 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 subgiant 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 abundances 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. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.