2013MNRAS.431.1425M

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] < -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±150K and log g = 4.0 ± 0.5, respectively. 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 luminosity ranges are estimated to be M = 0.72±0.06M☉ and L = 0.45±0.10L☉, placing it at a distance of 1.35±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 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.