  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.
