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

J. MacDonald, T. M. Lawlor, N. Anilmis, N. F. Rufo

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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 ± 0.10 L, 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.

Original languageEnglish (US)
Pages (from-to)1425-1431
Number of pages7
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - May 11 2013

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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