J0815+4729: A chemically primitive dwarf star in the Galactic Halo observed with Gran Telescopio Canarias

TL;DR

This paper reports the discovery of J0815+4729, a potentially the most iron-poor unevolved star with extreme carbon enrichment, providing insights into early universe star formation and supernovae.

Contribution

It presents the identification and initial characterization of a highly primitive, ultra metal-poor dwarf star with unprecedented carbon overabundance, using medium-resolution spectroscopy.

Findings

Upper limit to metallicity [Fe/H]<~-5.8

Extreme carbon overabundance [C/Fe]>~+5.0

Possible most iron-poor unevolved star known

Abstract

We report the discovery of the carbon-rich ultra metal-poor unevolved star J0815+4729. This dwarf star was selected from SDSS/BOSS as a metal-poor candidate and follow-up spectroscopic observations at medium-resolution were obtained with ISIS at William Herschel Telescope and OSIRIS at Gran Telescopio de Canarias. We use the FERRE code to derive the main stellar parameters, Teff=6215+-82 K, and logg=4.7+-0.5, an upper limit to the metallicity of [Fe/H]<~-5.8, and a carbon abundance of [C/Fe]>~+5.0, while [alpha/Fe]=0.4 is assumed. The metallicity upper limit is based on the Ca II K line, that at the resolving power of the OSIRIS spectrograph cannot be resolved from possible interstellar calcium. The star could be the most iron-poor unevolved star known, and is also amongst the ones with the largest overabundances of carbon. High-resolution spectroscopy of J0815+4729 will certainly help to derive other important element abundances, possibly providing new fundamental constraints on the early stages of the Universe, the formation of the first stars and the properties of the first supernovae.