2MASS J18082002-5104378: The brightest (V=11.9) ultra metal-poor star

TL;DR

This paper reports the discovery and detailed analysis of 2MASS J18082002-5104378, the brightest ultra metal-poor star known, providing insights into early galaxy conditions and chemical evolution.

Contribution

The study introduces a new candidate selection method based on spectral type mismatches and presents the first high-resolution analysis of the brightest ultra metal-poor star.

Findings

Star has [Fe/H] = -4.1, confirming its ultra metal-poor status.

Star exhibits a typical abundance pattern of non-CEMP ultra metal-poor stars.

Star has a binary companion, offering opportunities for further study.

Abstract

Context. The most primitive metal-poor stars are important for studying the conditions of the early galaxy and are also relevant to big bang nucleosynthesis. Aims. Our objective is to find the brightest (V<14) most metal-poor stars. Methods. Candidates were selected using a new method, which is based on the mismatch between spectral types derived from colors and observed spectral types. They were observed first at low resolution with EFOSC2 at the NTT/ESO to obtain an initial set of stellar parameters. The most promising candidate, 2MASS J18082002-5104378 (V=11.9), was observed at high resolution (R=50 000) with UVES at the VLT/ESO, and a standard abundance analysis was performed. Results. We found that 2MASS J18082002-5104378 is an ultra metal-poor star with stellar parameters Teff = 5440 K, log g = 3.0 dex, vt = 1.5 km/s, [Fe/H] = -4.1 dex. The star has [C/Fe]<+0.9 in a 1D analysis, or [C/Fe]<=+0.5 if 3D effects are considered; its abundance pattern is typical of normal (non-CEMP) ultra metal-poor stars. Interestingly, the star has a binary companion. Conclusions. 2MASS J1808-5104 is the brightest (V=11.9) metal-poor star of its category, and it could be studied further with even higher S/N spectroscopy to determine additional chemical abundances, thus providing important constraints to the early chemical evolution of our Galaxy.