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

¹ Universidade de São Paulo, IAG, Departamento de Astronomia, Rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, SP Brazil
e-mail: jorge.melendez@iag.usp.br
² University of Notre Dame, Department of Physics and JINA Center for the Evolution of the Elements, Notre Dame, IN 46556, USA
³ University of Texas at Austin, McDonald Observatory and Department of Astronomy, Austin, TX 78712, USA
⁴ Texas A&M University, Department of Physics and Astronomy, TX 77843, USA
⁵ Universidade de São Paulo, IAG, Departamento de Ciências Atmosféricas, Rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, SP, Brazil

Received: 27 September 2015
Accepted: 25 November 2015

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 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, and a standard abundance analysis was performed.

Results. We found that 2MASS J18082002−5104378 is an ultra metal-poor star with stellar parameters T_eff = 5440 K, log g = 3.0 dex, v_t = 1.5 km s^-1, [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.