Chemical abundances of giant stars in the Crater stellar system^⋆,⋆⋆

¹ GEPI, Observatoire de Paris, PSL Research University, CNRS, Univ. Paris Diderot, Sorbonne Paris Cité, Place Jules Janssen, 92195 Meudon, France
e-mail: Piercarlo.Bonifacio@obspm.fr
² Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ UPJV, Université de Picardie Jules Verne, 33 rue St Leu, 80080 Amiens, France
⁴ Millennium Institute of Astrophysics, Casilla 306, Santiego 22, Chile
⁵ Pontificia Universidad Católica de Chile Vicuña MacKenna 4860, Macul, Santiago, Chile
⁶ Department of Astronomy and Astronomical Observatory, Odessa National University, and Isaac Newton Institute of Chile Odessa branch, Shevchenko Park, 65014 Odessa, Ukraine

Received: 20 April 2015
Accepted: 9 June 2015

Abstract

Aims. We obtained spectra for two giants of Crater (Crater J113613-105227 and Crater J113615-105244) using X-Shooter at the VLT, with the purpose of determining their radial velocities and metallicities.

Methods. Radial velocities were determined by cross-correlating the spectra with that of a standard star. The spectra were analysed with the MyGIsFOS code using a grid of synthetic spectra computed from one-dimensional, local thermodynamic equilibrium (LTE) model atmospheres. Effective temperature and surface gravity were derived from photometry measured from images obtained by the Dark Energy Survey.

Results. The radial velocities are 144.3 ± 4.0 km s^-1 for Crater  J113613-105227 and and 134.1 ± 4.0km s^-1 for Crater J113615-105244. The metallicities are [Fe/H] = −1.73 and [Fe/H] = −1.67, respectively. In addition to the iron abundance, we were able to determine abundances for nine elements: Na, Mg, Ca, Ti, V, Cr, Mn, Ni, and Ba. For Na and Ba we took into account deviations from LTE because the corrections are significant. The abundance ratios are similar in the two stars and resemble those of Galactic stars of the same metallicity. In the deep photometric images we detected several stars that lie to the blue of the turn-off.

Conclusions. The radial velocities imply that both stars are members of the Crater stellar system. The difference in velocity between the two taken at face value implies a velocity dispersion >3.7 km s^-1 at a 95% confidence level. Our spectroscopic metallicities agree excellently well with those determined by previous investigations using photometry. Our deep photometry and the spectroscopic metallicity imply an age of 7  Gyr for the main population of the system. The stars to the blue of the turn-off can be interpreted as a younger population that is of the same metallicity and an age of 2.2 Gyr. Finally, spatial and kinematical parameters support the idea that this system is associated with the galaxies Leo IV and Leo V. All the observations favour the interpretation of Crater as a dwarf galaxy.