Abundance ratios of red giants in low-mass ultra-faint dwarf spheroidal galaxies

¹ GEPI, Observatoire de Paris, PSL Research University, CNRS, Univ. Paris Diderot, Sorbonne Paris Cité, 61 avenue de l’Observatoire, 75014 Paris, France
e-mail: patrick.francois@obspm.fr
² Université de Picardie Jules Verne, 33 rue St. Leu, 80000 Amiens, France
³ Departamento de Ciencias Fisicas, Universidad Andres Bello, 220 Republica, Santiago, Chile
⁴ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610 Antofagasta, Chile
⁵ Department of Astronomy, Faculty of Physical and Mathematical Sciences, University of Concepción, Chile
⁶ Millennium Institute of Astrophysics, Pontificia Universidad Católica de Chile, 4860 Vicuña Mackenna, Macul Santiago, Chile

Received: 13 August 2015
Accepted: 11 December 2015

Abstract

Context. Low-mass dwarf spheroidal galaxies are key objects for our understanding of the chemical evolution of the pristine Universe and the Local Group of galaxies. Abundance ratios in stars of these objects can be used to better understand their star formation and chemical evolution.

Aims. We report on the analysis of a sample of 11 stars belonging to five different ultra-faint dwarf spheroidal galaxies (UfDSph) that is based on X-Shooter spectra obtained at the VLT.

Methods. Medium-resolution spectra have been used to determine the detailed chemical composition of their atmosphere. We performed a standard 1D LTE analysis to compute the abundances.

Results. Considering all the stars as representative of the same population of low-mass galaxies, we found that the [α/Fe] ratios vs.s [Fe/H] decreases as the metallicity of the star increases in a way similar to that which is found for the population of stars that belong to dwarf spheroidal galaxies. The main difference is that the solar [α/Fe] is reached at a much lower metallicity for the UfDSph than for the dwarf spheroidal galaxies. We report for the first time the abundance of strontium in CVn II. The star we analyzed in this galaxy has a very high [Sr/Fe] and a very low upper limit of barium which makes it a star with an exceptionally high [Sr/Ba] ratio.