Letter to the Editor

Evidence of enrichment by individual SN from elemental abundance ratios in the very metal-poor dSph galaxy Boötes I^*

¹ Lund Observatory, Box 43, 221 00 Lund, Sweden e-mail: sofia@astro.lu.se
² Department of Astronomy and Space Physics, Uppsala University, Box 515, 751 20 Uppsala, Sweden e-mail: kjell.eriksson@astro.uu.se
³ Institute for Astronomy, Honululu, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁴ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK e-mail: miw6@astro.le.ac.uk

Received: 6 July 2009
Accepted: 18 September 2009

Abstract

Aims. We establish the mean metallicity from high-resolution spectroscopy for the recently found dwarf spheroidal galaxy Boötes I and test whether it is a common feature for ultra-faint dwarf spheroidal galaxies to show signs of inhomogeneous chemical evolution (e.g. as found in the Hercules dwarf spheroidal galaxy).

Methods. We analyse high-resolution, moderate signal-to-noise spectra for seven red giant stars in the Boötes I dSph galaxy using standard abundance analysis techniques. In particular, we assume local thermodynamic equilibrium and employ spherical model atmospheres and codes that take the sphericity of the star into account when calculating the elemental abundances.

Results. We confirm previous determinations of the mean metallicity of the Boötes I dwarf spheroidal galaxy to be -2.3 dex. Whilst five stars are clustered around this metallicity, one is significantly more metal-poor, at -2.9 dex, and one is more metal-rich at, -1.9 dex. Additionally, we find that one of the stars, Boo-127, shows an atypically high [Mg/Ca] ratio, indicative of stochastic enrichment processes within the dSph galaxy. Similar results have previously only been found in the Hercules and Draco dSph galaxies and appear, so far, to be unique to this type of galaxy.