Chemical compositions of six metal-poor stars in the ultra-faint dwarf spheroidal galaxy Boötes I^{⋆,⋆⋆,⋆⋆⋆}

¹ Kavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institute for Advanced Study, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8583 Chiba, Japan
e-mail: miho.ishigaki@ipmu.jp
² National Astronomical Observatory of Japan, Mitaka, 181-8588 Tokyo, Japan
e-mail: aoki.wako@nao.ac.jp; arimoto.n@nao.ac.jp
³ Department of Astronomical Science, The Graduate University of Advanced Studies (SOKENDAI), Mitaka, 181-8588 Tokyo, Japan
⁴ Subaru Telescope, National Astronomical Observatory of Japan, 650 North A’ohoku Place, Hilo, HI 96720, USA
⁵ Kavli Institute for Astronomy and Astrophysics, Peking University, 100871 Beijing, PR China
e-mail: okamoto@pku.edu.cn

Received: 4 October 2013
Accepted: 24 December 2013

Abstract

Context. Ultra-faint dwarf galaxies recently discovered around the Milky Way (MW) contain extremely metal-poor stars, and might represent the building blocks of low-metallicity components of the MW. Among them, the Boötes I dwarf spheroidal galaxy is of particular interest because of its exclusively old stellar population. Detailed chemical compositions of individual stars in this galaxy are a key to understanding formation and chemical evolution in the oldest galaxies in the Universe and their roles in building up the MW halo.

Aims. Previous studies of the chemical abundances of Boötes I show discrepancies in elemental abundances between different authors, and thus a consistent picture of its chemical enrichment history has not yet been established. In the present work, we independently determine chemical compositions of six red giant stars in Boötes I, some of which overlap with those analyzed in the previous studies. Based on the derived abundances, we re-examine trends and scatters in elemental abundances and make comparisons with MW field halo stars and other dwarf spheroidal galaxies in the MW.

Methods. High-resolution spectra of a sample of stars were obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope. Abundances of 12 elements, including C, Na, α, Fe-peak, and neutron capture elements, were determined for the sample stars. The abundance results were compared to those in field MW halo stars previously obtained using an abundance analysis technique similar to the present study.

Results. We confirm the low metallicity of Boo-094 ([Fe/H] = −3.4). Except for this star, the abundance ratios ([X/Fe]) of elements lighter than zinc are generally homogeneous with small scatter around the mean values in the metallicities spanned by the other five stars (−2.7 < [Fe/H] < −1.8). Specifically, all of the sample stars with [Fe/H] > −2.7 show no significant enhancement of carbon. The [Mg/Fe] and [Ca/Fe] ratios are almost constant with a modest decreasing trend with increasing [Fe/H] and are slightly lower than the field halo stars. The [Sr/Fe] and [Sr/Ba] ratios also tend to be lower in the Boötes I stars than in the halo stars.

Conclusions. Our results of small scatter in the [X/Fe] ratios for elements lighter than zinc suggest that these abundances were homogeneous among the ejecta of prior generation(s) of stars in this galaxy. The lower mean [Mg/Fe] and [Ca/Fe] ratios relative to the field halo stars and the similarity in these abundance ratios with some of the more luminous dwarf spheroidal galaxies at metallicities [Fe/H] < −2 can be interpreted as star formation in Boötes I having lasted at least until Type Ia supernovae started to contribute to the chemical enrichment in this galaxy.