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Astron. Astrophys. 359, 663-668 (2000)

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1. Introduction

In the recent years our ideas on galaxy formation and evolution have considerably developed, and it is generally acknowledged that it is a complex process which may well take different paths in different galaxies. Much attention is being devoted to dwarf spheroidal galaxies, essentially for two reasons: 1) they seem to be relatively simple systems, typically characterized by a single stellar population; in such a system we hope to be able to isolate some of the key ingredients of the phenomenon; 2) Interaction of these dwarf galaxies with large galaxies (such as our own or the Andromeda galaxy) could, in principle, play an important role in shaping the morphology of the large galaxies. The nearest members of the class, the dwarf spheroidals of the Local Group, are close enough that their stars are amenable to detailed analysis with the same techniques employed to study Galactic stars, with the advent of the new 8m class telescopes. In this paper we report on such an observation: the first detailed chemical analysis of two stars in the Sgr dwarf spheroidal based on high resolution spectra obtained with the UVES spectrograph on the ESO 8.2m Kueyen telescope. Ever since the discovery of Sgr (Ibata et al. 1994) photometric studies have shown the red giant branch (RGB) of Sgr to be wider than expected for a population with a single age and metallicity. This has been generally interpreted as evidence that Sgr displays a spread in metallicity which is likely due to different bursts of star formation. Ibata et al. (1995) found a mean metallicity of [Fe/H]=-1.25 and their metallicity distribution displays a spread of over 1 dex. Sarajedini & Layden (1995) found a main population with [Fe/H]= -0.52 and suggested the possible existence of a population of [Fe/H][FORMULA]. Mateo et al. (1995) provide a mean metallicity of [FORMULA], Ibata et al. (1997) estimate metallicities in the range [FORMULA], Marconi et al. (1998) [FORMULA], Bellazzini et al. (1999) [FORMULA]. The age of Sgr may not be disentangled from its metallicity, from Main Sequence fitting, Fahlman et al. (1996) found acceptable solutions for an age of 10 Gyr and metallicity -0.8 or an age of 14 Gyr and a metallicity -1.3. Clearly if Sgr may not be described as a single population the concepts of age and metallicity loose some of their significance; Bellazzini et al. (1999) proposed an extreme scenario in which star formation began rather early and continued for a period longer than 4 Gyr. Foreseeing the potentiality of UVES to perform detailed abundance analysis of these stars, to confirm or refute the photometrically inferred spread in metallicity, we undertook already in 1995 observations of low resolution spectra of photometrically identified (Marconi et al. 1998) Sgr candidates. The main purpose was to obtain confirmed radial velocity members of Sgr for subsequent high resolution follow-up with UVES. From the low resolution spectra we also devised a method to obtain crude metallicity estimates from spectral indices defined in the Mg I b triplet region. The two stars were selected from this low-resolution study of Sgr with photometry and abundance estimates which suggested these stars to differ by at least 0.5 dex in metallicity.

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© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000
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