The 300km/s stellar stream near Segue 1: Insights From high-resolution spectroscopy of its brightest star

We present a chemical abundance analysis of 300S-1, the brightest likely member star of the 300 km/s stream near the faint satellite galaxy Segue 1. From a high-resolution Magellan/MIKE spectrum we determine a metallicity of [Fe/H] = -1.46 +- 0.05 +- 0.23 (random and systematic uncertainties) for star 300S-1, and find an abundance pattern similar to typical halo stars at this metallicity. Comparing our stellar parameters to theoretical isochrones, we estimate a distance of 18 +- 7 kpc. Both the metallicity and distance estimates are in good agreement with what can be inferred from comparing the SDSS photometric data of the stream stars to globular cluster sequences. While several other structures overlap with the stream in this part of the sky, the combination of kinematic, chemical and distance information makes it unlikely that these stars are associated with either the Segue 1 galaxy, the Sagittarius stream or the Orphan stream. Streams with halo-like abundance signatures, such as the 300 km/s stream, present another observational piece for understanding the accretion history of the Galactic halo.Comment: 13 pages, emulateapj, accepted for publication in Ap

Good abundances from bad spectra; 1, techniques

We have developed techniques to extract true iron abundances and surface gravities from spectra of the type provided by the multiple-object fibre-fed spectroscopic radial-velocity surveys underway with 2dF, HYDRA, NESSIE, and the forthcoming Sloan survey. Our method is optimised for low S/N, intermediate resolution blue spectra of G stars. Spectroscopic indices sensitive to iron abundance and gravity are defined from a set of narrow (few Angstrom) wavelength intervals, and calibrated using synthetic spectra. We have also defined a single abundance indicator which is able to provide useful iron abundance information from spectra having S/N ratios as low as 10 per Angstrom. The theoretical basis and calibration using synthetic spectra are described in this paper. The empirical calibration of these techniques by application to observational data is described in Jones, Wyse and Gilmore (PASP July 1995). The technique provides precise iron abundances, with zero-point correct to \sim 0.1 dex, and is reliable, with typical uncertainties being \approxle 0.2 dex. A derivation of the {\it in situ\/} thick disk metallicity distribution using these techniques is presented by Gilmore, Wyse and Jones (AJ 1995 v109 p1095)

The Faint Optical Stellar Luminosity Function in the Ursa Minor Dwarf Spheroidal Galaxy

Analyses of their internal stellar kinematics imply that the dwarf spheroidal (dSph) companion galaxies to the Milky Way are among the most dark matter-dominated systems known. Should there be significant dark matter in the form of faint stars in these systems, the stellar luminosity function must be very different from that of a similar metallicity globular cluster, for which there is no evidence for dark matter. We present the faint stellar luminosity function in the Ursa Minor dSph, down to a luminosity corresponding to ~0.45 M_solar, derived from new deep Hubble Space Telescope/WFPC2 data. We find a remarkable similarity between this luminosity function, and inferred initial mass function, and those of the globular cluster M92, a cluster of similar age and metallicity to the Ursa Minor dSph