The 300 km s–1 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 −1 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 Sloan Digital Sky Survey 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 −1 stream, present another observational piece for understanding the accretion history of the Galactic halo.