We present Magellan/MIKE and Keck/HIRES high-resolution spectra of six red giant stars in the dwarf galaxy Segue 1 .
Including one additional Segue 1 star observed by Norris et al . , high-resolution spectra have now been obtained for every red giant in Segue 1 .
Remarkably , three of these seven stars have metallicities below \mbox { [ Fe / H ] } = -3.5 , suggesting that Segue 1 is the least chemically evolved galaxy known .
We confirm previous medium-resolution analyses demonstrating that Segue 1 stars span a metallicity range of more than 2 dex , from \mbox { [ Fe / H ] } = -1.4 to \mbox { [ Fe / H ] } = -3.8 .
All of the Segue 1 stars are \alpha -enhanced , with \mbox { [ $ \alpha$ / Fe ] } \sim 0.5 .
High \alpha -element abundances are typical for metal-poor stars , but in every previously studied galaxy [ \alpha /Fe ] declines for more metal-rich stars , which is typically interpreted as iron enrichment from supernova Ia .
The absence of this signature in Segue 1 indicates that it was enriched exclusively by massive stars .
Other light element abundance ratios in Segue 1 , including carbon-enhancement in the three most metal-poor stars , closely resemble those of metal-poor halo stars .
Finally , we classify the most metal-rich star as a CH star given its large overabundances of carbon and s-process elements .
The other six stars show remarkably low neutron-capture element abundances of \mbox { [ Sr / H ] } < -4.9 and \mbox { [ Ba / H ] } < -4.2 , which are comparable to the lowest levels ever detected in halo stars .
This suggests minimal neutron-capture enrichment , perhaps limited to a single r-process or weak s-process synthesizing event .
Altogether , the chemical abundances of Segue 1 indicate no substantial chemical evolution , supporting the idea that it may be a surviving first galaxy that experienced only one burst of star formation .