HE 1327-2326, An unevolved star with [Fe/H] < -5.0. I. A comprehensive abundance analysis

We present the elemental abundances of HE 1327-2326, the most iron-deficient star known, determined from a comprehensive analysis of spectra obtained with the Subaru Telescope High Dispersion Spectrograph. HE 1327-2326 is either in its main-sequence or subgiant phase of evolution. Its non-LTE-corrected iron abundance is [Fe/H] = -5.45, 0.2 dex lower than that of HE 0107-5240, the previously most iron-poor object known, and more than 1 dex lower than those of all other metal-poor stars. Both HE 1327-2326 and HE 0107-5240 exhibit extremely large overabundances of carbon ([C/Fe] ∼ +4). The combination of extremely high carbon abundance with outstandingly low iron abundance in these objects clearly distinguishes them from other metal-poor stars. The large carbon excesses in these two stars are not the result of a selection effect. There also exist important differences between HE 1327-2326 and HE 0107-5240. While the former shows remarkable overabundances of the light elements (N, Na, Mg, and Al), the latter shows only relatively small excesses of N and Na. The neutron-capture element Sr is detected in HE 1327-2326, but not in HE 0107-5240; its Sr abundance is significantly higher than the upper limit for HE 0107-5240. The Li I λ6707 line, which is detected in most metal-poor dwarfs and warm subgiants having the same temperature as HE 1327-2326, is not found in this object. The upper limit of its Li abundance [log ε(Li) < 1.5] is clearly lower than the Spite plateau value. These data provide new constraints on models of nucleosynthesis processes in the first-generation objects that were responsible for metal enrichment at the earliest times. We discuss possible scenarios to explain the observed abundance patterns.