The Oxygen Abundance of HE 1327 − 2326 1

From a newly obtained VLT/UVES spectrum we have determined the oxygen abundance of HE 1327−2326, the most iron-poor star known to date. UVOH lines yield a 1D LTE abundance of [O/Fe]OH = 3.7 (subgiant case) and [O/Fe]OH = 3.4 (dwarf case). Using a correction of −1.0 dex to account for 3D effects on OH line formation, the abundances are lowered to [O/Fe] = 2.8 and [O/Fe] = 2.5, respectively, which we adopt. Without 3D corrections, the UV-OH based abundance would be in disagreement with the upper limits derived from the O I triplet lines: [O/Fe]trip < 2.8 (subgiant) and [O/Fe]trip < 3.0 (dwarf). We also correct the previously determined carbon and nitrogen abundances for 3D effects. Knowledge of the O abundance of HE 1327−2326 has implications for the interpretation of its abundance pattern. A large O abundance is in accordance with HE 1327−2326 being an early Population II star which formed from material chemically enriched by a first generation supernova. Our derived abundances, however, do not exclude other possibilities such as a Population III scenario. Subject headings: Galaxy: abundances — Galaxy: halo — stars: abundances — stars: individual: HE 1327−2326 Research School of Astronomy & Astrophysics, The Australian National University, Cotter Road, Weston, ACT 2611, Australia; [email protected], [email protected], [email protected] Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany; [email protected] National Astronomical Observatory of Japan, 2-1-21 Osawa, Mitaka, Tokyo, 181-8588 Japan; [email protected] Based on observations collected at the European Southern Observatory, Paranal, Chile (Proposal ID 075.D-0048).