HE 0107-5240, a chemically ancient star. I. A detailed abundance analysis

We report on a detailed abundance analysis of HE 0107-5240, a halo giant with [Fe/H]_NLTE = -5.3. This star was discovered in the course of follow-up medium-resolution spectroscopy of extremely metal-poor candidates selected from the digitized Hamburg/ESO objective-prism survey. On the basis of high-resolution VLT/UVES spectra, we derive abundances for eight elements (C, N, Na, Mg, Ca, Ti, Fe, and Ni) and upper limits for another 12 elements. A plane-parallel LTE model atmosphere has been specifically tailored for the chemical composition of HE 0107-5240. Scenarios of the origin of the abundance pattern observed in the star are discussed. We argue that HE 0107-5240 is most likely not a post-asymptotic giant branch star and that the extremely low abundances of the iron-peak and other elements are not due to selective dust depletion. The abundance pattern of HE 0107-5240 can be explained by preenrichment from a zero-metallicity Type II supernova (SN II) of 20-25 M⊙, plus either self-enrichment with C and N or production of these elements in the asymptotic giant branch phase of a formerly more massive companion, which is now a white dwarf. However, significant radial velocity variations have not been detected within the 52 days covered by our moderate- and high-resolution spectra. Alternatively, the abundance pattern can be explained by enrichment of the gas cloud from which HE 0107-5240 formed by a 25 M⊙ first-generation star exploding as a subluminous SN II, as proposed by Umeda & Nomoto. We discuss consequences of the existence of HE 0107-5240 for low-mass star formation in extremely metal-poor environments and for currently ongoing and future searches for the most metal-poor stars in the Galaxy.