Is HE 0107−5240 A Primordial Star? The Characteristics of Extremely Metal‐Poor Carbon‐Rich Stars

We discuss the origin of HE0107-5240 which is the most metal poor star yetobserved ([Fe/H] = -5.3). Its discovery has an important bearing on thequestion of the observability of "first generation" stars. In common with othermetal-poor stars (-4 < [Fe/H] < -2.5), HE0107-5240 shows a peculiar abundancepattern (CNO rich, moderate Na rich). We elaborate the binary scenario on thebasis of the evolution and nucleosynthesis of extremely metal-poor, low-massmodel stars and discuss the possibility of discriminating this scenario fromothers. In our picture, iron peak elements arise in surface layers of thecomponent stars by accretion of gas from the polluted primordial cloud. Toexplain the observed C, N, O, and Na enhancements as well as 12C/13C ratio, wesuppose that the currently observed star accreted matter from a AGB companion.To estimate the abundances in the matter transferred in the binary, we rely onthe results of computations of model stars constructed with up-to-date inputphysics. Nucleosynthesis in the helium flash convection with hydrogen injectedis followed, allowing us to discuss the abundances of s-process elements, inaddition to explaining the origin of the observed O and Na enrichments. Fromthe observed abundances, we conclude that HE0107-5240 has evolved from a widebinary with a primary of initial mass, 1.2 ~ 3 Msun. We estimated the presentbinary separation of ~ 34 AU and period of ~ 150 years. We also conclude thatthe abundance distribution of the heavy s-process elements, may hold the key tounderstand the origin of HE0107-5240. An enhancement of [Pb/Fe] = 1 ~ 2 shouldbe observed. If the enhancement of s-process elements is not detected,HE0107-5240 may be a first generation secondary in a binary system with aprimary of mass less than 2.5 Msun.Comment: 50 pages, 10 figures in ApJ preprint style, accepted for Ap