Is HE0107-5240 A Primordial Star? -- The Characteristics of Extremely Metal-Poor Carbon-Rich Stars --

Abstract

We discuss the origin of HE0107-5240 which is the most metal poor star yet observed ([Fe/H] = -5.3). Its discovery has an important bearing on the question of the observability of "first generation" stars. In common with other metal-poor stars (-4 < [Fe/H] < -2.5), HE0107-5240 shows a peculiar abundance pattern (CNO rich, moderate Na rich). We elaborate the binary scenario on the basis of the evolution and nucleosynthesis of extremely metal-poor, low-mass model stars and discuss the possibility of discriminating this scenario from others. In our picture, iron peak elements arise in surface layers of the component stars by accretion of gas from the polluted primordial cloud. To explain the observed C, N, O, and Na enhancements as well as 12C/13C ratio, we suppose that the currently observed star accreted matter from a AGB companion. To estimate the abundances in the matter transferred in the binary, we rely on the results of computations of model stars constructed with up-to-date input physics. Nucleosynthesis in the helium flash convection with hydrogen injected is followed, allowing us to discuss the abundances of s-process elements, in addition to explaining the origin of the observed O and Na enrichments. From the observed abundances, we conclude that HE0107-5240 has evolved from a wide binary with a primary of initial mass, 1.2 ~ 3 Msun. We estimated the present binary separation of ~ 34 AU and period of ~ 150 years. We also conclude that the abundance distribution of the heavy s-process elements, may hold the key to understand the origin of HE0107-5240. An enhancement of [Pb/Fe] = 1 ~ 2 should be observed. If the enhancement of s-process elements is not detected, HE0107-5240 may be a first generation secondary in a binary system with a primary of mass less than 2.5 Msun.