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تسجيل مستخدم جديدIs HE0107-5240 A Primordial Star? -- The Characteristics of Extremely Metal-Poor Carbon-Rich Stars --
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T. Suda
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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.
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objective-prism survey. On the basis of high-resolution VLT/UVES spectra, we derive abundances for 8 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}. Scenarios for the origin of the abundance pattern observed in the star are discussed. We argue that HE0107-5240 is most likely not a post-AGB 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 HE0107-5240 can be explained by pre-enrichment from a zero-metallicity type-II supernova of 20-25M_Sun, plus either self-enrichment with C and N, or production of these elements in the AGB 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 HE0107-5240 formed by a 25M_Sun first-generation star exploding as a subluminous SNII, as proposed by Umeda & Nomoto (2003). We discuss consequences of the existence of HE0107-5240 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.
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