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Register a new userIs 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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We have determined the oxygen abundance of HE0107-5240 from UV-OH lines detected in VLT/UVES spectra. Using a plane-parallel LTE model atmosphere, we derive [O/Fe] = +2.4, and a similar analysis of CD -38 245 yields [O/Fe] = +1.0. We estimate systematic errors due to 3D effects to be in the order of 0.3 to 0.4 dex. That is, our derived O abundances are likely overestimates: effects from thermal inhomogeneities due to convection may require that the abundances should be reduced by 0.3-0.4 dex or even more. Radial velocity data for HE0107-5240 based on high-resolution spectra show that over a time span of 373 days the radial velocity was constant at 44.5 km/s, with a 1 sigma scatter of the measurements of 0.5 km/s. However, it can not yet be ruled out that HE0107-5240 is a very long period and/or low amplitude binary. These results provide new constraints on scenarios for the origin of the abundance pattern of HE0107-5240. In particular, it seems unlikely that the large overabundances of CNO have been produced in a medium-mass AGB star which later evolved to a white dwarf. The oxygen abundance of HE0107-5240 is significantly smaller than the prediction of Umeda & Nomoto (2003) from calculated yields of a ~25 solar mass Population III star exploding as a supernova of low explosion energy (E_exp = 3 x 10^50 erg) with mixing and fallback. The scenario of Limongi et al. (2003), involving two Population III supernovae, predicts an oxygen abundance of [O/Fe] = +4.1 for HE0107-5240, in strong contradiction with the observed value. In conclusion, none of the above mentioned scenarios, in their present realizations, can satisfactorly explain the abundance pattern of HE0107-5240.
The origin of carbon-enhanced metal-poor (CEMP) stars plays a key role in characterising the formation and evolution of the first stars and the Galaxy since the extremely-poor (EMP) stars with [Fe/H] leq -2.5 share the common features of carbon enhancement in their surface chemical compositions. The origin of these stars is not yet established due to the controversy of the origin of CEMP stars without the enhancement of s-process element abundances, i.e., so called CEMP-no stars. In this paper, we elaborate the s-process nucleosynthesis in the EMP AGB stars and explore the origin of CEMP stars. We find that the efficiency of the s-process is controlled by O rather than Fe at [Fe/H] lesssim -2. We demonstrate that the relative abundances of Sr, Ba, Pb to C are explained in terms of the wind accretion from AGB stars in binary systems.
High-resolution spectra obtained with three ground-based facilities and the Hubble Space Telescope (HST) have been combined to produce a new abundance analysis of CS 22892-052, an extremely metal-poor giant with large relative enhancements of neutron-capture elements. A revised model stellar atmosphere has been derived with the aid of a large number of Fe-peak transitions, including both neutral and ionized species of six elements.Several elements, including Mo, Lu, Au, Pt and Pb, have been detected for the first time in CS 22892-052, and significant upper limits have been placed on the abundances of Ga, Ge, Cd, Sn, and U in this star. In total, abundance measurements or upper limits have been determined for 57 elements, far more than previously possible. New Be and Li detections in CS 22892-052 indicate that the abundances of both these elements are significantly depleted compared to unevolved main-sequence turnoff stars of similar metallicity. Abundance comparisons show an excellent agreement between the heaviest n-capture elements (Z >= 56) and scaled solar system r-process abundances, confirming earlier results for CS 22892-052 and other metal-poor stars. New theoretical r-process calculations also show good agreement with CS 22892-052 abundances as well as the solar r-process abundance components.The abundances of lighter elements (40<= Z <= 50), however, deviate from the same scaled abundance curves that match the heavier elements, suggesting different synthesis conditions or sites for the low-mass and high-mass ends of the abundance distribution. The detection of Th and the upper limit on the U abundance together imply a lower limit of 10.4 Gyr on the age of CS 22892-052, quite consistent with the Th/Eu age estimate of 12.8 +/- ~= 3 Gyr. An average of several chronometric ratios yields an age 14.2 +/- ~= 3 Gyr.
Extremely metal-poor galaxies with metallicity below 10% of the solar value in the local universe are the best analogues to investigating the interstellar medium at a quasi-primitive environment in the early universe. In spite of the ongoing formation of stars in these galaxies, the presence of molecular gas (which is known to provide the material reservoir for star formation in galaxies, such as our Milky Way) remains unclear. Here, we report the detection of carbon monoxide (CO), the primary tracer of molecular gas, in a galaxy with 7% solar metallicity, with additional detections in two galaxies at higher metallicities. Such detections offer direct evidence for the existence of molecular gas in these galaxies that contain few metals. Using archived infrared data, it is shown that the molecular gas mass per CO luminosity at extremely low metallicity is approximately one-thousand times the Milky Way value.
We report a detailed abundance analysis for HE0107-5240, 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 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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