* Astronomy

Title: The formation of the extremely primitive star SDSS J102915+172927 relies on dust
Authors: Raffaella Schneider, Kazuyuki Omukai, Marco Limongi, Andrea Ferrara, Ruben Salvaterra, Alessandro Chieffi, Simone Bianchi

The relative importance of metals and dust grains in the formation of the first low-mass stars has been a subject of debate. The recently discovered Galactic halo star SDSS J102915+172927 (Caffau et al. 2011) has a mass less than 0.8 solar masses and a metallicity of Z = 4.5 10^{-5} Zsun. We investigate the origin and properties of this star by reconstructing the physical conditions in its birth cloud. We show that the observed elemental abundance trend of SDSS J102915+172927 can be well fitted by the yields of core-collapse supernovae with metal-free progenitors of 20 solar masses and 35 solar masses. Using these selected supernova explosion models, we compute the corresponding dust yields and the resulting dust depletion factor taking into account the partial destruction by the supernova reverse shock. We then follow the collapse and fragmentation of a star forming cloud enriched by the products of these SN explosions at the observed metallicity of SDSS J102915+172927. We find that [0.05 - 0.1] solar masses mass fragments, which then lead to the formation of low-mass stars, can occur provided that the mass fraction of dust grains in the birth cloud exceeds 0.01 of the total mass of metals and dust. This, in turn, requires that at least 0.4 solar masses of dust condense in the first supernovae, allowing for moderate destruction by the reverse shock. If dust formation in the first supernovae is less efficient or strong dust destruction does occur, the thermal evolution of the SDSS J102915+172927 birth cloud is dominated by molecular cooling, and only > 8 solar mass fragments can form. We conclude that the observed properties of SDSS J102915+172927 support the suggestion that dust must have condensed in the ejecta of the first supernovae and played a fundamental role in the formation of the first low-mass stars.

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Title: An extremely primitive halo star
Authors: E. Caffau (1,2), P. Bonifacio (2), P. François (2,3) . L. Sbordone (1,4,2) . L. Monaco (5), M. Spite (2), F. Spite (2), H.-G. Ludwig (1,2), R. Cayrel (2), S. Zaggia (6), F. Hammer (2), S. Randich (7), P. Molaro (8), V. Hill (9) ((1) ZAH-LSW Heidelberg, (2) GEPI - Obs. de Paris, CNRS, Univ. Paris Diderot, (3) Univ. de Picardie Jules Verne,(4) Max Planck Institut für Astrophysik, (5) ESO - Santiago, (6) INAF - Osservatorio Astronomico di Padova, (7) INAF - Osservatorio Astrofisico di Arcetri, (8) INAF - Osservatorio Astronomico di Trieste, (9) Univ. de Nice Sophia Antipolis, CNRS, OCA, Lab. Cassiopée)

The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium1, almost all other elements were created in stars and supernovae. The mass fraction, Z, of elements more massive than helium, is called "metallicity". A number of very metal poor stars have been found some of which, while having a low iron abundance, are rich in carbon, nitrogen and oxygen. For theoretical reasons and because of an observed absence of stars with metallicities lower than Z=1.5E-5, it has been suggested that low mass stars (M<0.8 solar masses, the ones that survive to the present day) cannot form until the interstellar medium has been enriched above a critical value, estimated to lie in the range 1.5E-8\leqZ\leq1.5E-6, although competing theories claiming the contrary do exist. Here we report the chemical composition of a star with a very low Z\leq6.9E-7 (4.5E-5 of that of the Sun) and a chemical pattern typical of classical extremely metal poor stars, meaning without the enrichment of carbon, nitrogen and oxygen. This shows that low mass stars can be formed at very low metallicity. Lithium is not detected, suggesting a low metallicity extension of the previously observed trend in lithium depletion. Lithium depletion implies that the stellar material must have experienced temperatures above two million K in its history, which points to rather particular formation condition or internal mixing process, for low Z stars.

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Title: An extremely primitive halo star
Authors: Elisabetta Caffau, Piercarlo Bonifacio, Patrick François, Luca Sbordone, Lorenzo Monaco, Monique Spite, François Spite, Hans­G. Ludwig, Roger Cayre, Simone Zaggia, François Hammer, Sofia Randich, Paolo Molaro, Vanessa Hill

The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium, almost all other elements were created in stars and supernovae. The mass fraction, Z, of elements more massive than helium, is called "metallicity". A number of very metal­poor stars have been found, some of which, while having a low iron abundance, are rich in carbon, nitrogen and oxygen. For theoretical reasons and because of an observed absence of stars with metallicities lower than Z=1.5 x 10^­5, it has been suggested that low mass stars (M<0.8 solar masses, the ones that survive to the present day) cannot form until the interstellar medium has been enriched above a critical value, estimated to lie in the range 1.5 x 10^­8 {\lesseq}Z{\lesseq}1.5 x 10^­6, although competing * Gliese Fellowtheories claiming the contrary do exist. Here we report the chemical composition of a star with a very low Z{\lesseq}6.9 x 10^­7 (4.5 x 10^­5 of that of the Sun) and a chemical pattern typical of classical extremely metal poor stars, meaning without the enrichment of carbon, nitrogen and oxygen. This shows that low­mass stars can be formed at very low metallicity. Lithium is not detected, suggesting a low metallicity extension of the previously observed trend in lithium depletion. Lithium depletion implies that the stellar material must have experienced temperatures above two million K in its history, which points to rather particular formation condition or internal mixing process, for low Z stars

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