95 research outputs found
The IMF of Extremely Metal-Poor Stars and the Probe into the Star-Formation Process of the Milky Way
We discuss the star formation history of the Galaxy, based on the
observations of extremely metal-poor stars (EMP) in the Galactic halo, to gain
an insight into the evolution and structure formation in the early universe.
The initialmass function (IMF) of EMP stars is derived from the observed
fraction of carbon-enhanced EXP (CEMP) stars among the EMP survivors, which are
thought to originate from the evolution in the close binary systems with mass
transfer. Relying upon the theory of the evolution of EMP stars and of their
binary evolution, we find that stars of metallicity [Fe/H]<-2.5 were formed at
typical mass of ~10M_sun. The top heavy IMF thus obtained is applied to study
the early chemical evolution of the Galaxy. We construct the merging history of
our Galaxy semi-analytically and derive the metallicity distribution function
(MDF) of low-mass EMP stars that survive to date with taking into account the
contribution of binary systems. It is shown that the resultant MDF can well
reproduce the observed distribution of EMP survivors, and, in particular, that
they almost all stem from a less-mass companion in binary systems. We also
investigate how first stars affect the MDF of EMP stars.Comment: 5 pages, 4 figures, conference proceedings of First Star II
The Stellar Abundances for Galactic Archeology (SAGA) Database - Compilation of the Characteristics of Known Extremely Metal-Poor Stars
We describe the construction of a database of extremely metal-poor (EMP)
stars in the Galactic halo whose elemental abundances have been determined. Our
database contains detailed elemental abundances, reported equivalent widths,
atmospheric parameters, photometry, and binarity status, compiled from papers
in the recent literature that report studies of EMP halo stars with [Fe/H] <
-2.5. The compilation procedures for this database have been designed to
assemble the data effectively from electronic tables available from online
journals. We have also developed a data retrieval system that enables data
searches by various criteria, and permits the user to explore relationships
between the stored variables graphically. Currently, our sample includes 1212
unique stars (many of which are studied by more than one group) with more than
15000 individual reported elemental abundances, covering all of the relevant
papers published by December 2007. We discuss the global characteristics of the
present database, as revealed by the EMP stars observed to date. For stars with
[Fe/H] < -2.5, the number of giants with reported abundances is larger than
that of dwarfs by a factor of two. The fraction of carbon-rich stars (among the
sample for which the carbon abundance is reported) amount to ~30 % for [Fe/H] <
-2.5. We find that known binaries exhibit different distributions of orbital
period, according to whether they are giants or dwarfs, and also as a function
of metallicity, although the total sample of such stars is still quite small.Comment: 24 pages, 10 figures, accepted by PASJ, final version. The SAGA
database is available at http://saga.sci.hokudai.ac.j
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