187 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
Transition of the Stellar Initial Mass Function Explored with Binary Population Synthesis
The stellar initial mass function (IMF) plays a crucial role in determining
the number of surviving stars in galaxies, the chemical composition of the
interstellar medium, and the distribution of light in galaxies. A key unsolved
question is whether the IMF is universal in time and space. Here we use
state-of-the-art results of stellar evolution to show that the IMF of our
Galaxy made a transition from an IMF dominated by massive stars to the
present-day IMF at an early phase of the Galaxy formation. Updated results from
stellar evolution in a wide range of metallicities have been implemented in a
binary population synthesis code, and compared with the observations of
carbon-enhanced metal-poor (CEMP) stars in our Galaxy. We find that applying
the present-day IMF to Galactic halo stars causes serious contradictions with
four observable quantities connected with the evolution of AGB stars.
Furthermore, a comparison between our calculations and the observations of CEMP
stars may help us to constrain the transition metallicity for the IMF which we
tentatively set at [Fe/H] = -2. A novelty of the current study is the inclusion
of mass loss suppression in intermediate-mass AGB stars at low-metallicity.
This significantly reduces the overproduction of nitrogen-enhanced stars that
was a major problem in using the high-mass star dominated IMF in previous
studies. Our results also demonstrate that the use of the present day IMF for
all time in chemical evolution models results in the overproduction of Type I.5
supernovae. More data on stellar abundances will help to understand how the IMF
has changed and what caused such a transition.Comment: 8 pages, 2 figures, accepted by MNRAS Lette
Formation History of Metal-Poor Halo Stars with Hierarchical Model and the Effect of ISM accretion on the Most Metal-Poor Stars
We investigate the star formation and chemical evolution in the early
universe by considering the merging history of the Galaxy in the {\Lambda}CDM
scenario according to the extended Press-Schechter theory. We give some
possible constraints from comparisons with observation of extremely metal-poor
(EMP) stars. We demonstrate that (1) The hierarchical structure formation can
explain the characteristics of the observed metallicity distribution function
(MDF) including a break around [Fe/H]~-4. (2) A high mass IMF of peak mass
~10Msun with the contribution of binaries, derived from the statistics of
carbon enhanced EMP stars (Komiya et al. 2007), predicts the frequency of
low-mass survivors consistent with the number of EMP stars observed for
-4~<[Fe/H]~<-2.5. (3) The stars formed from primordial gas before the first
supernova explosions in their host mini-halos are assigned to the HMP stars
with [Fe/H]~-5. (4) There is no indication of significant changes in the IMF
and the binary contribution at metallicity -4~<[Fe/H]~<-2.5, or even larger as
long as the field stars of Galactic halo are concerned. We further study the
effects of the surface pollution through the accretion of ISM along the
chemical and dynamical evolution of the Galaxy for low-mass Pop.III and EMP
survivors. Because of shallower potential of smaller halos, the accretion of
ISM in the mini-halos in which these stars were born dominates the surface
metal pollution. This can account for the surface iron abundances as observed
for the HMP stars if the cooling and concentration of gas in their birth
mini-halos is taken into account. We also study the feedback effect from the
very massive Pop. III stars. The metal pre-pollution by PISNe is shown to be
compatible with the observed lack of their nucleosynthetic signatures when some
positive feedback on gas cooling works and changes IMF from being very massive
to being high mass.Comment: 20 pages, 14 figures. ApJ accepte
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