19 research outputs found
The s-Process Nucleosynthesis in Extremely Metal-Poor Stars as the Generating Mechanism of Carbon Enhanced Metal-Poor Stars
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.Comment: The Proceedings of the Nuclei in the Cosmos XIV, in pres
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
Transition of the stellar initial mass function explored using binary population synthesis
The stellar initial mass function (IMF) plays a crucial role in the determination of the number of surviving stars in galaxies, of the chemical composition of the interstellar medium and of the distribution of light in galaxies. A key unsolved question i
Tidal Interactions of Red Giants with Environment Stars in Globular Clusters
We investigate the tidal interactions of a red giant with a main sequence in
the dense stellar core of globular clusters by Smoothed Particle Hydrodynamics
method. Two models of 0.8 \msun red giant with the surface radii 20 and 85
R_\sun are used with 0.6 or 0.8M_\sun main sequence star treated as a point
mass. We demonstrate that even for the wide encounters that two stars fly
apart, the angular momentum of orbital motion can be deposited into the red
giant envelope to such an extent as to trigger rotational mixing and to explain
the fast rotation observed for the horizontal branch stars, and also that
sufficient mass can be accreted on the main sequence stars to disguise their
surface convective zone with the matter from the red giant envelope. On the
basis of the present results, we discuss the parameter dependence of these
transfer characteristics with non-linear effects taken into account, and derive
fitting formulae to give the amounts of energy and angular momentum deposited
into the red giant and of mass accreted onto the perturber as functions of
stellar parameters and the impact parameter of encounter. These formulae are
applicable to the encounters not only of the red giants but also of the main
sequence stars, and useful in the study of the evolution of stellar systems
with the star-star interactions taken into account.Comment: 36 pages, 11 figures, accepte
The Stellar Abundances for Galactic Archaeology (SAGA) Database II - Implications for Mixing and Nucleosynthesis in Extremely Metal-Poor Stars and Chemical Enrichment of the Galaxy
We discuss the characteristics of known extremely metal-poor (EMP) stars in
the Galaxy using the Stellar Abundances for Galactic Archaeology (SAGA)
database (Suda et al. 2008, PASJ, 60, 1159).The analyses of carbon-enhanced
stars in our sample suggest that the nucleosynthesis in AGB stars can
contribute to the carbon enrichment in a different way depending on whether the
metallicity is above or below [Fe/H] ~ -2.5, which is consistent with the
current models of stellar evolution at low metallicity. We find the transition
of the initial mass function at [Fe/H] ~ -2 in the viewpoint of the
distribution of carbon abundance and the frequency of carbon-enhanced stars.
For observed EMP stars, we confirmed that some, not all, of observed stars
might have undergone at least two types of extra mixing to change their surface
abundances. One is to deplete the lithium abundance during the early phase of
red giant branch. Another is to decrease the C/N ratio by one order of
magnitude during the red giant branch phase. Observed small scatters of
abundances for alpha-elements and iron-group elements suggest that the chemical
enrichment of our Galaxy takes place in a well-mixed interstellar medium. We
find that the abundance trends of alpha-elements are highly correlated with
each other, while the abundances of iron-group elements are subject to
different slopes relative to the iron abundance. This implies that the
supernova yields of alpha-elements are almost independent of mass and
metallicity, while those of iron-group elements have a metallicity dependence
or mass dependence with the variable initial mass function.The occurrence of
the hot bottom burning in the mass range of 5 <~ M / Msun <~ 6 is consistent
with the initial mass function of the Galaxy peaked at ~ 10 - 12 Msun to be
compatible with the statistics of carbon enhanced stars with and without
s-process element (truncated)Comment: 35 pages, 27 figures, 6 tables, accepted by MNRAS, database to
reproduce figures is available at http://saga.sci.hokudai.ac.j