110 research outputs found
SAGA: Stellar Abundances for Galactic Archaeology
A tutorial for the Stellar Abundances for Galactic Archaeology (SAGA)
database is presented. This paper describes the outline of the database,
reports the current status of the data compilation and known problems, and
presents plans for future updates and extensions.Comment: Proceedings of the conference, "Nuclei in the Cosmos XII", invited
talk at Online Support sessio
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
Stellar Evolution Constraints on the Triple-Alpha Reaction Rate
We investigate the quantitative constraint on the triple-alpha reaction rate
based on stellar evolution theory, motivated by the recent significant revision
of the rate proposed by nuclear physics calculations. Targeted stellar models
were computed in order to investigate the impact of that rate in the mass range
of 0.8 < M / Msun < 25 and in the metallicity range between Z = 0 and Z = 0.02.
The revised rate has a significant impact on the evolution of low- and
intermediate-mass stars, while its influence on the evolution of massive stars
(M >~ 10 Msun) is minimal. We find that employing the revised rate suppresses
helium shell flashes on AGB phase for stars in the initial mass range 0.8 < M /
Msun < 6, which is contradictory to what is observed. The absence of helium
shell flashes is due to the weak temperature dependence of the revised
triple-alpha reaction cross section at the temperature involved. In our models,
it is suggested that the temperature dependence of the cross section should
have at least nu > 10 at T = 1 - 1.2 x 10^8 K where the cross section is
proportional to T^{nu}. We also derive the helium ignition curve to estimate
the maximum cross section to retain the low-mass first red giants. The
semi-analytically derived ignition curves suggest that the reaction rate should
be less than ~ 10^{-29} cm^6 s^{-1} mole^{-2} at ~ 10^{7.8} K, which
corresponds to about three orders of magnitude larger than that of the NACRE
compilation. In an effort to compromise with the revised rates, we calculate
and analyze models with enhanced CNO cycle reaction rates to increase the
maximum luminosity of the first giant branch. However, it is impossible to
reach the typical RGB tip luminosity even if all the reaction rates related to
CNO cycles are enhanced by more than ten orders of magnitude.Comment: 14 pages, 8 figures, accepted by the Ap
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
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