Search

Binary systems are important probes of the origin of stars with peculiar chemical features through the interactions between two stars. We have studied the evolution of a metal-free massive binary after the collision of the supernova ejecta with a low-mass companion. Theoretical models are developed using simulations of binaries after core-collapse supernovae using stellar evolution models, supernova ejecta models, and hydrodynamic simulations of the system consisting of supernova ejecta and companion stars. We find that these first star survivors will be observed as metal-rich halo stars in our Galaxy. In combination with the theoretical research, we looked for observational counterparts in the Galactic halo using the observational database where elemental abundances and kinematic data are available. We have also searched for the progenitor binary systems based on radial velocity monitoring. We report the current status of the search for massive binaries in the solar vicinity. The proposed scenario demands a new channel of star formation in the early universe and is a supplementary scenario for the origin of the known metal-poor stars.

The search for the first stars formed from metal-free gas in the universe is one of the key issues in astronomy because it relates to many fields, such as the formation of stars and galaxies, the evolution of the universe, and the origin of elements. It is not still clear if metal-free first stars can be found in the present universe. These first stars are thought to exist among extremely metal-poor stars in the halo of our Galaxy. Here we propose a new scenario for the formation of low-mass first stars that have survived until today and observational counterparts in our Galaxy. The first stars in binary systems, consisting of massive- and low-mass stars, are examined using stellar evolution models, simulations of supernova ejecta colliding with low-mass companions, and comparisons with observed data. These first star survivors will be observed as metal-rich halo stars in our Galaxy. We may have identified a candidate star in the observational database where elemental abundances and kinematic data are available. Our models also account for the existence of several solar-metallicity stars in the literature having space velocities equivalent to the halo population. The proposed scenario demands a new channel of star formation in the early universe and is a supplementary scenario for the origin of the known metal-poor stars., 41 pages, 14 figures, accepted for publication in PASJ (to be published in open access)

The chemical abundances of very metal-poor stars provide important constraints on the nucleosynthesis of the first generation of stars and early chemical evolution of the Galaxy. We have obtained high-resolution spectra with the Subaru Telescope for candidates of very metal-poor stars selected with a large survey of Galactic stars carried out with LAMOST. In this series of papers, we report on the elemental abundances of about 400 very metal-poor stars and discuss the kinematics of the sample obtained by combining the radial velocities measured in this study and recent astrometry obtained with Gaia. This paper provides an overview of our survey and follow-up program, and reports radial velocities for the whole sample. We identify seven double-lined spectroscopic binaries from our high-resolution spectra, for which radial velocities of the components are reported. We discuss the frequency of such relatively short-period binaries at very low metallicity., Comment: 24 pages, 9 figures, 5 tables, to appear in ApJ

We combine asteroseismology, optical high-resolution spectroscopy, and kinematic analysis for 26 halo red giant branch stars in the \textit{Kepler} field in the range of $-2.52$), which constrains a relative age dispersion to $-1.7$ into two [{Mg}/{Fe}] groups. While [$\alpha$/{Fe}] and [{Eu}/{Mg}] ratios are different between the two subsamples, [$s$/Eu], where $s$ stands for Ba, La, Ce, and Nd, does not show a significant difference. These abundance ratios suggest that the chemical evolution of the low-Mg population is contributed by type~Ia supernovae, but not by low-to-intermediate mass asymptotic giant branch stars, providing a constraint on its star formation timescale as $100\,\mathrm{Myr}, Comment: 44 pages. accepted version

Mixing and fallback models in faint supernova models are supposed to reproduce the abundance patterns of observed carbon-enhanced metal-poor (CEMP) stars in the Galactic halo. A fine tuning of the model parameters for individual stars is required to reproduce the observed ratios of carbon to iron. We focus on extremely metal-poor stars formed out of the ejecta from the mixing and fallback models using a chemical evolution model. Our chemical evolution models take into account the contribution of individual stars to chemical enrichment in host halos together with their evolution in the context of the hierarchical clustering. Parametrized models of mixing and fallback models for Pop. III faint supernovae are implemented in the chemical evolution models with merger trees to reproduce the observed CEMP stars. A variety of choices for model parameters on star formation and metal-pollution by faint supernovae is unable to reproduce the observed stars with [Fe/H] < -4 and [C/H] > -2, which are the majority of CEMP stars among the lowest metallicity stars. Only possible solution is to form stars from small ejecta mass, which produces an inconsistent metallicity distribution function. We conclude that not all the CEMP stars are explicable by the mixing and fallback models. We also tested the contribution of binary mass transfers from AGB stars that are also supposed to reproduce the abundances of known CEMP stars. This model reasonably reproduces the distribution of carbon and iron abundances simultaneously only if we assume that long-period binaries are favored at [Fe/H] < -3.5., 17 pages, 19 figures, accepted for publication in the Astrophysical Journal

We report on the very low $\alpha$-element abundances of a group of metal-poor stars with high orbital energy and with large retrograde motion in the Milky Way halo, whose excess has been reported recently from metallicity and kinematics. We constructed a sample of halo stars with measured abundances and precise kinematics, including $\sim 880$ stars with [{{Fe}/{H}}]$, Comment: accepted for publication in ApJL

Lithium is a unique element that is produced in the Big Bang nucleosynthesis but is destroyed by nuclear reactions inside stars. As a result, almost constant lithium abundance is found in unevolved main-sequence metal-poor stars, although the value is systematically lower than that expected from the standard Big Bang nucleosynthesis models, whereas lithium abundances of red giants are more than one order of magnitudes lower than those of unevolved stars. There are, however, a small fraction of metal-poor stars that show extremely high lithium abundances, which is not explained by standard stellar evolution models. We have discovered 12 new very metal-poor stars that have enhancement of lithium by more than 10 times compared with typical metal-poor stars at similar evolutionary stages by the large-scale spectroscopic survey with LAMOST and the follow-up high-resolution spectroscopy with the Subaru Telescope. The sample shows a wide distribution of evolutionary stages from subgiants to red giants with the metallicity of −3.3

Li abundances in the bulk of low-mass metal-poor stars are well reproduced by stellar evolution models adopting a constant initial abundance. However, a small number of stars have exceptionally high Li abundances, for which no convincing models have been established. We report on the discovery of 12 very metal-poor stars that have large excesses of Li, including an object having more than 100 times higher Li abundance than the values found in usual objects, which is the the largest excess in metal-poor stars known to date. The sample is distributed over a wide range of evolutionary stages, including five unevolved stars, showing no abundance anomaly in other elements. The results indicate the existence of an efficient process to enrich Li in a small fraction of low-mass stars at the main-sequence or subgiant phase. The wide distribution of Li-rich stars along the red giant branch could be explained by dilution of surface Li by mixing that occurs when the stars evolve into red giants. Our study narrows down the problem to be solved to understand the origins of Li-excess found in low-mass stars, suggesting the presence of unknown process that affects the surface abundances preceding red giant phases., 12 pages, 3 figures, 2 tables, ApJ Letters in press

We have constructed the database of stars in the local group using the extended version of the SAGA (Stellar Abundances for Galactic Archaeology) database that contains stars in 24 dwarf spheroidal galaxies and ultra faint dwarfs. The new version of the database includes more than 4500 stars in the Milky Way, by removing the previous metallicity criterion of [Fe/H], Comment: 48 pages in single column format, 14 figures, accepted by PASJ. The SAGA database is available at http://sagadatabase.jp

We explore the possibility of observing Population III (Pop~III) stars, born of the primordial gas. Pop~III stars with masses below $0.8 M_\odot$ should survive to date though are not observed yet, but the existence of stars with low metallicity as [Fe/H]$ < -5$ in the Milky Way halo suggests the surface pollution of Pop~III stars with accreted metals from the interstellar gas after birth. In this paper, we investigate the runaway of Pop~III stars from their host mini-halos, considering the ejection of secondary members from binary systems when their massive primaries explode as supernovae. These stars save them from the surface pollution. By computing the star formation and chemical evolution along with the hierarchical structure formation based on the extended Press--Schechter merger trees, we demonstrate that several hundreds to tens of thousands of low-mass Pop~III stars escape from the building blocks of the Milky Way. The second and later generations of extremely metal-poor (EMP) stars are also escaped from the mini-halos. We discuss the spatial distributions of these escaped stars by evaluating the distances between the mini-halos in the branches of merger trees under the spherical collapse model of dark matter halos. It is demonstrated that the escaped stars distribute beyond the stellar halo with a density profile close to the dark matter halo, while the Pop~III stars are slightly more centrally concentrated . Some escaped stars leave the Milky Way and spread into the intergalactic space. Based on the results, we discuss the feasibility of observing the Pop~III stars with the pristine surface abundance., 11 pages, 11 figures, Accepted for publication in ApJ

We investigate star formation and chemical evolution in the early universe by considering the merging history of the Galaxy in the Λ cold dark matter scenario according to the extended Press-Schechter theory. We give some possible constraints from comparisons with observation of extremely metal-poor (EMP) stars, made available by the recent large-scale surveys and by the follow-up high-resolution spectroscopy. We demonstrate that (1) the hierarchical structure formation can explain the characteristics of the observed metallicity distribution function including a break around [Fe/H] = –4; (2) a high-mass initial mass function (IMF) of peak mass ~10 M ☉ with the contribution of binaries, derived from the statistics of carbon-enhanced EMP stars, 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 (SN) explosions in their host mini-halos are assigned to the hyper metal-poor (HMP) stars with [Fe/H] ~ –5; and (4) there is no indication of significant changes in the IMF and the binary contribution at metallicities –4 [Fe/H] –2.5, or even larger, as far as the field stars of the Galactic halo are concerned. We further study the effects of surface pollution through the accretion of interstellar matter (ISM) along the chemical and dynamical evolution of the Galaxy for low-mass Population III and EMP survivors. Because of the 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 are taken into account. We also study the feedback effect from the very massive Population III stars. The metal pre-pollution by pair-instability SNe is shown to be compatible with the observed lack of their nucleosynthetic signatures when some positive feedback on gas cooling works and changes the IMF from being very massive to being high mass.

Extremely metal-poor (EMP) stars in the Galactic halo are unique probes into the early universe and the first stars. We construct a new program to calculate the formation history of EMP stars in the early universe with the chemical evolution, based on the merging history of the Galaxy. We show that the hierarchical structure formation model reproduces the observed metallicity distribution function and also the total number of observed EMP stars, when we take into account the high-mass initial mass function and the contribution of binaries, as proposed by Komiya et al. The low-mass survivors divide into two groups of those born before and after the mini-halos are polluted by their own first supernovae. The former has observational counterparts in the hyper metal-poor (HMP) stars below [Fe/H] - 4. In this Letter, we focus on the origin of the extremely small iron abundances of HMP stars. We compute the change in the surface abundances of individual stars through the accretion of the metal-enriched interstellar gas along with the dynamical and chemical evolution of the Galaxy, to demonstrate that after-birth pollution of Population III stars is sufficiently effectivemore » to explain the observed abundances of HMP stars. Metal pre-enrichment by possible pair instability supernovae is also discussed, to derive constraints on their roles and on the formation of the first low-mass stars.« less

We exploit the recent observations of extremely metal-poor (EMP) stars in the Galactic halo and investigate the constraints on the IMF of the stellar population that left these low-mass survivors of [Fe/H], Comment: 19 pages, 9 figures, accepted in ApJ

Extremely metal-poor (EMP) stars are thought to be formed in the low-mass protogalaxies as building blocks of the Milky Way and can be probes to investigate the early stage of galaxy formation and star formation in the early universe. We study the formation history of EMP stars in the Milky Way halo using a new model of chemical evolution based on the hierarchical theory of the galaxy formation. We construct the merging history of the Milky Way halo based on the extended Press-Schechter formalism, and follow the star formation and chemical evolution along the merger tree. The abundance trends and number of low-mass stars predicted in our model are compared with those of observed EMP stars. Additionally, in order to clarify the origin of hyper metal poor stars, we investigate the change of the surface metal abundances of stars by accretion of interstellar matter. We also investigate the pre-enrichment of intergalactic matter by the first supernovae.

It is known that the carbon-enhanced, extremely metal-poor (CEMP) stars constitute a substantial proportion in the extremely metal-poor (EMP) stars of the Galactic Halo, by far larger than CH stars in Population II stars. We investigate their origin with taking into account an additional evolutionary path to the surface carbon-enrichment, triggered by hydrogen engulfment by the helium flash convection, in EMP stars of $[Fe/H] \lesssim -2.5$. This process is distinct from the third dredge-up operating in more metal-rich stars and also in EMP stars. In binary systems of EMP stars, the secondary stars become CEMP stars through mass transfer from the primary stars of low and intermediate masses, which have developed the surface carbon-enhancement. Our binary scenario can predict the variations in the abundances not only for carbon but also for nitrogen and s-process elements and reasonably explain the observed properties such as the stellar distributions with respect to the carbon abundances, the binary periods, and the evolutionary stages. Furthermore, from the observed frequencies of CEMP stars with and without s-process element enhancement, we demonstrate that the initial mass function of EMP stars need to give the mean mass $~10\msun$ under the reasonable assumptions on the distributions of orbital separations and mass ratio of binary components. This also indicates that the currently observed EMP stars were exclusively born as the secondary members of binaries, making up $\sim 10%$ remnants of EMP binary systems of mass $~10^8\msun$ in total; in addition to CEMP stars with white dwarf companions, a significant fraction of them have experienced supernova explosions of their companions. We discuss the implications of the present results in relation to the formation of Galactic halo., Comment: 66 pages, 12 figures, 2 tables Accepted for publication in ApJ

The progenitor systems for type Ia supernovae are still controversial. One of the methods to test the proposed scenario for the progenitor systems is to identify companions that are supposed to survive according to the so-called single degenerate scenario. These companions might be affected by supernova ejecta. We present several numerical simulations of surviving red-giant companions whose envelopes were stripped and heated. We find that red-giants with less-massive helium cores ($\lesssim0.30\,M_{\odot}$) can be so faint after the supernovae that we cannot detect them. In addition, we apply the results to the case of SNR 0509-67.5, and put constraints on the helium core mass, envelope stripping, and energy injection under the single degenerate scenario for type Ia supernovae., 10 pages, 8 figures, accepted for publication in PASJ

Carbon-enhanced metal-poor (CEMP) stars play a key role in characterising the star formation history of the Galactic halo. In particular, most stars show carbon enhancement among the lowest metallicity range. In this paper, we explore the origin of the three most iron-poor stars (called hyper metal-poor or HMP stars) with [Fe/H] < −5 (which are all CEMP stars) in terms of binary hypothesis. We examined the metallicity distribution function of extremely metal-poor stars ([Fe/H]≲−2.5) using the merger tree models with chemical evolution. We also tried to reproduce the abundance patterns of these stars using the nucleosynthesis models of a hydrogen ingestion into the helium-flash convective zones of metal-free AGB stars. Both experiments suggest that the most iron-poor stars are likely to be the first generation stars born out of primordial gas with their surface polluted by the mass loss wind from AGB stars and the ejecta of supernovae in the early universe.

We investigate the origin of the most iron-poor stars including SMSS J031300.36-670839.3 with [Fe/H] < -7.52. We compute the change of surface metallicity of stars with the accretion of interstellar matter (ISM) after their birth using the chemical evolution model within the framework of the hierarchical galaxy formation. The predicted metallicity distribution function agrees very well with that observed from extremely metal-poor stars. In particular, the lowest metallicity tail is well reproduced by the Population III stars whose surfaces are polluted with metals through ISM accretion. This suggests that the origin of iron group elements is explained by ISM accretion for the stars with [Fe/H]$\lesssim -5$. The present results give new insights into the nature of the most metal-poor stars and the search for Population III stars with pristine abundances., Accepted for publication in ApJ Letters

The fraction of binary systems in various stellar populations of the Galaxy and the distribution of their orbital parameters are important but not well-determined factors in studies of star formation, stellar evolution, and Galactic chemical evolution. While observational studies have been carried out for a large sample of nearby stars, including some metal-poor, Population II stars, almost no constraints on the binary nature for extremely metal-poor (EMP; [Fe/H] < -3.0) stars have yet been obtained. Here we investigate the fraction of double-lined spectroscopic binaries and carbon-enhanced metal-poor (CEMP) stars, many of which could have formed as pairs of low-mass and intermediate-mass stars, to estimate the lower limit of the fraction of binary systems having short periods. The estimate is based on a sample of very metal-poor stars selected from the Sloan Digital Sky Survey, and observed at high spectral resolution in a previous study by Aoki et al. That survey reported three double-lined spectroscopic binaries and 11 CEMP stars, which we consider along with a sample of EMP stars from the literature compiled in the SAGA database. We have conducted measurements of the velocity components for stacked absorption features of different spectral lines for each double-lined spectroscopic binary. Our estimate indicates that the fraction of binary stars having orbital periods shorter than 1000 days is at least 10 %, and possibly as high as 20 %, if the majority of CEMP stars are formed in such short-period binaries. This result suggests that the period distribution of EMP binary systems is biased toward short periods, unless the binary fraction of low-mass EMP stars is significantly higher than that of other nearby stars., 20 Pages, 3 figures, accepted publication in Astronomical Journal

We investigate the chemical enrichment of r-process elements in the early evolutionary stages of the Milky Way halo within the framework of hierarchical galaxy formation using a semi-analytic merger tree. In this paper, we focus on heavy r-process elements, Ba and Eu, of extremely metal-poor (EMP) stars and give constraints on their astronomical sites. Our models take into account changes of the surface abundances of EMP stars by the accretion of interstellar matter (ISM). We also consider metal-enrichment of intergalactic medium (IGM) by galactic winds and the resultant pre-enrichment of proto-galaxies. The trend and scatter of the observed r-process abundances are well reproduced by our hierarchical model with $\sim 10\%$ of core-collapse supernovae in low-mass end ($\sim 10M_{\odot}$) as a dominant r-process source and the star formation efficiency of $\sim 10^{-10} \hbox{yr}^{-1}$. For neutron star mergers as an r-process source, their coalescence timescale has to be $ \sim 10^7$yrs, and the event rates $\sim 100$ times larger than currently observed in the Galaxy. We find that the accretion of ISM is a dominant source of r-process elements for stars with [Ba/H] < -3.5. In this model, a majority of stars at [Fe/H] < -3 are formed without r-process elements but their surfaces are polluted by the ISM accretion. The pre-enrichment affects $\sim 4\%$ of proto-galaxies, and yet, is surpassed by the ISM accretion in the surface of EMP stars., 21 pages 15 figures, accepted by ApJ

Nuclear reaction rates play a crucial role in the evolution of stars. For low-mass stars, the triple-α reaction controls the helium burning stars in the red giant and asymptotic giant branch (AGB) phase. More importantly, the cross section of the triple-α reaction has a great impact on the helium ignition at the center of the electron degenerate helium core of red giants and on the helium shell flashes of AGB stars. It is to be noted that stellar evolution models are influenced not only by the value of the cross section, but also by the temperature dependence of the reaction rate. In this paper, I present the impact of the triple-α reaction rates on the evolution of low-mass metal-free stars and intermediate-mass AGB stars. According to the previous study, the constraint on the triple-α reaction rate is derived based on stellar evolution theory. It is found that the recent revisions of the rate proposed by nuclear physics calculations satisfy the condition for the ignition of the helium core flash in low-mass stars.

Aims. Our main goals are to get a deeper insight into the evolution and final fates of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to investigate the C, N, and O yields of these stars. Methods. Using the Monash University Stellar Evolution code MONSTAR we computed and analysed the evolution of stars of metallicity Z = 10-5 and masses between 4 and 9 M¿, from their main sequence until the late thermally pulsing (super) asymptotic giant branch, TP-(S)AGB phase. Results. Our model stars experience a strong C, N, and O envelope enrichment either due to the second dredge-up process, the dredge-out phenomenon, or the third dredge-up early during the TP-(S)AGB phase. Their late evolution is therefore similar to that of higher metallicity objects. When using a standard prescription for the mass loss rates during the TP-(S)AGB phase, the computed stars are able to lose most of their envelopes before their cores reach the Chandrasekhar mass (mCh), so our standard models do not predict the occurrence of SNI1/2 for Z = 10-5 stars. However, we find that the reduction of only one order of magnitude in the mass-loss rates, which are particularly uncertain at this metallicity, would prevent the complete ejection of the envelope, allowing the stars to either explode as an SNI1/2 or become an electron-capture SN. Our calculations stop due to an instability near the base of the convective envelope that hampers further convergence and leaves remnant envelope masses between 0.25 M¿ for our 4 M¿ model and 1.5 M¿ for our 9 M¿ model. We present two sets of C, N, and O yields derived from our full calculations and computed under two different assumptions, namely, that the instability causes a practically instant loss of the remnant envelope or that the stars recover and proceed with further thermal pulses. Conclusions. Our results have implications for the early chemical evolution of the Universe and might provide another piece for the puzzle of the carbon-enhanced EMP star problem.

Chemical compositions are determined based on high-resolution spectroscopy for 137 candidate extremely metal-poor (EMP) stars selected from the Sloan Digital Sky Survey (SDSS) and its first stellar extension, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). High-resolution spectra with moderate signal-to-noise (S/N) ratios were obtained with the High Dispersion Spectrograph of the Subaru Telescope. Most of the sample (approximately 80%) are main-sequence turn-off stars, including dwarfs and subgiants. Four cool main-sequence stars, the most metal-deficient such stars known, are included in the remaining sample. Good agreement is found between effective temperatures estimated by the SEGUE stellar parameter pipeline, based on the SDSS/SEGUE medium-resolution spectra, and those estimated from the broadband $(V-K)_0$ and $(g-r)_0$ colors. Our abundance measurements reveal that 70 stars in our sample have [Fe/H] $ < -3$, adding a significant number of EMP stars to the currently known sample. Our analyses determine the abundances of eight elements (C, Na, Mg, Ca, Ti, Cr, Sr, and Ba) in addition to Fe. The fraction of carbon-enhanced metal-poor stars ([C/Fe]$> +0.7$) among the 25 giants in our sample is as high as 36%, while only a lower limit on the fraction (9%) is estimated for turn-off stars. This paper is the first of a series of papers based on these observational results. The following papers in this series will discuss the higher-resolution and higher-S/N observations of a subset of this sample, the metallicity distribution function, binarity, and correlations between the chemical composition and kinematics of extremely metal-poor stars., 11 Figures, 8 Tables, to appear in AJ

Extremely metal-poor (EMP) stars are cosmic stellar relics in the Milky Way halo. We investigate their formation history and elemental abundances using a hierarchical chemical evolution model. We focus on the r-process elements in this study. We show that distribution of the rprocess element abundances of EMP stars are well reproduced by our hierarchical model. Electron capture supernovae of which progenitor mass is 8 - 10M⊙ thought to be dominant source of rprocess elements. We also show that surface pollution by accretion of interstellar matter significantly changes the surface r-process element abundance of EMP stars.

We investigate the quantitative constraint on the triple-α 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/M⊙≤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 > 10M⊙) 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/M⊙≤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-α 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 ν > 10 at T = 1−1.2×108K wher...

We construct a binary population synthesis model to explore the star formation history of the Galaxy. The model includes the effects of AGB evolution and binary mass transfer for a given IMF and binary period distribution function. We discuss the origins of extremely metal-poor stars with enhancement of carbon or nitrogen, with the possible effect of mass loss at low-metallicity taken into account. Our results strongly support high-mass dominated star formation during the early epoch of the Galaxy in order to explain the observed frequency of carbon and nitrogen enhancements that are thought to result from mass transfer from a former AGB binary companion. Our model also suggests that the IMF had a transition phase at [Fe/H] ∼ −2.

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., 14 pages, 8 figures, accepted by the ApJ

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, Comment: 35 pages, 27 figures, 6 tables, accepted by MNRAS, database to reproduce figures is available at http://saga.sci.hokudai.ac.jp

Extremely metal‐poor (EMP) stars in the Galactic halo are the remaining survivors of the early generations of stars. Their surface element abundances are signature of the nucleosynthesis in the first and second generation of stars. From comparison between the observationsof EMP stars and calculations of stellar evolution, it is suggested that typical mass of EMP stars are much higher than more metal‐rich one and the majority of observed EMP stars are formed in binary systems. I review the effect of binarity and difference of initial mass function on the EMP stars and chemical evolution of galaxy. We calculate chemical enrichment history in the context of hierarchical Galaxy formation with high mass IMF and binary contribution. We compare resultant abundance distributions with observations. Additionally, I discuss the origin of the most metal‐deficient stars known to date.

Extremely metal‐poor (EMP) stars in the Galactic halo are stellar relics from the early universe and can be probes into formation of the Galaxy. We construct a hierarchical chemical evolution model for the early stages of the Galaxy formation and investigate about element abundances of EMP stars theoretically. Especially, we consider the possible difference of IMF of EMP stars and contribution of binary. We compare the predicted MDFs and abundance distributions with observations, and discuss about IMF and contribution from hypernovae in the early universe.

The extremely luminous supernova SN2006gy is explained in the same way as other SNIIn events: light is produced by a radiative shock propagating in a dense circumstellar envelope formed by a previous weak explosion. The problems in the theory and observations of multiple-explosion SNe IIn are briefly reviewed., 9 pages, 6 figures, LateX aipproc.cls. A bit more details and color added to Fig.3. The 10th International Symposium on Origin of Matter and Evolution of Galaxies (OMEG07), Sapporo, Japan, December 2007

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]

We investigate accretion flows around supermassive binary black holes (BBHs) with the orbital eccentricity e = 0.5, the semi‐major axis a = 0.01 pc, and the low mass ratio of the secondary black hole (BH) to the primary BH q = 0.1. In the simulations we consider a triple‐disk system composing of two accretion disks around BHs and one circumbinary disk surrounding the two. The circumbinary disk works as a mass reservoir. We confirm that a non‐axisymmetric accretion disk is formed around each BH. The X‐ray luminosity of the secondary BH exhibits the double peaks every binary orbit, whereas that of the primary BH shows a single peak. Such properties can not be seen in the case of equal mass BBHs and therefore provide a potentially important observational signature of supermassive BBHs with low mass ratios.

The standard cosmological model predicts that first cosmological objects are formed when the age of the universe is a few hundred million years. Recent theoretical works and numerical simulations consistently suggests that the first objects are very massive primordial stars. We introduce the key physics and explain why the first stars are thought to be massive, rather than to be low‐mass stars. The state‐of‐the‐art simulation includes all the relevant atomic and molecular physics to follow the thermal evolution of a prestellar gas cloud to very high densities. We also show the result of a simulation of the formation of primordial stars in a reionized gas.

The Lithium (Li) abundances measured for very metal‐poor turn‐off (unevolved) stars have been interpreted as the result of Big Bang nucleosynthesis. However, the value is lower by a factor of two or three than the prediction of standard Big Bang nucleosynthesis models, adopting the cosmological parameters determined by the measurements of cosmic microwave background radiation with the WMAP satellite. Moreover, the recent measurements for extremely metal‐poor stars (objects having iron abundances less than 1/1000th solar) suggest a scatter of the Li abundance, or a possible decreasing trend with decreasing metallicity. In order to further investigate the Li production and destruction processes in the very early universe, we have determined Li abundances for extremely metal‐poor stars based on high‐resolution spectra for the resonance line of neutral Li. The result of our analysis, combined with previous measurements, indicates that the Li abundances of extremely metal‐poor stars are, on average, lower than those of stars with higher metallicity, while the scatter or trend of the Li abundance remains unclear. We discuss possible reasons for the lower Li abundances in extremely metal‐poor stars, such as depletion of Li in low‐mass unevolved stars, or destruction of Li by the first generations of massive progenitors.

Extremely metal‐poor (EMP) stars in the Milky Way halo are the low‐mass survivors of the first stars and their descendants that were born and first lit the universe during the “cosmological dark age”. We use their properties to study the star formation and galaxy evolution process in the early universe. From the analysis of their surface abundances, it has been shown that EMP stars with metallicity [Fe/H]≲−2.5 were born under the initial mass function (IMF) peaked around 10 M⊙ and that most of their relic stars currently observed are the secondary members of binary systems. We investigate the star formation and early chemical evolution with the derived IMF in the context of the hierarchical formation process of the Galaxy to demonstrate that the metallicity distribution function (MDF) of these halo stars is well reproduced in terms of the high‐mass IMF and the binary origin of low‐mass survivors. Our results suggest that the three most iron‐poor stars of [Fe/H]

Primitive meteorites and interplanetary dust particles contain presolar grains: pristine solid samples of stellar material. These grains of Stardust are a new source of information on nuclear astrophysics, complementary to traditional astronomical observations. Identified phases include silicates, oxides, SiC, graphite (including sub‐grains of metal and carbides), and silicon nitride. Most grains can be attributed to either asymptotic giant branch (AGB) stars or Type II supernovae, based on their isotopic compositions. Detailed high‐precision isotopic data for the grains provide important and unique constraints on nuclear processes occurring in these stars.

This paper reviews the recent progress in the research of equation of state of dense matter for supernova simulations. The equation of state plays an important role to clarify the mechanism of core‐collapse supernovae, the formation of compact objects and the properties of supernova neutrinos. We describe the problems of explosion mechanism in current supernova simulations, putting emphasis on the equation of state. We discuss also the topics on the birth of neutron stars (or black holes) and the associated neutrino bursts, which reflect the properties of dense matter.

We have obtained infrared spectra of carbon stars in four nearby galaxies—the Large and Small Magellanic Clouds, and the Fornax dwarf spheroidal galaxy. Our primary aim is to investigate mass‐loss rate and gas compositions of these stars as a function of metallicity, by comparing AGB stars in several galaxies with different metallicities. These stars were observed using the Infrared Spectrometer (IRS) onboard the Spitzer Space Telescope which covers 5–35 μm region, and the Infrared Spectrometer And Array Camera (ISAAC) on the Very Large Telescope which covers the 2.9–4.1 μm region. HCN, CH and C2H2 molecular bands, as well as SiC and MgS dust features are identified in the spectra. We find no evidence that mass‐loss rates depend on metallicity. Carbon stars are strongly affected by carbon production during the AGB phase; primarily mass‐loss of carbon‐rich stars are driven by amorphous carbon dust grains, that explains the little metallicity dependence of mass‐loss rate for carbon‐rich stars. We found that...

We summarize evidence that the halo of the Milky Way comprises two different, and broadly overlapping, stellar components. The two structures exhibit different chemical compositions, spatial distributions, and kinematics. These results were obtained through an analysis of more than 20,000 calibration stars from the Sloan Digital Sky Survey (SDSS). The duality of the stellar halo directly impacts galaxy formation models, for the Milky Way and other large spirals., Comment: 6 pages, 4 figures, to appear in Proceedings of the OMEG07 Conference, held in December 2007

The information on surface abundances in extremely metal‐poor (EMP) stars in the Galactic halo is a useful probe of the early universe. Therefore, it is very important to understand the modification of abundances in EMP stars by internal and/or external pollution through mixing processes and binary interactions in stellar evolution. We computed various sets of model stars for 0.8–9.0–M⊙ in mass and −5< [Fe/H]

The origin of matter and many interesting phenomena occurred during the big bang between the first instants of cosmic expansion time of the surface of photon last scattering. Even so, there are only two means by which to probe the physics of the big bang epoch. One is the observed power spectrum of temperature fluctuations in the cosmic microwave background. This spectrum derives from a combination of physics from the first instants of cosmic inflation, and the physics of matter and radiation at the photon last scattering surface. The other probe is the observed ashes of primordial nucleosynthesis which occurred during the radiation dominated epoch from about 1 sec to 103 sec into the big bang. This paper summarizes these two cosmic probes and their roles in motivating and constraining new cosmological paradigms. Among the topics discussed are the limits which these probes place upon the nature of dark matter and dark energy, along with possible insights into physics beyond the standard model of particle physics.

Observations of metal poor halo stars exhibit a possible plateau of 6Li abundance as a function of metallicity similar to that for 7Li, suggesting a big bang origin. However, the inferred primordial abundance of 6Li is ∼1000 times larger than that predicted by standard big bang nucleosynthesis (BBN) for the baryon‐to‐photon ratio inferred from the WMAP data. On the other hand, the inferred 7Li primordial abundance is about 3 times smaller than the prediction. We study a possible simultaneous solution to both the problems of underproduction of 6Li and overproduction of 7Li in BBN. This solution involves a hypothetical massive, negatively‐charged leptonic particle that would bind to the light nuclei produced in BBN, but would decay long before it could be detected. Because the particle gets bound to the existing nuclei after the cessation of the usual big bang nuclear reactions, a second longer epoch of nucleosynthesis can occur among X‐nuclei which have reduced Coulomb barriers. We numerically carry out a ...

Our group has developed a new picture of the structure of Cas A's explosion using 5–40 micron images and spectra from the Spitzer Space Telescope. In this picture, two roughly spherical shocks (forward and reverse) were initially set up by the outer layers of the exploding star. Deeper layers were ejected in a highly flattened structure with large protrusions in the plane of the flattening; some of these are visible as jets. As these aspherical deeper layers encounter the reverse shock at different locations, they become visible across the electromagnetic spectrum, with different nucleosynthesis layers visible in different directions. In the infrared, we see the gas lines of Ar, Ne, O, Si, S, and Fe at different locations, along with higher ionization states of the same elements visible in the optical and X‐ray parts of the spectrum. These different nucleosynthesis layers appear to have formed characteristic types of dust, the deep layers producing dust rich in silicates, while dust from the upper layers ...

The first stars in our Universe (Population III) are the useful probes for the star formation history in the very early Universe. These stars were born in the primordial gas cloud produced soon after the Big Bang and considered to be metal‐free, i.e., comletely lack of elements heavier than lithium. In order to identify these survivors, we should consider the effect of changing surface abundances during their long lives. The surface abundances are modified by the accretion of gas from the interstellar matter and/or the binary mass transfer. The former possibly affects the iron group elements through the pollution of the parent cloud with the ejecta of supernovae. On the other hand, the latter can affect the abundance pattern of light and s‐process elements through the evolution of primary star that experienced the internal mixing and dredge‐up during thermally pulsating AGB before the final mass loss. The top three of the iron deficient stars HE1327‐2326, HE0107‐5240, and HE0557‐4840 are reported as the c...

We investigate the way how the total mass sum of neutrinos can be constrained from the neutrinoless double beta decay and cosmological probes with cosmic microwave background (WMAP 3-year results), large scale structures including 2dFGRS and SDSS data sets. First we discuss, in brief, on the current status of neutrino mass bounds from neutrino beta decays and cosmic constrain within the flat $\Lambda CMD$ model. In addition, we explore the interacting neutrino dark-energy model, where the evolution of neutrino masses is determined by quintessence scalar filed, which is responsable for cosmic acceleration today. Assuming the flatness of the universe, the constraint we can derive from the current observation is $\sum m_{\nu} < 0.87$eV at the 95 % confidence level, which is consistent with $\sum m_{\nu} < 0.68$eV in the flat $\Lambda CDM$ model. Finally we discuss the future prospect of the neutrino mass bound with weak-lensing effects.

We review our recent progresses on the presupernova evolution of massive stars in the range 11–120 M⊙ of solar metallicity. Special attention will be devoted to the effect of the mass loss rate during the Wolf‐Rayet stages in determining the structure and the physical properties of the star prior the supernova explosion. We also discuss the explosive yields and the initial mass‐remnant mass relation in the framework of the kinetic bomb induced explosion and hence the contribution of these stars to the global chemical enrichment of the interstellar medium

We propose a new explanation of some magnetic chemically peculiar (MCP) star anomalies, which is based on an assumption that such stars be the close binary systems with a secondary component being a neutron star. Within this hypothesis one can naturally explain the main anomalous features of MCP stars: first of all, an existence of the short‐lived radioactive isotopes detected in some stars (like Przybylski's star (PS) and HR465), and some others peculiarities. Also we can assume the presence of the electron‐positron annihilation emission lines (0.511 MeV) in the gamma spectrum of some MCP stars.

The origin of a double main-sequence (MS) in omega Centauri is explored. We have shown from theoretical calculations on the stellar evolution that the colors of MS stars are shifted to those of the observed blue MS if the surface layers are polluted by He-rich materials with the mass of ~ 0.1 solar mass. Stars are supposed to be polluted through numerous encounters with the ejecta descended from massive asymptotic giant-branch (AGB) stars. Two populations of stars with different kinematics exceptionally observed in omega Cen indicate that kinematically cooler stars are more polluted through encounters with AGB ejecta than kinematically hotter ones because the accretion rate is inversely proportional to the cube of the relative velocity. We propose that both of these factors split the MS in omega Cen. This theoretical scheme explains why only omega Cen exhibit a double MS and matches the amount of He necessary to produce the blue MS with that supplied from massive AGB stars. Furthermore, we predict that even if globular clusters (GCs) possess only one generation of stars, the velocity dispersion of stars broaden the MS in the color-magnitude diagram as long as the GCs are massive enough to keep the AGB ejecta after the burst of star formation. This view explains the broad MS recently found in the GC NGC 2808 which exhibits no scatter in [Fe/H] and thus is likely to consist of a single generation of stars unlike the case of omega Cen., 5 pages including 2 figures, to appear in ApJ Letters

It has been known for a long time that some red giants in globular clusters exhibit large star-to-star variations in the abundances of light elements that are not exhibited by field giants. This fact can be taken as evidence that the extra mixing mechanism(s) that operate in globular cluster giants may be consequences of star-star interactions in the dense stellar environment. In order to constrain the extra mixing mechanism(s), we study the influence of helium enrichment along the red giant branch (RGB) on the evolution of stars through the horizontal branch. Three possible modes of helium enrichment are considered, associated with close encounters of stars in the globular clusters. We show that as a consequence of the variations in the core mass as well as in the total mass due to mass loss, the color of horizontal branch models are distributed over almost all range of horizontal branch. The results are discussed in relation to the scenario for the origin of the abundance anomalies and for the effects on the morphology of horizontal branch. We argue that the star-star interactions can explain not only the source of angular momentum of rapid rotation but also provide a mechanism for the bimodal distribution of rotation rates in some globular clusters. We also propose the time elapsed from the latest core collapse phase during the gravo-thermal oscillations as the second parameter to explain the variations in HB morphology among the globular clusters., 57 pages, 16 figures, to be published on June 2006 in ApJ