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AP Lyn and V837 Her are long-period Mira variable stars in the Milky Way. We performed VLBI Exploration of Radio Astrometry (VERA) phase-referenced observations towards H2O masers associated with AP Lyn and V837 Her. The annual parallaxes of AP Lyn and V837 Her were obtained to be 2.008 ± 0.038 mas and 1.090 ± 0.014 mas, corresponding to distances of 498 ± 10 pc and 917 ± 12 pc, respectively. From our multi-epoch infrared observations using the Kagoshima University 1 m telescope, we derived the mean J-, H-, and K′-band magnitudes of AP Lyn, V837 Her, and an additional long-period Mira variable BX Cam, whose parallax is known. We derived their pulsation periods to be 433 ± 1 d, 520 ± 1 d, and 458 ± 1 d, respectively, using the K′-band light curves. The MK–log P relation of long-period Mira variables seem to be violated by Mira variable stars with larger-than-expected MK values (like OZ Gem) in the Milky Way because of circumstellar extinction leading to an observed dimming effect. AP Lyn, V837 Her, and BX Cam (like OZ Gem) are dimming from the trend to O-rich stars in the Large Magellanic Cloud. This implies that the high metallicity of the Milky Way galaxy increases the opacity of the Mira-type variable stars and strengthens mass loss.

OZ Geminorum (OZ Gem) is a galactic Mira variable in the Milky Way (MW). We measured its annual parallax with VLBI Exploration of Radio Astrometry to be π = 0.806 ± 0.039 mas, corresponding to a distance of D = 1.24 ± 0.06 kpc. Based on multi-epoch infrared observations with the Kagoshima University 1 m telescope, we also derived the mean J-, H-, and K′-band magnitudes of OZ Gem to be 5.75 ± 0.47 mag, 4.00 ± 0.16 mag, and 2.65 ± 0.16 mag, respectively. We derived a pulsation period of OZ Gem as 592 ± 1 d from the K′-band lightcurve. From the period–luminosity (P–L) relation and two-color diagram of the Large Magellanic Cloud (LMC), the property of OZ Gem suggests that OZ Gem is assigned among the carbon-rich Mira variables. However, our optical spectroscopic observational results (with the 1.5 m Kanata telescope) confirmed OZ Gem to be an oxygen-rich Mira star with the detection of multiple titanium oxide transition absorption lines. We suggest that OZ Gem is a low-mass star evolving to an OH/IR star with large mass loss and dust formation. It is predicted that the lower limit to the initial mass of AGB stars for developing the C-rich surface chemistry is larger in the MW than in the LMC because of larger metallicity, and OZ Gem is likely to be the first example to prove this. Our results highlight the necessity of deriving the PL relation of the Milky Way with high accuracy.

We investigate X-ray bursts during the thermal evolution of an accreting neutron star which corresponds to the X-ray burster GS\ 1826-24. Physical quantities of the neutron star are included using an equation of state below and above the nuclear matter density. We adopt an equation of state and construct an approximate network that saves the computational time and calculates nuclear energy generation rates accompanying the abundance evolutions. The mass and radius of the neutron star are got by solving the stellar evolution equations from the center to the surface which involve necessary information such as the nuclear energy generation in accreting layers, heating from the crust, and neutrino emissions inside the stellar core. We reproduce the light curve and recurrence time of the X-ray burst from GS 1826-24 within the standard deviation of 1$\sigma$ for the assumed accretion rate, metallicity, and equation of state. It is concluded that the observed recurrence time is consistent with the theoretical model having metallicity of the initial CNO elements $Z_{\rm CNO}$ = 0.01. We suggest that the nuclear reaction rates responsible for the $rp$-process should be examined in detail, because the rates may change the shape of the light curve and our conclusion., Comment: 11 pages, 11 figures, accepted for publication in PTEP

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 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

MAXI J0556-332 is the hottest transient accreting neutron star at the beginning of its quiescence. A theoretical model with crustal heating indicates that an additional shallow heat source of Qshallow > 6 MeV per accreted nucleon is required in the shallow outer crust with respect to the deeper star crust by considering the observed decline in accretion rate at the end of outburst. However, the physical source of this shallow heating is still unclear. In the present investigation, we performed stellar evolutionary calculations, adopting the effects of outburst behavior of the accretion rate. As a consequence, we find that the quiescent light curve of MAXI J0556-332 can be well explained by the nuclear energy generation due to the hot CNO cycle., 9 pages, 2 figures

We show that the superburst would be originated from thermonuclear burning ignited by accumulated fuels in the deep layers compared to normal X-ray bursts. Two cases are investigated for models related to superbursts by following thermal evolution of a realistic neutron star: helium flash and carbon flash accompanied with many normal bursts. For a helium flash, the burst shows the long duration when the accretion rate is low compared with the observation. The flash could become a superburst if the burning develops to the deflagration and/or detonation. For a carbon flash accompanied with many normal bursts, after successive 2786 normal bursts during 1.81 × 109 s, the temperature reaches the deflagration temperature. This is due to the produced carbon which amount reaches to ≈0.1 in the mass fraction. The flash will develop to dynamical phenomena of the deflagration and/or detonation, which may lead to a superburst.

We analyse the chemical abundances of stars in the local group dwarf galaxies using the SAGA database. The inspection of the relationship between Eu and Ba abundances confirms an anomalously Ba-rich population in Fornax, which indicates a pre-enrichment of interstellar gas with r-process elements.

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

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 press

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.

The metal-free (Pop. III) and extremely metal-poor (EMP) stars of low- and intermediate-masses experience mixing of hydrogen into the helium convection during the early TP-AGB phase, differently from the meal-rich stars. We study the nucleosynthesis in the helium convective zone with 13C formed from mixed protons as neutron source by using a nuclear network from H through S. In the absence or scarcity of the pristine metals, the neutron-recycling reactions, 12C(n,g)13C(a,n)16O and also 16O(n,g)17O(a,n)20Ne promote the synthesis of O and light elements, including their neutron-rich isotopes and the odd atomic number elements. Based on the results, we demonstrate that the peculiar abundance patterns of C through Al observed for the three most iron-deficient, carbon-rich stars can be reproduced in terms of the nucleosynthesis in Pop. III, AGB stars in the different mass range. We argue that these three stars were born as the low-mass members of Pop. III binaries and later subject to the surface pollution by the mass transfer in the binary systems. It is also shown that the AGB nucleosynthesis with hydrogen mixing explains the abundances of C, O, Na, Mg and Al observed for most of carbon-enhanced EMP (CEMP) stars, including all CEMP-s stars with s-process elements. In addition the present results are used to single out other nucleosynthetic signatures of early generations of stars., Comment: 46 pages, 13 figures, accepted for PASJ

We explore the general characteristics of 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 overall trend of EMP stars suggests that there are at least two types of extra mixing to change the surface abundances of EMP stars. One is to deplete lithium abundance during the early phase of giant branch and another is to decrease C/N ratio by one order of magnitude during the red giant branch or AGB phase. On the other hand, these mixing processes are different from those suggested in the Galactic globular clusters because of the different relations between O, Na, Mg, and Al abundances.

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

Large-area observations of dense molecular gas were made in NH3 (J , K) = (1, 1), (2, 2), and (3, 3) inversion lines with 4: 0 5 resolution toward the molecular cloud complex of the W 51 region. The observations were a part of a NH3 survey of the Galactic star-forming regions using the Tomakomai 11-m radio telescope. NH3 (J , K) = (1, 1) and (2, 2) emission was detected in the W 51 A and B complexes, while the detection of (J , K) = (2, 2) emission was marginal in W 51 B. The rotation temperature was � 40 K in the central part of the W 51 A complex, while being � 20 K in the other positions. A weak correlation is found that the ortho-to-para ratio decreases with increasing the rotation temperature, the far-infrared luminosity and the index of the star-formation efficiency. This tendency is explained if star formation has continued for more than the time scale of the transformation between ortho- and para-NH3: active star formation on a large scale, such as the interaction of molecular clouds with a spiral arm, has made the molecular gas warmer, even in a scale of � 10 pc, and the proceeding transformation has made the ortho-to-para ratio lower.

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.

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., 36 pages, 11 figures, accepted

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

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 evolution of a zero metallicity 9 M_s star is computed, analyzed and compared with that of a solar metallicity star of identical ZAMS mass. Our computations range from the main sequence until the formation of a massive oxygen-neon white dwarf. Special attention has been payed to carbon burning in conditions of partial degeneracy as well as to the subsequent thermally pulsing Super-AGB phase. The latter develops in a fashion very similar to that of a solar metallicity 9 M_s star, as a consequence of the significant enrichment in metals of the stellar envelope that ensues due to the so-called third dredge-up episode. The abundances in mass of the main isotopes in the final ONe core resulting from the evolution are X(^{16}O) approx 0.59, X(^{20}Ne) approx 0.28 and X(^{24}Mg) approx 0.05. This core is surrounded by a 0.05 M_s buffer mainly composed of carbon and oxygen, and on top of it a He envelope of mass 10^{-4} M_s, 11 pages, 11 figures, accepted for publication in A&A

The Na, Mg, and Al abundance anomalies that have been observed in globular cluster red giants are hard to understand in terms of standard evolutionary scenarios, especially with regard to the depletion of 24Mg. In previous papers, we have proposed a flash-assisted deep-mixing model that can burn 24Mg and help explain the observed abundance anomalies. Relevant reaction rates have considerable uncertainties at low energies. Using NACRE reaction rates, we recompute the nucleosynthesis predicted by our model and, by adjusting reaction rates within current uncertainties, find that agreement with the observations is considerably improved for almost all isotopes except for 25Mg, which remains overabundant. A resolution of the latter discrepancy by nuclear physics alone would require that the 25Mg(p,γ)26Al reaction rate be substantially larger than that given by the experiments. This suggests that our model may require burning of 25Mg (coupled with some extra mixing) during quiescent hydrogen-burning phases.

We have calculated models for low-mass, metal-deficient ([Fe/H] = -2.7) stars from the zero-age main sequence through the thermally pulsing asymptotic giant branch (TP-AGB) phase. We confirm that the entropy barrier between the H-rich envelope and the He intershell can be surmounted by the energy released by thermal pulses during the early phase of the TP-AGB. For models in the mass range of 1 ≤ M/M☉ < 3, this energy release causes the top of the flash-driven convective shell to reach into the bottom of the overlying H-rich envelope. Protons are then carried downward into the hotter He- and 12C-rich layer, while He intershell material is mixed upward. This phenomenon causes the surface chemical composition to change dramatically. In particular, surface abundances are enriched in CNO elements by as much as 1 to 3 orders of magnitude. Lithium is also enhanced by this event in the 1, 1.5, and 2 M☉ models. We have also studied the formation and reactions of 13C as protons are mixed into the He intershell. We find that this mixed material experiences the s-process through the α-capture reaction on newly synthesized 13C under convective conditions during the thermal pulse. This results in neutron-capture nucleosynthesis under relatively high neutron density environments. In lower mass models, the s-abundance distributions would be characterized by the small number of neutron irradiations through the standard s-process, which occurs under radiative conditions in a 13C pocket as a result of the immediate termination of the third dredge-up. Accordingly, in extremely metal-poor stars, we may observe the s-element distributions mainly created by the s-processing relevant to the proton-mixing event. Furthermore, we discuss possible observational signatures of the mixing of protons into He-burning regions.

We report on the first measurement of the Eu isotope fractions (151Eu and 153Eu) in s-process-element-enhanced, metal-poor stars. We use these ratios to investigate the 151Sm branching of s-process nucleosynthesis. The measurement was made by detailed study of Eu II lines that are significantly affected by hyperfine splitting and isotope shifts in spectra of the carbon-rich, very metal-poor stars LP625-44 and CS31062-050, observed with the Subaru Telescope High Dispersion Spectrograph. The 151Eu fractions [fr(151Eu) = 151Eu/(151Eu+153Eu)] derived for LP625-44 and CS31062-050 are 0.60 and 0.55, respectively, with uncertainties of about +/- 0.05. These values are higher than found in solar-system material, but agree well with the predictions of recent s-process models. We derive new constraints on the temperature and neutron density during the s-process based on calculations of pulsed s-process models for the 151Eu fraction., 10 pages, 2 figures, ApJL in press

The evolution of stars with masses ranging between 1 and 3M⊙, metallicity [ Fe H ] = − 2.7 and He mass fraction Y = 0.24 is followed from the zero-age main sequence through core He burning up to the thermally pulsing asymptotic giant branch phase. We find that the second thermal pulse leads to the injection of protons into a flash-driven convective shell, the outer part of which reaches the lower part of the hydrogen tail. The energy produced by proton capture reactions then induces various changes in the stellar interior. The main results of this event are that the surface abundances of CNO and 7Li become highly enhanced relative to solar abundance. We describe a new production mechanism for 7Li by this phenomenon.

We have investigated evolution of low-mass, low-metallicity AGB stars. We evolved models with masses ranging from 1 to 3 M ⊙ and metallicity of [Fe/H] = −2.7. We found for M ≲ 2.5 M ⊙ models that in the early phase of the AGB evolution convective shell triggered by thermal runaway of He shell burning extends upwards and reaches into the bottom of the overlying H-rich envelope. Protons are mixed into hotter He intershell. Proton capture reactions occur under He burning conditions and lead to release of huge amounts of energy. This phenomenon causes the surface chemical composition to change dramatically. We present surface abundance changes of CNO elements and lithium in this event.

We have made a parametric study of s-process nucleosynthesis in the metal poor ([Fe/H]=-2.7) stars LP625-44 and LP706-7. We find that a high neutron exposure and a small overlap factor are necessary to fit the abundance pattern observed in these two metal-deficient stars, particularly the abundance ratios, Pb/Ba $\approx 1$ and Ba/Sr $\approx 10$. We have also constructed stellar models to better understand how such s-process conditions could arise. We have calculated a 2$M_\odot$ model star with metallicity [Fe/H]=-2.7 from the ZAMS up to AGB phase. We find that for such low-metallicity stars the He convective shell reaches the bottom of the overlying H-rich envelope at the second thermal pulse. Protons are then carried into the hotter He burning layers and $^{13}$C is formed as protons mix into the He shell. Subsequently, material in the H-flash driven convective zone experiences a high neutron exposure due to the $^{13}$C($\alpha$, n) reaction. This results in a new neutron-capture s-process paradigm in which the abundances are characterized by only one neutron exposure. We suggest that this new s-process site may be a significant contributor to the s-process abundances in low-metallicity ([Fe/H] $\le -2.5$) stars., Comment: 4 pages, 4 figures, uses jnstcite.sty, nd2001_2.sty and article-nd2001_2.cls, J. Nucl. Sci. Technol., in press

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

We study the enrichment histories for nine elements, C, the four α-elements Mg, Si, Ca and Ti, Sc, and the three iron-peak elements Co, Ni and Zn, by using a large amount of stellar data, collected in the Stellar Abundances for Galactic Archaeology (SAGA) database. We find statistically significant changes, or breaks, of the mean abundance ratios to iron, [X/Fe]m, among these elements at the three metallicities of [Fe/H] ≃−1.8,−2.2 or −3.3. Across the first break, [X/Fe]m decreases with metallicity by similar amounts of ≃0.08 dex, common to all the elements with sufficient data in the relevant metallicity range. Across the latter two, downward trends with metallicity are also detected but only for some elements, i.e., for C, Co, Zn and possibly Sc across the second break, and for Co and Zn across the third one. These breaks define four stellar populations with different abundance patterns, Populations IIa, IIb, IIc and IId in order of increaesing metallicity, which dominate over each metallicity range divided by the breaks. We explore the correlations with the vertical distance of stars from the disk to reveal that the break of [Zn/Fe] at [Fe/H] ≃ −2.2 is detected for distant stars of |Z|≳1 kpc but not for nearby stars and that two populations of Pop IIb and IIc, characterized by [Zn/Fe]m = 0.22 and 0.09 respectively, coexist in the same metallicity range of −2.6≲[Fe/H]≲ −2.2. This indicates that the break cannot be ascribed to the metallicity dependence of supernova yields, but is explicable only in terms of the change of initial mass function (IMF) from high-mass/top-heavy to low-mass one, coupled with hypernova yielding high-[Co, Zn/Fe] (Umeda & Nomoto 2005), which is also claimed from a large fraction of carbon enhanced metal-poor stars (Komiya et al. 2007). For the two higher-metallicity populations, Pops. IIc and IId, on the other hand, the difference in the abundance patterns is attributable to the delayed contribution of Type Ia supernovae, indicative of a low-mass IMF and a specific star formation ra te comparable to that in the present-day Galactic disc. We discuss the relevance to the kinematically resolved structures of Galactic halo and the possible sites of these populations within the framework of the hierarchical structure formation scenario.

We discuss a new model of deep mixing in red giants caused by shell flashes triggered by the inward penetration of hydrogen into the helium core. The objective is to explain the large star-to-star variations of proton-capture elements that are observed among globular cluster red giants (Fujimoto, Aikawa, & Kato). The characteristics of this mixing model are explored by computing the products of nucleosynthesis during hydrogen shell flashes in red giants. It is shown that, during sufficiently strong flashes, both 24Mg and 23Na are depleted by burning. For moderately and very metal-poor clusters, the burned 24Mg is converted mostly into 27Al. Thus, the model not only resolves the difficulties arising from the observed underabundance of 24Mg, but also removes the problem of sodium overproduction, which would otherwise accompany the aluminum enrichment observed. For moderately metal-poor cluster giants, 25Mg and 26Mg survive with nearly scaled-solar abundances, while for very metal-poor cluster giants, all the magnesium isotopes are depleted and aluminum also burns appreciably into 28Si. For metal-rich cluster giants, shell flashes are quenched before 25Mg and 26Mg can burn to synthesize aluminum. These properties agree well with the manner in which observed abundance anomalies vary with cluster metallicity, including the absence of large Al-enrichments for red giants in metal-rich clusters. From a detailed comparison with observed abundances in M13 giants, we deduce constraints on the strength of the mixing mechanism responsible for the inward penetration of hydrogen into the helium core. We discuss further implications of the present results, in particular the importance of star-star interactions for the proper understanding of the evolution of stars in globular clusters.

The evolution of stars to red giants is revisited in order to promote a better understanding of the behavior of stars on the H-R diagram by separating the essential nonlinear characteristics of stellar structure from detailed effects of input physics such as chemical compositions, opacity, convection criterion, etc. It is shown that the red giant and dwarf structures are clearly discriminated in terms of the variation through the stellar interior of the ratio, W, between the mass interior to the relevant shell and the mass contained in the pressure scale height at the shell (≡ GM/4πr4P). For the simplest structures of dwarfs such as main-sequence stars, W increases monotonically from zero at the center to the greatest value at the surface. In later stages of evolution such as the red giants, W evolves to have a local extremum near the hydrogen-burning shell. Such a change in the distribution of W is a consequence of the increase in the ratio, Θ, of thermal and/or Fermi energy (P/ρ) between the core center and the envelope. In fact there exists a lower bound to Θ that is required by a red giant structure. During stellar evolution, the increase in Θ is brought about by the gravitational contraction of the core and by the fact that shell burning prevents the envelope from following the core contraction. Structures with an extremum of W correspond to an envelope of the condensed type, the properties of which are strongly regulated by the polytropic index N and the ratio, V [= (GMr/r)/(P/ρ)], between the local gravitational potential to the thermal energy in the lower envelope. A large polytropic index N 3 makes the envelope expanded, particularly when V N + 1 is realized in the lower envelope. On the basis of such an understanding, we are able to analyze the effects of input physics on the excursion of the star to a red giant. A key role is played by the gradients of the opacity and of the mean molecular weight. The stellar radius can be larger if the opacity is increasing outward to make N larger than 3. A moderate gradient in the mean molecular weight also allows the value of N to come into the range of 3-5, which is appropriate for V N + 1 to be realized in the bottom of the envelope, while a steep gradient yields too large a value of N. The so-called peculiar evolution of the SN 1987A progenitor, Sk -69°202, can be understood in such a context. In particular, a constraint is derived for the mechanism of the extra mixing that is responsible for the final red-to-blue excursion of the star.

A long-standing problem of large star-to-star variations in the surface abundances of C, N, O, Na, and Al that is observed among red giant stars in globular clusters is discussed in the light of the recent finding by Shetrone that 24Mg is reduced among the Al-enhanced stars. A new scenario of nucleosynthesis and element mixing is proposed, whereby the surface anomalies are explained as the consequence of the inward mixing of hydrogen followed by the outward mixing of nuclear products of hydrogen burning. If hydrogen is carried down into the helium core by some extra mixing mechanism, a hydrogen-burning shell flash is ignited. The flash forces the formation of a convective shell whose outer edge extends into hydrogen-rich layers, bringing in fresh hydrogen to fuel the flash further. During a flash, the temperature at the base of the flash-driven shell can become larger than the temperature in the hydrogen-burning shell during quiescent interflash evolution, and during the decay phase of the flash the nuclear products are dredged up by surface convection, which becomes deeper in mass than during the quiescent phases. The properties of shell flashes induced by the inward mixing of hydrogen are investigated by means of a semianalytical method to derive the conditions required for burning 24Mg and producing Al via proton captures. The mixing inward of hydrogen at an abundance by mass X 0.001 is enough to ignite shell flashes. The deeper hydrogen is mixed and the more massive the core, the stronger is the resultant shell flash and the larger are the temperatures in the flashing region. The burning of 24Mg in the flash convective zone demands temperatures T 8.5 × 107 K because of short duration of the flash. The necessary depth of hydrogen mixing can be as small as ~1.5 times the pressure scale height for a core mass Mc = 0.45 M☉, while it must be twice this for Mc = 0.3 M☉. For Mc 0.25 M☉, the temperature never becomes large enough for 24Mg to burn during shell flashes. In stars in metal poor clusters with [Fe/H] < -1,27Al is produced via the subsequent burning of 25Mg,26Mg, and 26Al, while in stars in metal-rich clusters, the duration of a flash is too short for these burnings to occur. It is also predicted that significant enrichment of helium in the surface is necessarily attendant upon the Al/Mg abundance anomaly. In order to ignite shell flashes, the inward mixing of hydrogen has to take place on a timescale shorter than the lifetime of protons near the outer edge of the helium core; this lifetime varies from ~105 to ~104 yr, depending on the core mass. Hydrodynamical instabilities that are triggered by the inflow of angular momentum into the core are suggested as being responsible for the postulated inward mixing. The origin of this flow is attributed to star-star interactions in the dense environment of a globular cluster.

We study the enrichment histories for nine elements, C, four alpha-elements of Mg, Si, Ca, and Ti, Sc, and three iron-peak elements of Co, Ni, and Zn, by using a large number of stellar data, collected by the Stellar Abundances for Galactic Archaeology (SAGA) database. We find statistically significant changes, or breaks, of the mean abundance ratios to iron at three metallicities of [Fe/H]-1.8, -2.2, and -3.3. Across the first one, the mean abundance ratios decrease with the metallicity by similar extents for all the elements with the sufficient data. Across the latter two, downward trends with the metallicity are also detected but for limited elements, C, Co, Zn, and possibly Sc, and for two of Co and Zn, respectively. The breaks define four stellar populations with the different abundance patters which are dominant in each metallicity range divided by the breaks, Pop IIa, IIb, IIc, and IId in order of increasing metallicity. We also explore their spatial distributions with the spectroscopic distances to demonstrate that Pops IIa and IIb spread over the Galactic halo while Pops IIc and IId are observed near the Galactic plane. In particular, Pop IIc stars emerge around [Fe/H] -2.6 and coexist with Pop IIb stars, segregated by the spatial distributions. Our results reveal two distinct modes of star formation during the early stages of Galaxy formation, which are associated with the variations of IMF and the spatial distribution of remnant low-mass stars. For the two lower-metallicity populations, the enhancements of Zn and Co indicate a high-mass and top-heavy IMF together with the statistics on the carbon-enhanced stars. We discuss the relevance to the kinematically resolved structures of the Galactic halo and the possible sites of these populations within the framework of hierarchical structure formation scenario., 19 pages, 16 figures, submitted to MNRAS on January, 21 and accepted on August, 23

In the core accretion model for the formation of gas giant planets, runaway gas accretion onto a core is the primary requisite, triggered when the core mass reaches a critical value. The recently revealed wide diversity of the extrasolar giant planets suggests the necessity to further the understanding of the conditions resulting in the critical core mass that initiates runaway accretion. We study the internal structure of protoplanets under hydrostatic and thermal equilibria represented in terms of a polytropic equation of state to investigate what factors determine and affect the critical core mass. We find that the protoplanets, embedded in protoplanetary disks, have the same configuration as red giants, characterized by the envelope of the centrally-condensed type solution. Applying the theory of stellar structure with homology invariants, we demonstrate that there are three types of criteria for the critical core mass depending on the stiffness of polytrope and the nature of outer boundary condition. For the stiff polytropes of index $N \le 3$ with the Bondi radius as the outer boundary, the criterion governing the critical core mass occurs at the surface. For stiff polytropes with the Hill outer boundary and for soft polytropes of $N>3$, this criterion acts at the bottom of gaseous envelope. Further, we elucidate the roles and effects of coexistent radiative and convective zones in the envelope of critical core mass. Based on the results, we discuss the relevance of Bondi and Hill surface conditions and explore the parameter dependences of critical core mass., 54 pages, 9 figures, 3 tables, Accepted for publication in ApJ

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

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.

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 investigate the effects of a new 3-α reaction rate (OKK rate) on a He flash of a He accreting neutron star in a binary system. Since ignition points determine the properties of a thermonuclear flash of type I X-ray bursts, we examine the cases of different accretion rates, dM/dt (Ṁ). We find that, at low accretion rates, nuclear burning occurs at the He layers of rather low densities. As a consequence, He ignition would occur at accretion rate lower than M&dot ≃3×10−8M⊙yr−1 Furthermore we find that the OKK rate is consistent with the available observations of X-ray bursts on a He accreting neutron star. We advocate that the OKK rate is better than the previous rate for the astrophysical phenomena of X-ray burst due to He accretion.

We analyze the enrichment histories of chemical elements in the Galactic halo stars with use of large abundance data compiled in the Stellar Abundances for Galactic Archaeology (SAGA) database [4]. We find four different populations represented by stars divided at [Fe/H] ≃ −1.8, ≃ −2.2 and ≃ −3.3 across which mean enrichments of C, Mg, Si, Ca, Ni, Co and Zn decrease by ∼ 0.1 or ∼ 0.1-0.3 dex toward higher metallicity. The first one is common to both α-elements (C, Mg, Si and Ca) and iron-group elements (Ni and Zn). And the second one is seen for C, Co and Zn, but not for other α-elements and Ni, while the third one is only seen for Co and Zn. We also investigate the correlation of Zn enrichment with the spatial distribution of stars. The difference near [Fe/H] ≃ −1.8 is detected for stars with the vertical distance from the disk less than 500pc and unclear for stars greater than it. The offset near [Fe/H] ≃ −2.2 is detected for stars greater than 500pc, but does not exist for stars less than it.

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

Takuma Sudaa,b, Shimako Yamada,a, Yutaka Katsuta,a Yutaka Komiya,c, Wako Aoki,c and Masayuki Y. Fujimotoa aHokkaido University, Kita 10 Nishi 8, Kita-ku, Sapporo, 060-0810, Japan bKeele University, Keele, Staffordshire, ST5 5BG, UK cNational Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo, 181-8588, Japan E-mail: [suda,yamada,kat,fujimoto]@astro1.sci.hokudai.ac.jp, [yutaka.komiya,aoki.wako]@nao.ac.jp

The effects of a new triple-$\alpha$ reaction rate (OKK rate) on the helium flash of a helium accreting neutron star in a binary system have been investigated. Since the ignition points determine the properties of a thermonuclear flash of type I X-ray bursts, we examine the cases of different accretion rates, $dM/dt (\dot{M})$, of helium from $3\times10^{-10} M_{\odot} \rm yr^{-1}$ to $3\times10^{-8} M_{\odot} \rm yr^{-1}$, which could cover the observed accretion rates. We find that for the cases of low accretion rates, nuclear burnings are ignited at the helium layers of rather low densities. As a consequence, helium deflagration would be triggered for all cases of lower accretion rate than $\dot{M}\simeq 3\times10^{-8} M_{\odot} \rm yr^{-1}$. We find that OKK rate could be barely consistent with the available observations of the X-ray bursts on the helium accreting neutron star. However this coincidence is found to depend on the properties of crustal heating and the neutron star model.We suggest that OKK rate would be reduced by a factor of $10^{2-3}$ for $10^8$ K in the range of the observational errors., Comment: 10 pages, 4 figures

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

We present extensive sets of stellar models for 0.8-9.0Msun in mass and -5, Comment: 19 pages, 12 figures, accepted by MNRAS

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.

We investigate the effect of the triple‐α reaction rates on the evolution of low‐mass stars and massive stars. The former is compared with the observations of metal‐poor stars known to date. For the latter, we discuss the impact of recent calculation of triple‐α reaction rate by Ogata et al. (2009, PTP, 122, 1055) on the evolution until carbon burning.