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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

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

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.

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 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.

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 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...

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

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

The solar abundances show empirical scaling laws for p‐ and s‐nuclei with the same atomic number, which are a piece of evidence that most p‐nuclei are dominantly synthesized by supernova γ‐process. The scalings led to a novel concept of the universality of the γ‐process that scalings should hold for individual supernova nucleosyntheses. Supernova γ‐process calculations under various astrophysical conditions show the universality originated from three mechanism, shift of γ‐process layers, change of abundance distribution by weak s‐process, independence of nuclear reaction flows. Scaling show some deviations which may originate from contribution of other nucleosynthesis processes. The corrected abundance ratios show clear correlation, which means that the scaling is robust.

We investigate nucleosynthesis inside magnetically driven jets ejected from collapsars, or rotating magnetized stars collapsing to a black hole, based on two‐dimensional magnetohydrodynamic simulation of the collapsars during the core collapse. We follow the evolution of the abundances of about 4000 nuclides from the collapse phase to the ejection phase using a large nuclear reaction network. We find that the r‐process successfully operates only in the energetic jets (>1051 erg), so that U and Th are synthesized abundantly, even when the collapsars have a relatively small magnetic field (1010 G) and a moderately rotating core before the collapse. The abundance patterns inside the jets are similar to that of the r‐elements in the solar system. The higher energy jets have larger amounts of 56Ni. Less energetic jets, which have small amounts of 56Ni, could induce GRB without supernova, such as GRB060505 and GRB060614.

Recent observational studies show considerable small dispersions for abundance ratios of C–Zn relative to iron in metal‐poor halo stars. On the other hand, abundance ratios of neutron‐capture elements show large scatters, suggesting incomplete mixing of the interstellar medium at the beginning of the Galaxy. We attempt to explain these contradictory observations, using an inhomogeneous chemical evolution model, which is shown to well reproduce large scatters in neutron‐capture elements in our previous studies. In particular, taking into account of difference of observational trends among iron‐peak elements, we discuss nucleosynthesis of the very first stars.

Because of the still present uncertainties on its rate, the 17O(p,α)14N is one of the most important reaction to be studied in order to get more information about the fate of 17O in different astrophysical scenarios. The preliminary study of the three‐body reaction 2H(17O,α14N)n is presented here as a first stage of the indirect study of this important 17O(p,α)14N reaction through the Trojan Horse Method (THM)

SN 1006 is one of the supernova remnants (SNR) recorded in the Japanese diary “Meigetsuki”. From the historical records including Meigetsuki, we conclude that SN 1006 was the brightest type Ia supernova remnant. We report on the observations of SN 1006 with the X‐ray Imaging Spectrometers (XIS) on board the 5‐th Japanese X‐ray satellite Suzaku. We found that the ionization age of SN 1006 is the youngest among any Galactic SNRs, hence is the best SNR to study early phase of type Ia. In the X‐ray spectrum, we found the K‐shell emission lines from heavy elements, in particular that from iron, for the first time. The X‐ray emitting plasma is highly overabundant in heavy elements, hence are likely due to ejecta. The abundance pattern agrees well to the theoretical prediction of type Ia supernova.

The evolution of extremely metal‐poor (EMP) stars of low‐/intermediate‐masses is distinct from those of metal‐rich stars in that the convection driven by the helium shell flash can extend outward into the hydrogen‐rich layer during TP‐AGB phase. In the circumstance of [Fe/H]

A common method for calculating nucleosynthetic yields for Type Ia supernovae (SNe Ia) is the post‐processing of a large number of passively advected tracer particles with a nuclear reaction network. In this paper we present a novel and more computationally efficient method for calculating nucleosynthetic yields for material burned to nuclear statistical equilibrium (NSE). The method exploits a tight correlation that exists between the final entropy and neutron excess for material processed by a detonation. We generate a table of freeze out abundances by integrating a 443 nuclide nuclear reaction network along synthetic thermodynamic trajectories at a fixed entropy and neutron excess. Final yields are calculated by first interpolating in the table and then decaying short lived radionuclides back to stability.We apply this method to obtain Fe‐peak yields for a suite of 2D axisymmetric SNe Ia simulations in the framework of the gravitationally confined detonation (GCD) explosion mechanism developed at the F...

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.

Effect of the β‐delayed fission in r‐process abundance is investigated with a realistic model for the fission fragment mass distribution (FFMD). The data base for the FFMD is constructed based on the two‐center shell model and multi‐dimensional Langevin calculation. The β‐decay rates including neutron emission and β‐delayed fission are also newly calculated with 2nd version of the the gross theory. The differences appeared in the final element abundance calculated with and without fission process, with different β‐delayed fission rates are demonstrated.

Diamond is the first mineral type of presolar grains that were isolated from meteorites, yet it is one of the least understood presolar grain types. An isotopically anomalous component Xe‐HL is carried by diamond and is characterized by excesses in both the light, p‐process only isotopes (124 and 126) and the heavy, r‐process only isotopes (134 and 136). These excesses are always correlated although physical settings of these two processes are quite different and there is little reason to always correlate each other. Furthermore, the r‐process Xe and the p‐process Xe in diamond inferred from Xe‐HL are quite different from what is derived from the solar system abundance as well as stellar models. Further studies are needed to investigate these outstanding problems for over three decades.

The fission fragments mass distribution (FFMD) which is the important nuclear information in the study of the r‐process nucleosynthesis is estimated for the neutron‐rich nuclei (Z>85) according to the theoretical investigation of the potential energy surface. The details for determining FFMD are discussed. In this paper, the network calculation on the r‐process nucleosynthesis is also performed including the data of FFMD. The comparison of the results of the network calculation, with and without the fission processes, is shown.

For the key reactions 16O(n,γ)17O and 17O(n,γ)18O on nucleosyntheses in stellar interior, the theoretical estimate for the cross section and reaction rate have been made. The careful description of the nuclear structure is important in very low energy regions. The Cluster Orbital Shell Model is adopted for reproducing the structure of the nuclei. Our results for the cross section of the 16O(n,γ)17O reaction are consistent with that of the microscopic two cluster model. The straightforward application becames possible to the case of the 17O(n,γ)18O reaction with a substantial reliance.

The excitation functions of the 8Li(d,t)7Li and the 8Li(d,p)9Li reactions were directly measured using 8Li beams with Ecm = 0.3, 0.4, 0.5, 0.7, 0.8, 1.0, 1.1, and 1.2 MeV irradiating CD2 targets. These beam energies covered the Gamow peaks for 1−3×109 K. Large cross sections were observed at around Ecm = 0.8 MeV for the 8Li(d,t)7Li and Ecm = 1.0 MeV for the 8Li(d,p)9Li, implying the existence of a resonant state of 10Be at the excitation energy of around 22.4 MeV. Enhanced with the present work, the astrophysical reaction rates of the 8Li(d,t)7Li and the 8Li(d,p)9Li reactions are higher by one and two orders of magnitude than the previously evaluated ones, respectively.

The r‐process in a low temperature environment is explored, in which the neutron emission by photodisintegration does not play a role. A semi‐analytic neutrino‐driven wind model is utilized for this purpose. The temperature in a supersonically expanding outflow can quickly drop to a few 108 K, where the (n,γ)‐(γ,n) equilibrium is never achieved during the heavy r‐nuclei synthesis. In addition, the neutron capture competes with the β‐decay owing to the low matter density. Despite such non‐standard physical conditions, a solar‐like r‐process abundance curve can be reproduced. The cold r‐process predicts, however, the low lead production compared to that expected in the traditional r‐process conditions, which can be a possible explanation for the low lead abundances found in a couple of r‐process‐rich Galactic halo stars.

A new and simple discription of the three‐body scattering states, which is based on the Lippmann‐Schwinger equation and the complex scaling method, is developed. As applications of this method, the energy distribution of the E1 transition strength for 6He is calculated. It is also shown that the correlations of subsystems in 6He are well reproduced.

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.

We evolve thermally pulsing AGB star models in the mass range of 1–8 M⊙. The metallicity of the models is assumed to be [Fe/H]≃−3. Mass loss is taken into account to investigate the abundance patterns of the yields ejected from the AGB models. In the 1 and 2 M⊙ AGB models hot bottom burning (HBB) does not take place at the base of the convective envelope during interpulse phases, but the low‐mass models produce 7Li in an H‐flash event. The occurrence of this event is associated with the ingestion of protons from the overlying H‐rich envelope into the He‐flash driven convective shell during thermal pulse phase. The resulting 7Li abundances are higher than the primordial one based on the analysis of the WMAP data. The present investigation also confirms the efficient production of 7Li by the HBB in the intermediate‐mass (4–8 M⊙) AGB stars. If these AGB stars belong to a binary system with a low‐mass companion, mass loss from the primary AGB star transfers the materials enriched in 7Li to the surface of the ...

Real photon beams in the energy region up to several ten MeV provide a good opportunity for experimental studies of astrophysical nucleosynthesis caused by the electromagnetic interactions as well as the weak interactions. In this paper development of real photon sources at low energies is reviewed, and a recent experimental work on the photonuclear reactions of 4He with new‐generation γ‐ray sources by the laser Compton‐scattering method is presented.

In nova and X‐ray burst nucleosynthesis, waiting points are nuclei in the reaction path which delay the nuclear flow towards heavier nuclei, typically because of a weak proton capture reaction and a long β+ lifetime. Waiting points can influence the energy generation and final abundances synthesized in these explosions. We have constructed a systematic, quantitative set of criteria to identify rp‐process waiting points, and use them to search for waiting points in post‐processing simulations of novae and X‐ray bursts. These criteria have been incorporated into the Computational Infrastructure for Nuclear Astrophysics, online at nucastrodata.org, to enable anyone to run customized searches for waiting points.

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 ...

Photoneutron cross sections were measured for 80Se near the neutron separation energy with laser Compton‐backscattered γ‐ray beams at AIST. Neutron capture rates are evaluated for 79Se with the photoreaction data as experimental constraints on the El γ‐strength function, a key parameter in the Hauser‐Feshbach statistical model calculation. Solving the set of differential equations under a single neutron exposure, we analyzed the solar abundance ratio of the weak components of 80Kr and 82Kr in terms of the s‐process branching at 79Se. We discuss the region of temperature and neutron density allowed for the weak s‐process nucleosynthesis.

Neutrino‐induced reactions on 12C, 4He as well as Fe and Ni isotopes are studied based on new shell model Hamiltonians for p‐shell and fp‐shell. Gamow‐Teller and spin‐dipole transitions are investigated, and applied to neutrino‐nucleus reactions induced by both DAR and supernova neutrinos. The reaction cross sections are found to be enhanced compared with conventional Hamiltonians as well as previous calculations. The production yields of 7Li and 11B during supernova explosions are found to be enhanced, and the effects of neutrino oscillations and implications of the enhancement on the constraint on temperature for νμ,τ and νμ,τ are discussed. Production of other light elements such as 10Be and 10B by neutrino processes is also discussed. Neutral current reactions on Ni and Fe isotopes induced by supernova neutrinos are investigated. Effects of neutrino‐induced reactions on the production yields of heavy elements such as Mn are discussed.

We examine production of light elements (Li, Be, & B) on the surface of a low‐mass secondary in a black hole soft X‐ray transient (BHSXT) through the spallation of CNO nuclei by neutrons which are ejected from a hot (>10 MeV) advection‐dominated accretion flow (ADAF) around the black hole. Using updated binary parameters, cross sections of neutron‐induced spallation reactions, and mass accretion rates in ADAFs derived from the spectrum fitting of multi‐wavelength observations of quiescent BHSXTs, we obtain the equilibrium abundances of Li by equating the production rate of Li and the mass transfer rate through accretion to the black hole. We find that the resulting abundances are found to be in good agreement with the observed values in seven BHSXTs. Moreover, the isotopic ratio 6Li/7Li and the abundance ratios Be/Li and B/Li are calculated to be about 0.7–0.8, 1.0–1.3, and 1.8–2.1, respectively, on the secondaries.

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

The sources of nuclear uncertainties in nova nucleosynthesis have been identified using hydrodynamical nova models. Experimental efforts have followed and significantly reduced those uncertainties. This is important for the evaluation of nova contribution to galactic chemical evolution, gamma--ray astronomy and possibly presolar grain studies. In particular, estimations of expected gamma-ray fluxes are essential for the planning of observations with existing or future satellites., Comment: Invited contribution to the "Origin of Matter and Evolution of Galaxies" conference (OMEG07) with additional and color figures

Recent hydrodynamic studies of core-collapse supernovae imply that the neutrino-heated ejecta from a nascent neutron star develops to supersonic outflows. These supersonic winds are influenced by the reverse shock from the preceding supernova ejecta, forming the wind termination shock. We investigate the effects of the termination shock in neutrino-driven winds and its roll on the r-process. Supersonic outflows are calculated with a semi-analytic neutrino-driven wind model. Subsequent termination-shocked, subsonic outflows are obtained by applying the Rankine-Hugoniot relations. We find a couple of effects that can be relevant for the r-process. First is the sudden slowdown of the temperature decrease by the wind termination. Second is the entropy jump by termination-shock heating, up to several 100NAk. Nucleosynthesis calculations in the obtained winds are performed to examine these effects on the r-process. We find that 1) the slowdown of the temperature decrease plays a decisive roll to determine the r-process abundance curves. This is due to the strong dependences of the nucleosynthetic path on the temperature during the r-process freezeout phase. Our results suggest that only the termination-shocked winds with relatively small shock radii (~500km) are relevant for the bulk of the solar r-process abundances (A~100-180). The heaviest part in the solar r-process curve (A~180-200), however, can be reproduced both in shocked and unshocked winds. These results may help to constrain the mass range of supernova progenitors relevant for the r-process. We find, on the other hand, 2) negligible roles of the entropy jump on the r-process. This is a consequence that the sizable entropy increase takes place only at a large shock radius (~10,000km) where the r-process has already ceased., 11 pages, 7 figures, submitted to ApJ, revised following referee's comments,Accepted for publication in ApJ

In extremely metal‐poor stars ([Fe H]< 2 5), hydrogen is mixed into the convection driven by helium flash and induces neutron‐capture nucleosynthesis with the reactions, 12C(p γ)13N(e+ν)13C(α n)16O, as the neutron source. We investigate the progress of this nucleosynthesis with use of nuclear network for a wide range of model parameters such as the amount of mixed hydrogen and the strength of helium flash. We reveal the characteristic abundance pattern from the light elements through the s‐process elements, produced by alpha‐ and neutron‐capture reactions under the extremely metal‐poor condition. On the basis, we explore the possible modifications of surface abundances in the metal‐free, Population III stars and discuss their relevance to the two most iron‐deficient stars, HE0107‐5240 and HE1327‐2326, known to date.

We apply our Monte Carlo method to the s‐process element synthesis in the helium shell flash model for extremely metal‐poor stars. We emphasize the importance of the reaction 17O(n γ)18O. It is presented that the uncertainty of this reaction rate affects the heaver elements abundance. We also discuss the variation of the O‐Ne reaction paths with the uncertainty of the reaction rate of 17O(n γ)18O.

We present an elemental abundance analysis of HE 1327‐2326, the most iron‐deficient star known, based on a comprehensive investigation of spectra obtained with the Subaru Telescope. HE 1327‐2326 is either in its main sequence or subgiant phase of evolution, hence it is essentially unevolved. The chemical abundances of this star have the following properties, which provide new constraints on models of nucleosynthesis processes that occurred in first‐generation objects:(1)The iron abundance (NLTE) is [Fe/H]= −5.45. This value is 0.2 dex lower than that of HE 0107‐5240, the previously most iron‐poor object known. No object having [Fe/H]= −5 ∼ −4 is known to date.(2)This star, as well as HE 0107‐5240, exhibits extremely large overabundances of carbon relative to solar ratios ([C/Fe]∼ +4).(3)HE 1327‐2326 exhibits remarkable overabundances of the light elements (N, Na, Mg and Al), while HE 0107‐5240 shows only relatively small excesses of N and Na.(4)A large overabundance of Sr is found in HE 1327‐2326 as compa...

We investigate the nucleosynthesis in the helium flash convective zone, triggered by the hydrogen mixing, for extremely metal‐poor stars of low and intermediate mass. Mixed hydrogen is converted into neutron through 12C(p,γ)13N(e+ν)13C(α,n)16O and the doubly neutron‐recycling reactions 12C(n,γ)13C(α,n)16O(n,γ)17O(α,n)20Ne operate. In addition to oxygen and neon, not only light elements from sodium through phosphorus but also the s‐process elements, heavier than iron, are synthesized via successive neutron captures with 20Ne as seeds even in the stars originally devoid of metals. We follow the both the doubly neutron‐recycling reactions and the s‐process nucleosynthesis up to Pb and Bi by varying model parameters such as the amount of mixed 13C. The resultant abundance patterns is shown to reproduce the observed enhancement not only of oxygen, the light elements but also Sr observed from HE 0107‐5240 and HE 1327‐2326.

We propose a Monte Carlo method to study the reaction paths in nucleosynthesis during stellar evolution. Determination of reaction paths is important to obtain the physical picture of stellar evolution. The combination of network calculation and our method gives us a better understanding of physical picture. We apply our method to the case of the helium shell flash model in the extremely metal poor star.

We elaborate the binary scenario for the origin of HE0107-5240, the most metal-poor star yet observed ([Fe/H] = -5.3), using current knowledge of the evolution of extremely metal-poor stars. From the observed C/N value, we estimate the binary separation and period. Nucleosynthesis in a helium convective zone into which hydrogen has been injected allows us to discuss the origin of surface O and Na as well as the abundance distribution of s-process elements. We can explain the observed abundances of 12C, 13C, N, O, and Na and predict future observations to validate the Pop III nature of HE0107-5240., 4 pages, 3 figures, proceedings of the conference, "Nuclei in the Cosmos VIII", Nuclear Physics A in press

The chemically most primitive stars provide constraints on the nature of the first stellar objects that formed in the Universe; elements other than hydrogen, helium and traces of lithium within these objects were generated by nucleosynthesis in the very first stars. The relative abundances of elements in the surviving primitive stars reflect the masses of the first stars, because the pathways of nucleosynthesis are quite sensitive to stellar masses. Several models have been suggested to explain the origin of the abundance pattern of the giant star HE 0107-5240, which hitherto exhibited the highest deficiency of heavy elements known. Here we report the discovery of HE 1327-2326, a subgiant or main-sequence star with an iron abundance about a factor of two lower than that of HE 0107-5240. Both stars show extreme overabundances of carbon and nitrogen with respect to iron, suggesting a similar origin of the abundance patterns. The unexpectedly low Li and high Sr abundances of HE 1327-2326, however, challenge existing theoretical understanding: none predicts the high Sr abundance or provides a Li depletion mechanism consistent with data available for the most metal-poor stars., 9 pages, 2 figures, accepted for publication in Nature; typos corrected