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We model the quiescent luminosity of accreting neutron stars with several equation of states (EOSs), including the effect of pion condensation and superfluidity. As a consequence of comparison with the observations, we show that the results with Togashi EoS (the strong direct Urca process is forbidden) and TM1e EoS (mass at direct Urca process is $2.06 M_\odot$) can explain the observations well by considering pion condensation and the effect of superfluidity, while LS220 EoS and TM1 EoS can explain the observations well by considering the baryon direct Urca process and the effect of superfluidity. Besides, we compare the results with the observations of a neutron star RX J0812.4-3114 which has the low average mass accretion rate ($\langle\dot{M}\rangle\sim(4-15)\times 10^{-12}~M_\odot ~\rm yr^{-1}$) but high thermal luminosity ($L_q^\infty\sim(0.6-3)\times10^{33}~\rm erg ~ s^{-1}$), and we suggest that a low-mass neutron star ($, 12 pages, 8 figures, accepted by Phys. Rev. D

We study the quiescent luminosities of accreting neutron stars by a new mechanism as neutrino heating for additional deep crustal heating, where the neutrino heating is produced by charged pions decay from the nuclear collisions on the surface of neutron star during its active accretion. For low mass neutron star($\lesssim1.4~M_{\odot}$), as the neutrino heating is little($\lesssim1$ MeV per accreted nucleon) or there would be no neutrino heating, the quiescent luminsoity will be not affected or slightly affected. While for massive neutron star ($\gtrsim2~M_{\odot}$), the quiescent luminosity will be enhanced more obviously with neutrino heating in the range 2-6 MeV per accreted nucleon. The observations on cold neutron stars such as 1H 19605+00, SAX J1808.4-3658 can be explained with neutrino heating if a fast cooling and heavy elements surface are considered. The observations on a hot neutron star such as RX J0812.4-3114 can be explained with neutrino heating if the direct Urca process is forbidden for a massive star with light elements surface, which is different from the previous work that the hot observations should be explained with small mass neutron star and the effect of superfluidity., Comment: 9 pages, 5 figures, accepted for publication in Phys. Rev. D

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

Neutron stars are high-density objects formed by the gravitational collapse of massive stars, and the whole star can be likened to a giant nucleus. The interior of a neutron star is considered to contain exotic particles and states which do not appear in a normal nucleus. The internal states are constrained by observations of masses and radii via the equation of state of highly dense nuclear matter. Within these constraints, a variety of exotic states have been discussed. The internal state of neutron stars is closely related to its neutrino emission process, which cools the star from the inside. This effect can be compared with observations of the surface temperature of neutron stars. However, despite the wide range of observations of neutron stars, the nature of the neutron star matter remains uncertain. We consider quark matter as an exotic state and perform cooling calculations for neutron stars, incorporating the effects of nucleon superfluidity and quark colour superconductivity.We take into account the “quark-hadron continuity”, in which the neutron superfluidity is succeeded by thedquark pairing. Furthermore, we obtained the range of the neutron star cooling curve, taking into account the difference in surface temperature due to the composition of the surface layer. We found that the existence of quark matter causes strong neutrino emission from quarks, which is moderately suppressedbysuperfluidity and superconductivity, and canexplain the cold surface temperature of neutron stars.

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 reinvestigate effects of neutrino oscillations on the production of 7Li and 11B in core-collapse supernovae (SNe). During the propagation of neutrinos from the proto-neutron star, their flavors change and the neutrino reaction rates for spallation of 12C and 4He are affected. In this work corrected neutrino spallation cross sections for 4He and 12C are adopted. Initial abundances involving heavy s-nuclei and other physical conditions are derived in a new calculation of the SN 1987A progenitor in which effects of the progenitor metallicity are included. A dependence of the SN nucleosynthesis and final yields of 7Li and 11B on the neutrino mass hierarchy are shown in several stellar locations. In the normal hierarchy case, the charged current reaction rates of electron neutrinos are enhanced, and yields of proton-rich nuclei, along with 7Be and 11C, are increased. In the inverted hierarchy case, the charged current reaction rates of electron antineutrinos are enhanced, and yields of neutron-rich nuclei, along with 7Li and 11B, are increased. We find that variation of the metallicity modifies the yields of 7Li, 7Be, 11B, and 11C. This effect is caused by changes in the neutron abundance during SN nucleosynthesis. Therefore, accurate calculations of Li and B production in SNe should take into account the metallicity of progenitor stars., 25 pages, 22 figures, 3 tables, accepted for publication in ApJ, minor corrections added

We construct the quiescent neutron star models in the evolutionary calculations. The X-ray luminosities have been derived in terms of the time-averaged mass accretion rate for various neutron star masses and surface compositions. We compare the quiescent luminosities observed from X-ray transients in low mass X-ray binaries, where the stellar evolutionary calculations of accreting neutron stars include neutrino cooling due to strong pion condensations. Our results based on the evolutionary calculations suggest that stronger cooling process would be necessary to be consistent with observations.

We investigate the nucleosynthesis of a massive star whose mass in the main sequence stage is Mms = 70 M that corresponds to the helium core or helium star of Mα = 32 M . Numerical calculations of the nucleosynthesis are performed during the stage of hydrostatic stellar evolution until the core composed of iron-group nuclei (Fe core) begins to collapse. A collapsar model whose jets are driven by two-dimensional magnetohydrodynamical effects of a differentially rotating core is constructed. The explosive nucleosynthesis inside the jets of a specified collapsar model is followed along the trajectories of stream lines. We combine the results of both detailed hydrostatic and explosive nucleosyntheses to compare the solar system abundances. We show that the agreement is considerably improved compared with the case without the detailed hydrostatic nucleosynthesis included. We suggest that many radioactive nuclei such as Ti and Ni are produced if the jets continue for more than 10 s, which would become crucial for the observation of abundance distribution in young supernova remnants.

We investigate synthesis of heavy elements in a collapsar. We have calculated detailed composition of magnetically driven jets ejected from a collapsar, which is based on long-term, magneto-hydrodynamic simulations of a rapidly rotating massive star of 40Msun during core collapse. We follow evolution of abundances of about 4000 nuclides from the collapse phase to the ejection phase through the jet generation phase with use of two large nuclear reaction networks. We find that the r-process successfully operates in the jets, so that U and Th are synthesized abundantly when the progenitor has large magnetic field of 10^{12} G and rapidly rotating core. Abundance pattern inside the jets is similar compared to that of r-elements in the solar system. Heavy neutron-rich nuclei \sim 0.01Msun can be ejected from the collapsar. The detailed abundances depend on nuclear properties of mass model, beta-decay rate, and fission, for nuclei near the neutron drip line. Furthermore, we find that p-nuclei are produced without seed nuclei: not only light p-nuclei, such as Se74, Kr78, Sr84, and Mo92, but also heavy p-nuclei, In113, Sn115, and La138, can be abundantly synthesized in the jets. The amounts of p-nuclei in the ejecta are much greater than those in core-collapse supernovae (SNe). In particular, Mo92, In113, Sn115, and La138 deficient in the SNe, are significantly produced in the ejecta., Comment: 16 pages, 19 figures, submitted to the Astrophysical Journal

We perform two-dimensional, axisymmetric, magnetohydrodynamic simulations of the collapse of a rotating star of 40 Msun and in the light of the collapsar model of gamma-ray burst. Considering two distributions of angular momentum, up to \sim 10^{17} cm^2/s, and the uniform vertical magnetic field, we investigate the formation of an accretion disk around a black hole and the jet production near the hole. After material reaches to the black hole with the high angular momentum, the disk is formed inside a surface of weak shock. The disk becomes in a quasi-steady state for stars whose magnetic field is less than 10^{10} G before the collapse. We find that the jet can be driven by the magnetic fields even if the central core does not rotate as rapidly as previously assumed and outer layers of the star has sufficiently high angular momentum. The magnetic fields are chiefly amplified inside the disk due to the compression and the wrapping of the field. The fields inside the disk propagate to the polar region along the inner boundary near the black hole through the Alfv{��}n wave, and eventually drive the jet. The quasi-steady disk is not an advection-dominated disk but a neutrino cooling-dominated one. Mass accretion rates in the disks are greater than 0.01 Msun/sec with large fluctuations. The disk is transparent for neutrinos. The dense part of the disk, which locates near the hole, emits neutrino efficiently at a constant rate of < 8 \times 10^{51} erg/s. The neutrino luminosity is much smaller than those from supernovae after the neutrino burst., 42 pages, accepted for publication in the Astrophysical Journal. A paper with higher-resolution figures available at http://www.ec.knct.ac.jp/~fujimoto/collapsar/mhd-color.pdf

We investigate nucleosynthesis inside an accretion disk formed during a collapse of a massive star. The mass accretion rate and the mass of a central object vary with time according to a fall-back scenario. The disk is confirmed to be dominated by advection. A considerable amount of Fe-group elements is produced inside the disk through a sequence of ( α , γ ) reactions, and the processed material is ejected via outflows. The collapsar accretion disk is an promising site for production of heavy elements.

We investigate primordial nucleosynthesis in a Brans-Dicke model with a decaying Λ term using the Monte-Carlo method and likelihood analysis. The produced abundances of 4He and D are consistent with the observations when the baryon to photon ratio η 10 = 5.47 – 6.64 , which is in agreement with the value deduced from WMAP.

We propose a formation scenario of strange stars from rapidly rotating neutron stars after supernova explosions due to the spin-down of the compact stars. By considering a process where the total baryon mass is conserved but the angular momentum is lost due to the emission of gravitational waves and/or magnetic dipole radiation, we find that the transition from a rapidly rotating neutron star to a slowly rotating strange star is possible. The large amount of energy of 5 × 10 53 ergs can be released, which is large by some factors compared to the transition between spherical stars. The liberated energy might become a new energy source for a delayed explosion of supernova. Thus our scenario suggests that the supernova associated with gamma-ray bursts becomes promising targets of future observation of gravitational waves.

We investigate nucleosynthesis inside both a gamma-ray burst accretion disk and a wind launched from an inner region of the disk using one-dimensional models of the disk and wind and a nuclear reaction network. Far from a central black hole, the composition of accreting gas is taken to be that of an O-rich layer of a massive star before core collapse. We find that the disk consists of five layers characterized by dominant elements: O16, Si28, Fe54 (and Ni56), He4, and nucleons, and the individual layers shift inward with keeping the overall profiles of compositions as the accretion rate decreases. Ni56 are abundantly ejected through the wind from the inner region of the disk with the electron fraction \simeq 0.5. In addition to iron group, elements heavier than Cu, in particular Cu63 and Zn64, are massively produced through the wind. Various neutron-rich nuclei can be also produced in the wind from neutron-rich regions of the disk, though the estimated yields have large uncertainties.

Presolar grains have large isotopic anomalies revealing the traces of nucleosynthesis at their birth, and thus they are considered to originate from stellar outflows and supernova ejecta. These grains have been extracted from primitive meteorites, and the isotopic ratios of each individual grain have been analyzed in the laboratory. Several isotopic signatures of SiC X and low-density graphite grains indicate an origin in supernovae. We investigate the isotopic ratios of Type II supernova ejecta with large-scale mixing so as to reproduce the ratios of as many ratios from supernovae as possible. We pursued the evolution and the supernova explosion of a massive star using a 4 M☉ He star model and calculated detailed nucleosynthesis by postprocessing. We then show how many isotopic ratios of each individual grain agree with those of the mixtures of the supernova ejecta under appropriate mixing ratios. We assume two cases of mixtures with artificial mixing ratios: one consists of Si/S, O/Ne, He/C, and He/N layers, and the other consists of Ni, Si/S, He/C, and He/N layers. We select the isotopic ratios of 12C/13C, 14N/15N, 26Al/27Al, 29Si/28Si, 30Si/28Si, and 44Ti/48Ti from eleven SiC X grains and four low-density graphite grains that contain evidence for the original presence of the short-lived isotope 44Ti. For one SiC X grain and one low-density graphite grain, four isotopic ratios are reproduced by the corresponding mixtures. For seven SiC X grains and three graphite grains, three isotopic ratios agree with those of the mixtures. Most mixtures that reproduce the isotopic ratios of the grains have a large contribution from the He/N layer. The characteristics of the mixtures are found to be 0.01 0.1. For C/O < 0.1, the isotopic ratios of most grains are not reproduced.

We investigate p-process nucleosynthesis in a supercritical accretion disk around a compact object of 1.4 M_solar, using the self-similar solution of an optically thick advection dominated flow. Supercritical accretion is expected to occur in a supernova with fallback material accreting onto a new-born compact object. It is found that appreciable amounts of p-nuclei are synthesized via the p-process in supernova-driven supercritical accretion disks (SSADs) when the accretion rate m_dot = M_dot c^2/(16 L_Edd) >10^5, where L_Edd is the Eddington luminosity. Abundance profiles of p-nuclei ejected from SSADs have similar feature to those of the oxygen/neon layers in Type II supernovae when the abundance of the fallback gas far from the compact object is that of the oxygen/neon layers in the progenitor. The overall abundance profile is in agreement with that of the solar system. Some p-nuclei, such as Mo, Ru, Sn, and La, are underproduced in the SSADs as in Type II supernovae. If the fallback gas is mixed with a small fraction of proton through Rayleigh-Taylor instability during the explosion, significant amounts of Mo92 are produced inside the SSADs. Ru96 and La138 are also produced when the fallback gas contains abundant proton though the overall abundance profile of p-nuclei is rather different from that of the solar system. The p-process nucleosynthesis in SSADs contributes to chemical evolution of p-nuclei, in particular Mo92, if several percents of fallback matter are ejected via jets and/or winds., 15 pages, 7 figures included, 3 tables, LaTeX emulateapj5.sty, accepted for publication by the Astronomical Journal (March, 2003)

We have found that a new and nonexplosive evolutionary path from stars in the white dwarf region to stars in the neutron star region may be possible. Such a process can be realized if we incorporate both a large amount of rotation and the temperature effect on the equation of state. The large value of angular momentum is required to make stars secularly unstable because of gravitational radiation emission. For high temperature matter, the contribution of the temperature and the electron fraction to the pressure becomes large enough to make rotating stars dynamically stable against axisymmetric perturbations. Thus, equilibrium states may exist for rotating compact stars that are dynamically stable against axisymmetric collapse but secularly unstable. The evolution of dynamically stable and secularly unstable rotating stars can proceed as follows. The secular instability is caused by the emission of gravitational waves that carry away the angular momentum of the star. Stars with less angular momentum will contract to higher density states. This process occurs rather slowly, i.e., not on a dynamical timescale but on a secular timescale of gravitational radiation emission. Consequently, compact configurations such as white dwarfs in this category may undergo nonexplosive and slow contraction. This contraction leads some configurations to neutron stars and others to black holes, depending on the mass and the angular momentum. If the final outcomes are neutron stars, they are both dynamically and secularly stable because some of the angular momentum is lost. Therefore, we have succeeded in showing that fizzlers can exist, although Newtonian gravity is used. This evolution is likely to occur within the central region of massive stars. Since the central region of a massive star is hot, high-temperature effects become important. Concerning high angular momentum, massive stars in the main sequence stage usually rotate rather rapidly. It implies that the angular momentum of the core can also be large enough to lead to secular instability. Thus, cores of massive stars may contract on a long timescale without being accompanied by supernova explosions.

We have examined explosive nucleosynthesis of p-nuclei during a neutrino-driven, aspherical supernova aided by both convection and standing accretion shock instability, based on a twodimensional hydrodynamic simulation of the explosion of a 15M star, with an explosion energy of ∼ 1051ergs. We find that p-nuclei are mainly produced by γ-processes, and that the nuclei lighter than 92Mo are abundantly synthesized in slightly neutron-rich bubbles with electron fractions of Ye ≤ 0.48, although masses of the bubbles are highly uncertain in the current hydrodynamic simulation. Abundances of the p-nuclei are comparable to those in a spherical model, and nuclei, 94Mo, 96Ru, and 98Ru, are underproduced compared with the solar system for our aspherical models, as in a spherical model. 138La and 180Ta, which are underproduced in the model are abundantly produced by charged current neutrino-interactions with 138Ba and 180Hf, respectively.

We examine explosive nucleosynthesis during neutrino-driven, aspherical supernovae of Population III stars, based on two-dimensional (2D) hydrodynamic simulations of the explosion of 11-40M⊙ stars with zero metallicity. The magnitude and asymmetry of the explosion energy are estimated with the simulations. By post-processing calculations with a large nuclear reaction network, we have evaluated abundances and masses of ejecta from the aspherical SNe. We find that the evaluated abundance patterns are similar to those observed in extremely metal poor stars, as shown in spherical and 2D models, in which the explosion is manually and spherically initiated. Matter mixing induced via standing accretion shock instability is important for the abundances and masses of the SN ejecta.

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.

Effect of a new 3-alpha reaction rate on the s-process during the evolution of a massive star of 25 solar mass is investigated for the first time, because the s-process in massive stars have been believed to be established with only minor change. We find that the s-process with use of the new rate during the core helium burning is very inefficient compared to the case with the previous 3-alpha rate. However, the difference of the overproduction is found to be largely compensated by the subsequent carbon burning. Since the s-process in massive stars has been attributed so far to the neutron irradiation during core helium burning, our finding reveals for the first time the importance of the carbon burning for the s-process during the evolution of massive stars.

Effects of a new triple-{\alpha} reaction rate on the s-process during the evolution of a massive star of 25 M\odot are investigated for the first time. Although the s-process in massive stars has been believed to be established with only minor change, we find that the s-process with use of the new rate during the core He-burning is very inefficient compared to the case with the previous triple-{\alpha} rate. However, the difference of the overproduction is found to be largely compensated by the subsequent C-burning. Since the s-process in massive stars has been attributed so far to the neutron irradiation during core He-burning, our finding reveals the importance of C-burning for the s-process during the evolution of massive stars., Comment: 8 pages, 5 figures. Submitted to Prog. Theor. Phys. Letters on October 2, 2011

We investigate explosive nucleosynthesis in a non-rotating 15$M_\odot$ star with solar metallicity that explodes by a neutrino-heating supernova (SN) mechanism aided by both standing accretion shock instability (SASI) and convection. To trigger explosions in our two-dimensional hydrodynamic simulations, we approximate the neutrino transport with a simple light-bulb scheme and systematically change the neutrino fluxes emitted from the protoneutron star. By a post-processing calculation, we evaluate abundances and masses of the SN ejecta for nuclei with the mass number $\le 70$ employing a large nuclear reaction network. Aspherical abundance distributions, which are observed in nearby core-collapse SN remnants, are obtained for the non-rotating spherically-symmetric progenitor, due to the growth of low-mode SASI. Abundance pattern of the supernova ejecta is similar to that of the solar system for models whose masses ranges $(0.4-0.5) \Ms$ of the ejecta from the inner region ($\le 10,000\km$) of the precollapse core. For the models, the explosion energies and the \nuc{Ni}{56} masses are $ \simeq 10^{51} \rm erg$ and $(0.05-0.06) \Ms$, respectively; their estimated baryonic masses of the neutron star are comparable to the ones observed in neutron-star binaries. These findings may have little uncertainty because most of the ejecta is composed by matter that is heated via the shock wave and has relatively definite abundances. The abundance ratios for Ne, Mg, Si and Fe observed in Cygnus loop are well reproduced with the SN ejecta from an inner region of the $15\Ms$ progenitor., Comment: 15 pages, 1 table, 17 figures, accepted for publication in Astrophyscal Journal

Effects of a new triple-alpha reaction rate on the ignition of carbon-oxygen white dwarfs accreting helium in a binary systems have been investigated. The ignition points determine the properties of a thermonuclear explosion of a Type Ia supernova. We examine the cases of different accretion rates of helium and different initial masses of the white dwarf, which was studied in detail by Nomoto. We find that for all cases from slow to intermediate accretion rates, nuclear burnings are ignited at the helium layers. As a consequence, carbon deflagration would be triggered for the lower accretion rate compared to that of $dM/dt\simeq 4\times10^{-8} M_{\odot} \rm yr^{-1}$ which has been believed to the lower limit of the accretion rate for the deflagration supernova. Furthermore, off-center helium detonation should result for intermediate and slow accretion rates and the region of carbon deflagration for slow accretion rate is disappeared., 4 pages, 2 figures

Recently, the central source of Cassiopeia A (Cas A) has been observed, which indicates that the star has large mass and high effective temperature. We suspect that the compact object cools by the standard neutrino emission. We assume that the compact object contains quark matter with colour superconductivity and calculate cooling curves. Considering the Quark‐Hadron Mixed Phase, we obtain cooling curves which are found to be consistent with the observations.

We investigate r‐process nucleosynthesis during magneto‐hydrodynamic supernova explosions driven by rapid rotations strong magnetic fields. These types of supernovae are very important not only for magnetar formation sites, but also for astronomical r‐process sites in astrophysics. Our r‐process nucleosynthesis simulations are based on the astronomical supernova explosion models, which follow the long‐term evolution in special relativistic magneto‐hydrodynamic simulations. We perform an r‐process nucleosynthesis simulation for magneto‐hydrodynamic jet supernova models based on a large nuclear reaction network including fully nuclear reactions. In our results, we find that a jet‐like supernova explosion model with strong magnetic field can occur successful r‐process nucleosynthesis.

We investigate explosive nucleosynthesis in a delayed neutrino‐driven, supernova explosion aided by standing accretion shock instability (SASI), based on two‐dimensional hydrodynamic simulations of the explosion of a 15 M⊙ star. We take into accounts neutrino heating and cooling as well as change in electron fraction due to weak interactions appropriately, in the two‐dimensional simulations. We assume the isotropic emission of neutrinos from the neutrino spheres with given luminosities. and the Fermi‐Dirac distribution of given temperatures. We find that the stalled shock revives due to the neutrino heating aided by SASI for cases with Lνe≥3.9×1052ergss−1 and the as‐pherical shock passes through the outer layers of the star (≥10,000 km), with the explosion energies of ∼1051ergs.Next we examine abundances and masses of the supernova ejecta. We find that masses of the ejecta and 56Ni correlate with the neutrino luminosity, and 56Ni mass is comparable to that observed in SN 1987A. We also find that abundance pattern of the supernova ejecta is similar to that of the solar system, for cases with high explosion energies of >1051ergs. We emphasize that 64Zn, which is underproduced in the spherical case, is abundantly produced in slightly neutron‐rich ejecta.

Effects of a new triple‐α reaction rate on the ignition of carbon‐oxygen white dwarfs accreting helium in a binary systems have been investigated. The ignition points determine the properties of a thermonuclear explosion of a Type la supernova. We examine the cases of different accretion rates of helium and different initial masses of the white dwarf, which was studied in detail by Nomoto[4]. We find that for all cases from slow to intermediate accretion rates, nuclear burnings are ignited at the helium layers. As a consequence, carbon deflagration would be triggered for the lower accretion rate compared to that of dM/dt≃4×1O−8 M⊙ yr−1 which has been believed to the lower limit of the accretion rate for the deflagration supernova. Furthermore, off‐center helium detonation should result for intermediate and slow accretion rates and the region of carbon deflagration for slow accretion rate is disappeared.

We investigate effects of triple-$\alpha$ and $^{12}\rm C(\alpha,\gamma) ^{16}O$ reaction rates on the production of supernova yields for a massive star of 25 $M_{\odot}$. We combine the reaction rates to examine the rate dependence, where the rates are considered to cover the possible variation of the rates based on experiments on the earth and theories. We adopt four combinations of the reaction rates from two triple-$\alpha$ reaction rates and two $^{12}\rm C(\alpha,\gamma)^{16}O$ ones. First, we examine the evolution of massive stars of 20 and 25 $M_{\odot}$ whose helium cores correspond to helium stars of 6 and 8 $M_{\odot}$, respectively. While the 25 $M_{\odot}$ stars evolve to the presupernova stages for all combinations of the reaction rates, evolutionary paths of the 20 $M_{\odot}$ stars proceed significantly different way for some combinations, which are unacceptable for progenitors of supernovae. Second, we perform calculations of supernova explosions within the limitation of spherical symmetry and compare the calculated abundance ratios with the solar system abundances. We can deduce some constraints to the reaction rates. As the results, a conventional rate is adequate for a triple-$\alpha$ reaction rate and a rather higher value of the reaction rate within the upper limit for the experimental uncertainties is favorable for a $^{12}\rm C(\alpha,\gamma)^{16}O$ rate.

We investigate the big‐bang nucleosynthesis (BBN) in a Brans‐Dicke model with a varying Λ term. We find that the cosmic expansion rate differs appreciably from that of the Friedmann model during the BBN epoch . The produced abundances of 4He, D, and 7Li are consistent with both the observed ones and the baryon density obtained from WMAP.

For the rst time heavy element nucleosynthesis in the magneto‐hydrodynamical (MHD) explosions of core‐collapse supernovae are investigated using a massive star of 13M⊙ in a main sequence stage. Contrary to the case of the spherical explosion, jet‐like explosion due to the combined effects of the rotation and magnetic eld lowers the electron fraction signicantly inside the layers above the Fe‐core. Then, the anisotropic shock waves pass through the oxygen rich layers. As a consequence, we nd that the nucleosynthesis of the r‐ and p‐process proceeds appreciably compared to the models previously considered.

We present a scenario of formation of strange stars due to spin-down of {\it rapidly rotating} neutron stars left after supernova explosions . By assuming a process where the total baryon mass is conserved but the angular momentum is lost due to emission of gravitational waves and/or the magnetic braking, we find that the transition from rapidly rotating neutron stars to slowly rotating strange stars is possible; a large amount of energy $\sim 10^{53} ergs $ could be released. The liberated energy might become a new energy source for a delayed explosion of supernova. Furthermore, our scenario suggests that the supernova associated with gamma-ray bursts could become candidates for targets in the future observation of gravitational waves., 11 pages, 3 figures, Received November 5, 2004

We investigate nucleosynthesis inside both a gamma-ray burst accretion disk and a wind launched from an inner region of the disk using one-dimensional models of the disk and wind and a nuclear reaction network. Far from a central black hole, the composition of accreting gas is taken to be that of an O-rich layer of a massive star before core collapse. We find that the disk consists of five layers characterized by dominant elements: O16, Si28, Fe54 (and Ni56), He4, and nucleons, and the individual layers shift inward with keeping the overall profiles of compositions as the accretion rate decreases. Ni56 are abundantly ejected through the wind from the inner region of the disk with the electron fraction \simeq 0.5. In addition to iron group, elements heavier than Cu, in particular Cu63 and Zn64, are massively produced through the wind. Various neutron-rich nuclei can be also produced in the wind from neutron-rich regions of the disk, though the estimated yields have large uncertainties., Comment: 4 pages, 6 figures - OMEG03 - Int. Symposium on Origin of Matter and Evolution of the Galaxies November 17-19, 2003 RIKEN, Wako, Saitama, Japan

We examine the characteristics of nucleosynthesis in 'hypernovae', i.e., supernovae with very large explosion energies ($ \gsim 10^{52} $ ergs). We carry out detailed nucleosynthesis calculations for these energetic explosions and compare the yields with those of ordinary core-collapse supernovae. We find that both complete and incomplete Si-burning takes place over more extended, lower density regions, so that the alpha-rich freezeout is enhanced and produces more Ti in comparison with ordinary supernova nucleosynthesis. In addition, oxygen and carbon burning takes place in more extended, lower density regions than in ordinary supernovae. Therefore, the fuel elements O, C, Al are less abundant while a larger amount of Si, S, Ar, and Ca ("Si") are synthesized by oxygen burning; this leads to larger ratios of "Si"/O in the ejecta. Enhancement of the mass ratio between complete and incomplete Si-burning regions in the ejecta may explain the abundance ratios among iron-peak elements in metal-poor stars. Also the enhanced "Si"/O ratio may explain the abundance ratios observed in star burst galaxies. We also discuss other implications of enhanced [Ti/Fe] and [Fe/O] for Galactic chemical evolution and the abundances of low mass black hole binaries., Comment: Accepted for publication in the Astrophysical Journal (13 March 2001) Tables 6 - 9 are available at http://www.astron.s.u-tokyo.ac.jp/~nakamura/research/papers/nakamuratab.ps.gz

Nucleosynthesis in ONeMg novae has been investigated with the wide ranges of three parameters, i.e., the white dwarf mass, the envelope mass at ignition, and the initial composition. A quasi-analytic one-zone approach is used with an up-to-date nuclear reaction network. The nucleosynthesis results show correlation with the peak temperatures or the cooling timescales during outbursts. Among the combinations of white dwarf and envelope masses which give the same peak temperature, the explosion is more violent for a lower white dwarf mass owing to its smaller gravitational potential. Comparison of the nucleosynthesis results with observations implies that at least two-third of the white dwarf masses for the observed ONeMg novae are $\simeq 1.1 M_\odot$, which are significantly lower than estimated by previous hydrodynamic studies but consistent with the observations of V1974 Cyg. Moreover, the envelope masses derived from the comparison are $\gtrsim 10^{-4} M_\odot$, which are in good agreement with the ejecta masses estimated from observations but significantly higher than in previous hydrodynamic studies. With such a low mass white dwarf and a high mass envelope, the nova can produce interesting amounts of $\gamma$-ray emitters $^7$Be, $^{22}$Na, and $^{26}$Al. We suggest that V1974 Cyg has produced $^{22}$Na as high as the upper limit derived from the COMPTEL survey. In addition, a non-negligible part of the Galactic $^{26}$Al may originate from ONeMg novae, if not the major contributors. Both the future INTEGRAL survey for these $\gamma$-ray emitters and abundance estimates derived from ultraviolet, optical, and near infrared spectroscopies will impose a severe constraint on the current nova models., Comment: 21 pages, 23 figures, to appear in the Astrophysical Journal, Vol. 523, No.1, September 20, 1999; preprint with embedded images can be obtained from http://th.nao.ac.jp/~wanajo/journal/onenova.ps

Type II supernovae (SNe II) are linked to massive stars (M>8M⊙) at the end of their evolution. Gravitational collapse results in a central hot proto-neutron star which cools via neutrino emission. Neutrino opacities and transport determine the neutrino emission luminosity, which is responsible for the heating of outer layers, causing the explosion and ejection of matter. Nucleosynthesis products are effected via two main uncertainties: (i) the locations of the mass cut between the ejecta and the remaining neutron star; and (ii) the total explosion energy responsible for explosive nucleosynthesis. Observations of individual supernovae can constrain these quantities.

This review concentrates on nucleosynthesis processes in general and their applications to massive stars and supernovae. A brief initial introduction is given to the physics in astrophysical plasmas which governs composition changes. We present the basic equations for thermonuclear reaction rates and nuclear reaction networks. The required nuclear physics input for reaction rates is discussed, i.e. cross sections for nuclear reactions, photodisintegrations, electron and positron captures, neutrino captures, inelastic neutrino scattering, and beta-decay half-lives. We examine especially the present state of uncertainties in predicting thermonuclear reaction rates, while the status of experiments is discussed by others in this volume (see M. Wiescher). It follows a brief review of hydrostatic burning stages in stellar evolution before discussing the fate of massive stars, i.e. the nucleosynthesis in type II supernova explosions (SNe II). Except for SNe Ia, which are explained by exploding white dwarfs in binary stellar systems (which will not be discussed here), all other supernova types seem to be linked to the gravitational collapse of massive stars (M$>$8M$_\odot$) at the end of their hydrostatic evolution. SN1987A, the first type II supernova for which the progenitor star was known, is used as an example for nucleosynthesis calculations. Finally, we discuss the production of heavy elements in the r-process up to Th and U and its possible connection to supernovae.

We have performed 1-dimensional calculations for explosive nucleosynthesis in collapse-driven supernova and investigated its sensitivity to the initial form of the shock wave. We have found the tendency that the peak temperature becomes higher around the mass cut if the input energy is injected more in the form of kinetic energy rather than internal energy. Then, the mass cut becomes larger, and, as a result, neutron-rich matter is less included in the ejecta; this is favorable for producing the observational data compared with a previous model. Our results imply that the standard method to treat various processes for stellar evolution, such as convection and electron capture during the silicon burning stage, are still compatible with the calculation of explosive nucleosynthesis., 20 pages, 6 figures, LaTex

We perform two-dimensional axisymmetric hydrodynamic simulations of matter mixing in aspherical core-collapse supernova explosions of a 16.3 $M_{\odot}$ star with a compact hydrogen envelope. Observations of SN 1987A have provided evidence that $^{56}$Ni synthesized by explosive nucleosynthesis is mixed into fast moving matter ($\gtrsim$ 3,500 km s$^{-1}$) in the exploding star. In order to clarify the key conditions for reproducing such high velocity of $^{56}$Ni, we revisit matter mixing in aspherical core-collapse supernova explosions. Explosions are initiated artificially by injecting thermal and kinetic energies around the interface between the iron core and the silicon-rich layer. Perturbations of 5% or 30% amplitude in the radial velocities are introduced at several points in time. We found that no high velocity $^{56}$Ni can be obtained if we consider bipolar explosions with perturbations (5% amplitude) of pre-supernova origins. If large perturbations (30% amplitude) are introduced or exist due to some unknown mechanism in a later phase just before the shock wave reaches the hydrogen envelope, $^{56}$Ni with a velocity of 3,000 km s$^{-1}$ can be obtained. Aspherical explosions that are asymmetric across the equatorial plane with clumpy structures in the initial shock waves are investigated. We found that the clump sizes affect the penetration of $^{56}$Ni. Finally, we report that an aspherical explosion model that is asymmetric across the equatorial plane with multiple perturbations of pre-supernova origins can cause the penetration of $^{56}$Ni clumps into fast moving matter of 3,000 km s$^{-1}$. We show that both aspherical explosion with clumpy structures and perturbations of pre-supernova origins may be necessary to reproduce the observed high velocity of $^{56}$Ni. To confirm this, more robust three-dimensional simulations are required., 29 pages, 28 figures, 2 tables, submitted in ApJ

The maximum angular velocity of uniformly rotating nuetron stars is investigated. If we consider a hot neutron star stage after the birth from supernova explosions, an upper limit of the angular velocity for uniformly rotating neutron stars becomes a little smaller than that obtained by the equation of state for cold neutron matters.

Primordial nucleosynthesis is examined with evaporating black holes. A low density model of the universe is preferable because of the heating by the evaporation.

The final evolution of 8-10 M ○. stars in the AGB phase is recalculated to clarify whether these stars undergo collapse or thermonuclear explosion. New electron capture rates based on the shell model wave functions are employed for the evolution of the degenerate O+Ne+Mg core. It is found that the difference between the old and new rates are too small to alter the earlier conclusion that the O+Ne+Mg core collapses. The optical appearance of the resultant supernova explosion of AGB stars are discussed for observational diagnostics. In particular, a possible connection of the AGB star model with SN 1993J is examined

The ring discovered around SN 1987A is likely to be related to the rotation of the progenitor. Two rotational effects of the progenitor of SN 1987A have been studied : (1) mass-shedding from the equator and (2) an aspherical mass distribution in a stellar wind due to the direction-dependent mass loss. With approximate models for a rotating star and a wind, the following results have been obtained. First, if the rotational velocity of the progenitor at the main sequence exceeds 270 km/s, mass-shedding can occur (i.e., the angular velocity can exceed the critical angular velocity) at the blue supergiant stage. Second, the stellar wind combined with the rotation forms an aspherical mass distribution in the distant region from the star

The 20 solar mass model of Nomoto and Hashimoto (1988) is utilized with a 6 solar mass. He core is used to perform explosive nucleosynthesis calculations. The employed explosion energy of 10 to the 51st ergs lies within the uncertainty range inferred from the bolometric light curve. The nucleosynthesis processes and their burning products are discussed in detail. The results are compared with abundances from IR observations of SN 1987A and the average nucleosynthesis expected for Type II supernovae in Galactic chemical evolution. The abundances of long-lived radioactive nuclei and their importance for the late light curve and gamma-ray observations are predicted. The position of the mass cut between the neutron star and the ejecta is deduced from the total amount of ejected Ni-56. This requires a neutron star with a baryonic mass of 1.6 + or - 0.045 solar mass, which corresponds to a gravitational mass of 1.43 + or - 0.05 solar mass after subtracting the binding energy of a nonrotating neutron star.

If the accreting white dwarf increases its mass to the Chandrasekhar mass, it will either explode as a Type I supernova or collapse to form a neutron star. In fact, there is a good agreement between the exploding white dwarf model for Type I supernovae and observations. We describe various types of evolution of accreting white dwarfs as a function of binary parameters (i.e, composition, mass, and age of the white dwarf, its companion star, and mass accretion rate), and discuss the conditions for the precursors of exploding or collapsing white dwarfs, and their relevance to cataclysmic variables. Particular attention is given to helium star cataclysmics which might be the precursors of some Type I supernovae or ultrashort period X-ray binaries. Finally we present new evolutionary calculations using the updated nuclear reaction rates for the formation of O+Ne+Mg white dwarfs, and discuss the composition structure and their relevance to the model for neon novae.

Neutrino bursts from the supernova SN1987A have been reported1–3 which provide qualitatively new information as to the time interval between the core collapse and the optical flaring of the supernova up to about 6 mag4. The interval that we take as three hours1,2 imposes conditions on the radius of the progenitor star and the energetics of the explosion. As for the brightness and its time change, the relatively less bright visual magnitude at the plateau in the light curve during the initial eight days or so (refs 5, 29) as well. The subsequent light curve after eight days is quite unique; the bolometric luminosity increases until ∼65 days (ref. 29) but thereafter it seems to level off gradually (J. Walsh and R. Stathakis, personal communication; M.A. Dopita, personal communication). To maintain the bolometric luminosity of the star means that certain conditions should be imposed on the energy source. By computing the propagation of shock wave, the subsequent expansion of ejected matter and the optical light curve, we find that the early light curve can be accounted for by the diffusive release of energy deposited by the shock wave, that the progenitor's radius should be as small as ∼(1–3) × 1012 cm, and that the explosion energy per unit mass should be relatively large, ∼(2–3)× 1051 erg for the ejected matter of 7–10 M⊙. The light curve after eight days is well reproduced by using a model with constant energy input. The energy input may be due to the activity of an embedded pulsar. The light curves calculated by invoking the radioactive decay of 56Ni and 56Co are less satisfactory in accounting for the nearly flat portion observed after 65 days. Supposing Sk–69 202 is the progenitor of SN1987A13,14, its explosion energy should be >2×l051erg; this suggests that a prompt shock mechanism forms a neutron star.