Search

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

As some neutron star transients require an additional unknown heat source (referred to as “shallow heating”) to explain their high temperatures at the beginning of quiescence, we investigate the effect of shallow heating as well as compressional heating on the thermal state of transiently accreting neutron stars with the use of evolutionary calculations in the present work. Through comparing our theoretical predictions of the equilibrium redshifted luminosities $(L_{\gamma}^{\infty})$ produced by both deep crustal heating and shallow heating/compressional heating for different time-averaged mass-accretion rates $\langle\dot{M}\rangle$ with 35 updated observations of soft X-ray transients, the results show that both shallow heating and compressional heating make significant contributions to the equilibrium redshifted luminosity. The hotter sources (XTE J1701, MAXI J0556, EXO 0748, Aql X-1 etc.) with higher accretion rates are more likely to be explained with the effect of shallow heating or compressional heating. In addition, for a proper shallow heat $q_\mathrm{sh}$ and mass-accretion rate $\dot{M}$, the effect of shallow heating could be simulated by compressional heating.

We have proposed a short-lived radioisotope 98Tc as the nuclear cosmochronometer to evaluate the time from the last supernova neutirno-process to the solar system formation (SSF). We have calculated the supernova neutirno-process using a SN 1987A model with neutrino-induced reaction cross sections. The calculated result is consistent with the observed upper limit of 98Tc/98Ru at the SSF.

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

We calculate the abundances of 7Li, 11B, 92Nb, 98Tc, 138La, and 180Ta produced by neutrino (ν)-induced reactions in a core-collapse supernova explosion. We consider the modification by ν self-interaction (ν-SI) near the neutrinosphere and the Mikheyev–Smirnov–Wolfenstein (MSW) effect in the outer layers based on time-dependent neutrino energy spectra. Abundances of 7Li and the heavy isotopes 92Nb, 98Tc, and 138La are reduced by a factor of 1.5–2.0 by the ν-SI. In contrast, 11B is relatively insensitive to the ν-SI. We find that the abundance ratio of heavy to light nuclei, 138La/11B, is sensitive to the neutrino mass hierarchy, and the normal mass hierarchy is more likely to be consistent with the solar meteoritic abundances.

We investigate the significance of isomers on a kilonova associated with neutron star mergers (NSMs) for the first time. We calculate the evolution of abundances and energy generation rates ($\dot{\epsilon}_{\rm nuc}$) of ejecta from NSMs, taking into account $\beta^-$ decay through isomers. We find that for ejecta with electron fraction ($Y_e$) of $0.2-0.3$, $\dot{\epsilon}_{\rm nuc}$ is appreciably changed from those without isomers, due to the large change in timing of $\beta^-$ decay through the isomeric states of ${}^{123, 125, 127} \rm Sn$ and ${}^{128} \rm Sb$. In particular, the effects of the isomers on $\dot{\epsilon}_{\rm nuc}$ are prominent for ejecta of $Y_e \sim 0.25$, which could emit a fraction of an early, blue component of a kilonova observed in GW170817. When the excitation from a ground state to its isomeric state is important, the isomers of ${}^{129} \rm Sb$ and ${}^{129, 131} \rm Te$ also cause additional and appreciable change in $\dot{\epsilon}_{\rm nuc}$. Furthermore, we demonstrate that larger amounts of lanthanide-free ejecta result in a better fit of an observed light curve of the kilonova in GW170817 if the isomers are taken into account., Comment: 6 pages, 4 figures. Accepted for publication in MNRAS

Whether fast cooling processes occur or not is crucial for the thermal evolution of neutron stars. In particular, the threshold of the direct Urca process, which is one of the fast cooling processes, is determined by the interior proton fraction $Y_p$, or the nuclear symmetry energy. Since recent observations indicate the small radius of neutron stars, a low value is preferred for the symmetry energy. In this study, simulations of neutron star cooling are performed adopting three models for the equation of state (EoS): Togashi, Shen, and LS220 EoSs. The Togashi EoS has been recently constructed with realistic nuclear potentials under finite temperature, and found to account for the small radius of neutron stars. As a result, we find that, since the direct Urca process is forbidden, the neutron star cooling is slow with use of the Togashi EoS. This is because the symmetry energy of Togashi EoS is lower than those of other EoSs. Hence, in order to account for observed age and surface temperature of isolated neutron stars with the use of the Togashi EoS, other fast cooling processes are needed regardless of the surface composition.

The isotope $^{98}\mathrm{Tc}$ decays to $^{98}\mathrm{Ru}$ with a half-life of $4.2\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{ }\text{ }\mathrm{yr}$ and could have been present in the early Solar System. In this Letter, we report on the first calculations of the production of $^{98}\mathrm{Tc}$ by neutrino-induced reactions in core-collapse supernovae (the $\ensuremath{\nu}$ process). Our predicted $^{98}\mathrm{Tc}$ abundance at the time of solar system formation is not much lower than the current measured upper limit raising the possibility for its detection in the not too distant future. We show that, if the initial abundance were to be precisely measured, the $^{98}\mathrm{Tc}$ nuclear cosmochronometer could be used to evaluate a much more precise value of the duration time from the last core-collapse supernova to the formation of the solar system. Moreover, a unique and novel feature of the $^{98}\mathrm{Tc}$ $\ensuremath{\nu}$-process nucleosynthesis is the large contribution ($\ensuremath{\sim}20%$) from charged current reactions with electron antineutrinos. This means that $^{98}\mathrm{Tc}$ becomes a unique new $\ensuremath{\nu}$-process probe of the temperature of the electron antineutrinos.

We summarize physical and astronomical constants in cgs units. We also provide the conversion of peculiar units in nuclear physics and astronomy used in this book.

Concerning the decrease in the cosmological term Λ from a large magnitude in a very early universe to a small value at the present epoch, we explore the Brans-Dicke theory modified with a variable Λ term which is a function of the scalar field. The fundamental equations are derived for the gravitational and scalar fields from the variational principle of the action. In the framework of the Robertson-Walker metric, we obtain the expression for the expansion rate of the universe like the Friedmann equation. Then we turn to summarize the observational constraints on the intrinsic parameters contained in this theory. We confirm that the evolution of the universe deviates significantly from the standard model in the early stage. Calculations of Big Bang nucleosynthesis (BBN) are performed with the use of the nuclear reaction network. Comparing the resultant amounts of the light elements,4He, D, and 7Li, with the observed primordial abundances and using also the magnitude-redshift relation of Type Ia supernovae (SNe Ia), we derive reasonable ranges of the parameters.

Many alternative models are proposed, though remarkable agreement is obtained on the primordial abundance of standard BBN. As an approach to nonstandard models, first we investigate a possibility that neutrinos or antineutrinos are degenerate. An excess density of neutrinos speeds up the expansion of the universe, leaving more neutrons at the onset of nucleosynthesis. In addition, degenerate electron-neutrinos shift less neutrons through β-equilibrium. Performing χ2 analysis for the calculated and observed abundances of 4He and D, we determine the plausible ranges of the degeneracy parameter and the baryon-to-photon ratio. Next, we explore BBN under the brane-world cosmology. The Friedmann-like equation is derived in the five-dimensional universe. It is found that more 4He is produced than in standard BBN because of rapid expansion due to the interaction energy on the brane. Finally, we examine thermal evolution in the early universe including a decaying cosmological term which is treated as a source of the gravitational field.

We mention the fundamentals in the theory of general relativity, i.e., the principles of equivalence and general covariance. Based on the facts that our universe is homogeneous and isotropic at large scales, we derive the Robertson-Walker metric and subsequently the Friedmann equation which governs the expansion of the universe. The equation is expressed in a convenient form in terms of density parameters. Next, we describe thermonuclear reaction rates utilized in astrophysics, which involve resonant and nonresonant reactions, photodisintegration, electron capture, and β-decay. We review the significance of standard Big Bang nucleosynthesis (SBBN) and summarize the current situation in the observed primordial abundance of light elements,4He, D, and 7Li. Comparing the calculated abundance of the elements with observed values, we determine a reasonable range for the baryon-to-photon ratio. Moreover, we examine the dependence of the produced amount of 4He on the measured lifetimes of neutrons. Finally, we consider the magnitude-redshift relation of type Ia supernovae (SNe Ia) as an independent probe to cosmological models.

We review the recent progress in the Big-Bang nucleosynthesis which includes the standard and nonstandard theory of cosmology, effects of neutrino degeneracy, and inhomogeneous nucleosynthesis within the framework of a Friedmann model. As for a nonstandard theory of gravitation, we adopt a Brans–Dicke theory which incorporates a cosmological constant. We constrain various parameters associated with each subject.

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

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

We 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

This paper describes an exploratory study on the effects of temperature on the formation of oxide film and rolling contact fatigue life in hydrogen, argon and air. Rolling contact fatigue tests were conducted at 333 K and 363 K by using a three-ball-on-disk type apparatus. The rolling contact fatigue life in hydrogen was shorter than that in argon, and life in air was the longest. Relationship was found between fatigue life and hydrogen concentration in steel. Cross sections of the specimens show that iron oxide grew to larger grain size in the subsurface in hydrogen environment, which may have resulted in shorter fatigue life. It was also found that fatigue failure occurs on ball surface in hydrogen at 363 K.

We investigate the nucleosynthesis in a massive star of 70 M_solar with solar metallicity in the main sequence stage. The helium core mass after hydrogen burning corresponds to 32 M_solar. Nucleosynthesis calculations have been performed during the stellar evolution and the jetlike supernova explosion of a collapsar model, where the weak s-, p-, and r-processes are taken into account. We confirm that s-elements of 60 < A < 90 are highly overproduced relative to the solar abundances in the hydrostatic nucleosynthesis. During oxygen burning, p-elements of A > 90 are produced via photodisintegrations of seed s-elements. However, the produced p-elements are disintegrated in later stages except for ^{180}Ta. In the explosive nucleosynthesis, elements of 90 < A < 160 are significantly overproduced relative to the solar values owing to the r-process. Only heavy p-elements (N > 50) are overproduced via the p-process. Compared with the previous study of r-process nucleosynthesis calculations in the collapsar model of 40 M_solar by Fujimoto et al. 2007, 2008, our jet model cannot contribute to the third peak of the solar r-elements and intermediate p-elements. Averaging the overproduction factors over the progenitor masses with the use of Salpeter's IMF, we suggest that the 70 M_solar star could contribute to the solar weak s-elements of 60 < A < 90 and neutron-rich elements of 90 < A < 160. We confirm the primary synthesis of light p-elements in the ejected matter of high peak temperature. The ejected matter has [Sr/Eu] \sim -0.4, which is different from that of a typical r-process-enriched star CS22892-052 ([Sr/Eu] \sim -1). We find that Sr-Y-Zr isotopes are primarily synthesized in the explosive nucleosynthesis in a similar process of the primary production of light p-elements, which has been considered as one of the sites of a lighter element primary process (LEPP)., 25 pages, 13 figures, 2 tables, accepted for publication in Progress of Theoretical Physics

A novel technique was developed to control the concentration of water between 0.1 - 20 ppm and of oxygen between 0.3 - 2 ppm in hydrogen over a pin-on-disk apparatus. The developed technique simulates commercially available hydrogen gas at the point of use. This hydrogen usually contains sub-ppm to single-digit ppm levels of water and oxygen. The influence of impurities on the tribological properties of hardened chromium steel (JIS SUJ2) was obvious under the studied conditions, while the influence on austenitic stainless steel (JIS SUS316L) was not significant by comparison with that found in our previous study concerning higher water concentrations. The coefficient of friction of JIS SUJ2 increased with a decrease in the water concentration even at the smallest concentrations investigated in this study.

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 have made detailed calculations of the composition of magnetically driven jets ejected from collapsars, or rapidly rotating massive stars, based on long-term magnetohydrodynamic simulations of their core collapse with various distributions of magnetic field and angular momentum before collapse. We follow the evolution of the abundances of about 4000 nuclides from the collapse phase to the ejection phase and through the jet generation phase using a large nuclear reaction network. We find that the r-process successfully operates only in energetic jets (> 1e51 ergs), such that U and Th are synthesized abundantly, even when the collapsar has a relatively weak magnetic field (1e10 G) and a moderately rotating core before the collapse. The abundance patterns inside the jets are similar to those of the r-elements in the solar system. About 0.01-0.06 Msun neutron-rich, heavy nuclei are ejected from a collapsar with energetic jets. The higher energy jets have larger amounts of Ni56, varying from 0.00037 to 0.06Msun. Less energetic jets, which eject small amounts of Ni56, could induce a gamma-ray burst (GRB) a supernova, such as GRB 060505 or GRB 060614. Considerable amounts of r-elements are likely to be ejected from GRBs with hypernovae, if both the GRB and hypernova are induced by jets that are driven near the black hole., 10 pages, 14 figures, 1 table, Accepted for publication in the Astroph ysical Journal

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 $r$-process nucleosynthesis during the magnetohydrodynamical (MHD) explosion of supernova in a massive star of 13 $M_{\odot}$. Contrary to the case of the spherical explosion, jet-like explosion due to the combined effects of the rotation and magnetic field lowers the electron fraction significantly inside the layers above the iron core. We find that the ejected material of low electron fraction responsible for the $r$-process comes out from the silicon rich layer of the presupernova model. This leads to the production up to the third peak in the solar $r$-process elements. We examine whether the fission affects the $r$-process paths by using the full nuclear reaction network with both the spontaneous and $\beta$-delayed fission included. Moreover, we pay particular attention how the mass formula affects the $r$-process peaks with use of two mass formulae. It is found that both formulae can reproduce the global abundance pattern up to the third peak though detailed distributions are rather different. We point out that there are variations in the $r$-process nucleosynthesis if the MHD effects play an important role in the supernova explosion.

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 investigate the r-process that occurs due to the explosion from an inner region of the Fe-core. It is shown that the solar r-process abundances are rather well reproduced for a model of 13 M ⊙ . We also examine whether the fission affects final r-process abundances by using the full nuclear reaction network with both spontaneous and the β -delayed fission included. Moreover, we pay attention how the mass formula affects the r-process peaks by using two networks.

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 heavy element synthesis through thermally driven winds launched from an inner neutron-rich region of an accretion disk in a collapsar. Various heavy elements are found to be produced through the wind. There exist three types of winds characterized by their abundant nuclei: (i) 56 Ni, (ii) light ( Z = 30 − 45 ) neutron-rich nuclei, and (iii) light ( Z ⩽ 44 ) p-nuclei. We find that Y e , NSE > 0.49 , Y e , NSE ⩽ 0.45 , and 0.45 Y e , NSE ⩽ 0.49 for winds of types (i), (ii), and (iii), respectively, where Y e , NSE is the electron fraction of the wind at T = 9 × 10 9 K . The collasar winds may be a lighter element primary process, suggested from Galactic evolution of Sr, Y, and Zr. The p-nuclei, 92 Mo and 94 Mo, which are underproduced in oxygen layers in a supernova, can be abundantly synthesized in the collapsar winds.

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.

Using both shell-flash and realistic models on accreting neutron stars with the full nuclear reaction network up to Bi, we investigate the detailed relation between the final products of the rp-process and the ignition pressure. We find that nuclear fuels of H and 4 He are almost burned out after the flash and that the mass number of synthesized nuclei reaches to '100 in the pressure range from 10 23 to 10 23.5 dyn cm � 2 for an eutron star of 1.4Mand 10 km radius. Furthermore, p-nuclei up to 126 Xe are found to be produced after the flash, thanks to our large network. The postprocess nucleosynthesis for accretion rates of 3 � 10 � 10 ,3 � 10 � 9 , and 10 � 8 Myr � 1 , which corresponds to an ignition pressure from 10 22:7 to 10 22:9 dyn cm � 2 , reveals that H is exhausted completely during the burst. This is because H decreases significantly as a result of the steady burning before the burst and convective mixing at the initial stage; we find that 64 Zn is the most abundant element after the burst. Subject headings: accretion, accretion disks — nuclear reactions, nucleosynthesis, abundances — stars: neutron — X-rays: bursts

We investigate formation of molecules and first objects in a model with a decaying cosmological term (DCT). In particular, we show how the formation of important molecules such as H2 and HD is influenced during the epoch of the redshift z from 104 to 1. We find that for a phenomenological DCT molecular formation is significantly shifted to the earlier epoch by Δz ~ 103 compared to the case in the standard big bang cosmology, as a result of the rapid decrease in the matter temperature. The mass of first objects is estimated using the timescales relevant to their formation. We suggest that the formation scenarios of the objects studied so far could be revised if a DCT is taken into account.

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)

The thermal evolution of the universe with a variable cosmological term λ = λ10 + λ20a-m is investigated, where a is the scale factor. Energy densities of the radiation and matter are included to examine numerically the thermal evolution from the primordial nucleosynthesis to the matter-dominant era. It is found that the radiation temperature predicted from the variable λ models is lowered significantly compared with the constant λ model if λ20 0.0002 and z 104, which implies that the matter temperature decreases drastically. We stress that the strong decreases in both the radiation temperature and matter temperature affect the formation of the first objects at 10 z 1000.

The generalized scalar-tensor (GST) theory is modified. In the modified GST theory, the cosmological term ? is a function of the scalar field and of its time derivative {}2 as well. We obtain exact solutions for the scale factor, the scalar field and the cosmological term which evolves with a different dependence on time for each era of the universe. The cosmological `constant' and flatness problems are examined in the simple case of a flat universe.

We have investigated the possibility of nonexplosive core contraction of massive stars in the framework of general relativity by constructing equilibrium sequences of rapidly rotating compact stars with phenomenological equations of state for the high-density region. Since there are no satisfactory equations of state for the high-temperature/high-density region, we have devised a simplified equation of state that can be considered as representing an extremely soft equation of state for the high-density region. By analyzing the stabilities of equilibrium sequences with constant total angular momentum, we have shown that there is a chance of nonexplosive core contraction in from white dwarf to neutron star density regions. It is important to note that the pressure due to a high electron fraction and/or high temperature plays an essential role in making rapidly rotating stars dynamically stable against axisymmetric collapse. In general, both a very soft equation of state and general relativity tend to make the formation of "fizzlers" more difficult. Nevertheless, since there are parameter regions that allow for fizzlers even from general relativistic computations, the possibility of their existence becomes greater than ever in spite of our oversimplified equation of state.

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.

The large abundance ratio of 44Ti/56Ni in Cassiopeia A is puzzling. In fact, the ratio seems to be larger than the theoretical constraint derived by Woosley & Hoffman. However, this constraint is obtained on the assumption that the explosion is spherically symmetric, whereas Cas A is famous for the asymmetric form of the remnant. Recently, Nagataki et al. calculated the explosive nucleosynthesis of axisymmetrically deformed collapse-driven supernova. They reported that the ratio of 44Ti/56Ni was enhanced by the stronger alpha-rich freezeout in the polar region. In this Letter, we apply these results to Cas A and examine whether this effect can explain the large amount of 41Ti and the large ratio of 44Ti/56Ni. We demonstrate that the conventional, spherically symmetric explosion model cannot explain the 44Ti mass produced in Cas A if its lifetime is shorter than ~80 yr and the intervening space is transparent to the gamma-ray line from the decay of 44Ti. On the other hand, we show that the axisymmetric explosion models can solve the problem. We expect the same effect from a three-dimensionally asymmetric explosion, since the stronger alpha-rich freezeout will also occur in that case in the region where the larger energy is deposited.