Masa Aki Hashimoto

Presupernova evolution of massive stars

Abstract New presupernova evolutionary models of massive stars are presented. Improvements compared with the previous models are the inclusion of Coulomb interaction in the equation of state, electron capture from the beginning of oxygen burning, and many links between the silicon and iron quasi-equilibrium clusters during silicon burning. The iron core is found to be significantly smaller than in the previous models, especially for stars smaller than 15 M⊙.

Explosive nucleosynthesis in SN 1987A. II - Composition, radioactivities, and the neutron star mass

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. 135 refs.

Explosive Nucleosynthesis in Axisymmetrically Deformed Type II Supernovae

Explosive nucleosynthesis under the axisymmetric explosion in Type II supernovae has been examined by means of two-dimensional hydrodynamic calculations. We have compared the results with the observations of SN 1987A. Our chief findings are as follows: (1)44Ti is synthesized in a sufficient amount to explain the tail of the bolometric light curve of SN 1987A. We think this is because the alpha-rich freezeout takes place more actively under the axisymmetric explosion. (2)57Ni and 58Ni tend to be overproduced compared with the observations. However, this tendency relies strongly on the model of the progenitor. We have also compared the abundance of each element in the mass number range A = 16-73 with the solar values. We have found three outstanding features. (1) For the nuclei in the range A = 16-40, their abundances are insensitive to the initial form of the shock wave. This insensitivity is favored since the spherical calculations thus far can explain the solar system abundances in this mass range. (2) There is an enhancement around A = 45 in the axisymmetric explosion that compares fairly well with that of the spherical explosion. In particular,44Ca, which is underproduced in the present spherical calculations, is enhanced significantly. (3) In addition, there is an enhancement around A = 65. This feature relies on the form not of the mass cut but of the initial shock wave. This enhancement may cause the problem of overproduction in this mass range, although this effect would be relatively small since Type I supernovae are chiefly responsible for this mass number range.

A New Approach to Determine the Initial Mass Function in the Solar Neighborhood

Oxygen to iron abundance ratios of metal-poor stars provide information on nucleosynthesis yields from massive stars that end in Type II supernova (SN II) explosions. Using a standard model of chemical evolution of the Galaxy we have reproduced the solar neighborhood abundance data and estimated the oxygen and iron yields of genuine SN II origin. The estimated yields are compared with the theoretical yields to derive the relation between the lower and upper mass limits in each generation of stars and the initial mass function (IMF) slope. Independent of this relation, we furthermore derive the relation between the lower mass limit and the IMF slope from the stellar mass-to-light ratio in the solar neighborhood. These independent relations unambiguously determine the upper mass limit of mu = 50 ± 10 M☉ and the IMF slope index of 1.3-1.6 above 1 M☉. This upper mass limit corresponds to the mass beyond which stars end as black holes without ejecting processed matter into the interstellar medium. We also find that the IMF slope index below 0.5 M☉ cannot be much shallower than 0.8.

r-process nucleosynthesis in magnetohydrodynamic jet explosions of core-collapse supernovae

We investigate the r-process nucleosynthesis during a purely magnetohydrodynamic (MHD) explosion in a massive star of 13 M☉. The two-dimensional MHD simulations have been carried out from the onset of the core collapse to the shock propagation to the silicon-rich layers (~500 ms after bounce). Thereafter, using the compositions during the explosion, we calculate the r-process nucleosynthesis in the later phase by employing the two kinds of time extrapolations of the temperature and density. With these computations, we show that the jetlike explosion formed due to the combined effects of rapid rotation and strong magnetic field lowers the electron fraction significantly in the iron core, contrary to the spherical explosion. We demonstrate that the ejected material with low Ye in the jet coming out from the silicon layers is good for reproducing the third peak of the solar r-element pattern. In addition, we investigate the effects of fission using the full nuclear reaction network and the differences of two kinds of mass formulae on the r-process peaks obtained in the above MHD models. As a result, we find that both of them can reproduce the global abundance pattern up to the third peaks, although the detailed distributions are rather different. Finally, we discuss the effects of neutrino absorption reactions, which are not coupled to the above MHD simulations, on the possible reduction of Ye obtained in the above computations. We point out that there should be variations in the r-process nucleosynthesis in the supernova explosion if the MHD effects play an important role.

Low-mass helium star models for Type Ib supernovae : light curves, mixing, and nucleosynthesis

The applicability of theoretical models of He-star explosions to type Ib SN explosions is explored. Particular attention is given to light curves and mixing, Rayleigh-Taylor instabilities and mixing, and nucleosynthesis and the mass of Ni-56. Typical numerical results are presented in graphs, and it is concluded that the explosions of SN 1983N and SN 1983I can be accurately represented in terms of explosions of He stars with M(alpha) of 3-4 solar mass. A strong M(alpha) dependence of light-curve shape, photospheric velocity, and Ni-56 mass is found. 44 refs.

Magnetohydrodynamic simulations of a rotating massive star collapsing to a black hole

We perform two-dimensional, axisymmetric, magnetohydrodynamic simulations of the collapse of a rotating star of 40 M☉ in light of the collapsar model of gamma-ray bursts. Considering two distributions of angular momentum, up to ~1017 cm2 s-1, and the uniform vertical magnetic field, we investigate the formation of an accretion disk around a black hole and the jet production near the black hole. After material reaches the black hole with high angular momentum, the disk forms inside a surface of weak shock. The disk reaches a quasi-steady state for stars whose magnetic field is less than 1010 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 as long as the outer layers of the star have 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 Alfven 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 M☉ s-1 with large fluctuations. The disk is transparent for neutrinos. The dense part of the disk, which is located near the black hole, emits neutrinos efficiently at a constant rate of <8 × 1051 ergs s-1. The neutrino luminosity is much smaller than those from supernovae after the neutrino burst.

NUCLEOSYNTHESIS IN MAGNETICALLY DRIVEN JETS FROM COLLAPSARS

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 (>1051 ergs), such that U and Th are synthesized abundantly, even when the collapsar has a relatively weak magnetic field (1010 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 M☉ of neutron-rich, heavy nuclei are ejected from a collapsar with energetic jets. The higher energy jets have larger amounts of 56Ni, varying from 3.7 × 10−4 to 0.06 M☉. Less energetic jets, which eject small amounts of 56Ni, could induce a gamma-ray burst (GRB) without 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.

Heavy-element nucleosynthesis in a collapsar

We have made detailed calculations of the composition of magnetically driven jets ejected from a collapsar, based on long-term, magnetohydrodynamic simulations of a rapidly rotating, massive (40 M☉) star during core 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 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 a large magnetic field (1012 G) and a rapidly rotating core. The abundance pattern inside the jets is similar to that of the r-elements in the solar system. About 0.01 M☉ of heavy, neutron-rich nuclei can be ejected from the collapsar. The detailed abundances depend on the nuclear properties of the mass model, β-decay rate, and fission, for nuclei near the neutron drip line. Furthermore, we find that p-nuclei are produced without seeds: not only can light p-nuclei, such as 74Se, 78Kr, 84Sr, and 92Mo, be abundantly synthesized in the jets, but also heavy p-nuclei, 113In, 115Sn, and 138La. The amounts of p-nuclei in the ejecta are much greater than those in core-collapse supernovae. In particular, 92Mo, 113In, 115Sn, and 138La, which are deficient in these supernovae, are produced significantly in the collapsar ejecta.

Nucleosynthesis in ONeMg Novae: Models versus Observations to Constrain the Masses of ONeMg White Dwarfs and Their Envelopes

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 com- binations of white dwarf and envelope masses that 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-thirds of the white dwarf masses for the observed ONeMg novae are ^1.1 which is signi—cantly lower than estimated by previous M _ , hydrodynamic studies but consistent with the observations of V1974 Cyg. Moreover, the envelope masses derived from the comparison are which is in good agreement with the ejecta masses Z10~4 M _ , estimated from observations but signi—cantly higher than in previous hydrodynamic studies. With such a low-mass white dwarf and a high-mass envelope, a nova can produce interesting amounts of the c-ray emitters 7Be, 22Na, and 26Al. We suggest that V1974 Cyg has produced 22Na as high as the upper limit derived from the COMPTEL survey. In addition, a nonnegligible part, if not the majority, of the Galac- tic 26Al may originate from ONeMg novae. Both the future International Gamma-Ray Astrophysical L aboratory (INT EGRAL ) survey for these c-ray emitters and abundance estimates derived from ultra- violet, optical, and near-infrared spectroscopy will impose severe constraints on the current nova models. Subject headings: novae, cataclysmic variablesnuclear reactions, nucleosynthesis, abundances ¨ white dwarfs