arXiv subjects

Masa-aki Hashimoto

Publications and source records attributed to Masa-aki Hashimoto.

At least 37 records · Page 2Linked to original sources

arXiv2013

Matter mixing in aspherical core-collapse supernovae: a search for possible conditions for conveying $^{56}$Ni into high velocity regions

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.

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arXiv2013

Cooling of Compact Stars with Color Superconducting Phase in Quark Hadron Mixed Phase

We present a new scenario for the cooling of compact stars considering the central source of Cassiopeia A (Cas A). The Cas A observation shows that the central source is a compact star that has high effective temperature, and it is consistent with the cooling without exotic phases. The observation also gives the mass range of $M \geqslant 1.5 M_\odot$, which may conflict with the current plausible cooling scenario of compact stars. There are some cooled compact stars such as Vela or 3C58, which can be barely explained by the minimal cooling scenario, which includes the neutrino emission by nucleon superfluidity (PBF). Therefore, we invoke the exotic cooling processes, where a heavier star cools faster than lighter one. However, the scenario seems to be inconsistent with the observation of Cas A. Therefore, we present a new cooling scenario to explain the observation of Cas A by constructing models that include a quark color superconducting (CSC) phase with a large energy gap; this phase appears at ultrahigh density region and reduces neutrino emissivity. In our model, a compact star has CSC quark core with a low neutrino emissivity surrounded by high emissivity region made by normal quarks. We present cooling curves obtained from the evolutionary calculations of compact stars: while heavier stars cool slowly, and lighter ones indicate the opposite tendency without considering nucleon superfluidity. Furthermore, we show that our scenario is consistent with the recent observations of the effective temperature of Cas A during the last 10 years, including nucleon superfluidity.

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arXiv2012

Explosive Nucleosynthesis in Magnetohydrodynamical Jets from Collapsars II. Heavy-Element Nucleosynthesis of s, r, p-Processes

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).

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arXiv2012

Constraints on a vacuum energy from both SNIa and CMB temperature observations

We investigate the cosmic thermal evolution with a vacuum energy which decays into photon at the low-redshift. We assume that the vacuum energy is a function of the scale factor that increases toward the early universe. We put on the constraints using recent observations of both type Ia supernovae (SNIa) by Union-2 compilation and the cosmic microwave background (CMB) temperature at the range of the redshift 0.01 < z < 3. From SNIa, we find that the effects of a decaying vacuum energy on the cosmic expansion rate should be very small but could be possible for z < 1.5. On the other hand, we obtain the severe constraints for parameters from the CMB temperature observations. Although the temperature can be still lower than the case of the standard cosmological model, it should only affect the thermal evolution at the early epoch.

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arXiv2012

R-process Nucleosynthesis during the Magnetohydrodynamics Explosions of a Massive Star

We investigate the possibility of the r-process during the magnetohydrohynamical explosion of supernova in a massive star of 13 solar mass with the effects of neutrinos induced. We adopt five kinds of initial models which include properties of rotation and the toroidal component of the magnetic field . The simulations which succeed the explosions are limitted to a concentrated magnetic field and strong differential rotation. Low $Y_{e}$ ejecta produce heavy elements and the third peak can be reprocuced. However, the second peak is low because $Y_{e}$ distribution as a function of radius is steep and ejecta corresponding to middle $Y_{e}$ is very few.

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arXiv2011

Effects of a New Triple-α Reaction on the S-process in Massive Stars

Effects of a new triple-α 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-α 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.

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arXiv2011

Explosive nucleosynthesis in the neutrino-driven aspherical supernova explosion of a non-rotating 15$M_{\odot}$ star with solar metallicity

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.

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arXiv2010

Effects of a new triple-$α$ reaction rate on the helium ignition of accreting white dwarfs

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.

astro-ph.HE↗

arXiv2008

Reply to 'Comment on 'Heavy element production in inhomogeneous big bang nucleosynthesis''

This is a reply report to astro-ph/0604264. We studied heavy element production in high baryon density region in early universe astro-ph/0507439. However it is claimed in astro-ph/0604264 that small scale but high baryon density region contradicts observations for the light element abundance or in order not to contradict to observations high density region must be so small that it cannot affect the present heavy element abundance. In this paper we study big bang nucleosynthesis in high baryon density region and show that in certain parameter spaces it is possible to produce enough amount of heavy element without contradiction to CMB and light element observations.

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arXiv2008

Cosmic microwave background constraints on a decaying cosmological term related to the thermal evolution

We constrain the thermal evolution of the universe with a decaying cosmological term by using the method of the analysis for the Wilkinson Microwave Anisotropy Probe (WMAP) observation data. The cosmological term is assumed to be a function of the scale factor that increases toward the early universe, and the radiation energy density is lower compared to that in the model with the standard cosmological "constant" (LCDM). The decrease in the radiation density affects the thermal history of the universe; e.g. the photon decoupling occurs at higher-z compared to the case of the standard LCDM model. As a consequence, a decaying cosmological term affects the cosmic microwave background anisotropy. Thanks to the Markov-chain Monte Carlo method, we compare the angular power spectrum in the decaying LCDM model with the CMB data, and we get severe constraints on parameters of the model.

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arXiv2008

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 (> 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.

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arXiv2007

Observational constraints on a decaying cosmological term

We investigate the evolution of a universe with a decaying cosmological term (vacuum energy) that is assumed to be a function of the scale factor. In this model, while the cosmological term increases to the early universe, the radiation energy density is lower than the model with the cosmological "constant". We find that the effects of the decaying cosmological term on the expansion rate at the redshift z<2 is negligible. However, the decrease in the radiation density affects on the thermal history of the universe; e.g. the photon decoupling occurs at higher $z$ compared to the case of the standard ΛCDM model. As a consequence, a decaying cosmological term affects on the cosmic microwave background anisotropy. We show the angular power spectrum in DΛCDM model and compare with the Wilkinson Microwave Anisotropy Probe (WMAP) data.

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arXiv2007

Effects of QCD phase transition on gravitational radiation from two-dimensional collapse and bounce of massive stars

We perform two-dimensional, magnetohydrodynamical core-collapse simulations of massive stars accompanying the QCD phase transition. We study how the phase transition affects the gravitational waveforms near the epoch of core-bounce. As for initial models, we change the strength of rotation and magnetic fields. Particularly, the degree of differential rotation in the iron core (Fe-core) is changed parametrically. As for the microphysics, we adopt a phenomenological equation of state above the nuclear density, including two parameters to change the hardness before the transition. We assume the first order phase transition, where the conversion of bulk nuclear matter to a chirally symmetric quark-gluon phase is described by the MIT bag model. Based on these computations, we find that the phase transition can make the maximum amplitudes larger up to $\sim$ 10 percents than the ones without the phase transition. On the other hand, the maximum amplitudes become smaller up to $\sim$ 10 percents owing to the phase transition, when the degree of the differential rotation becomes larger. We find that even extremely strong magnetic fields $\sim 10^{17}$ G in the protoneutron star do not affect these results.

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arXiv2006

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 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{é}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.

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arXiv2006

Heavy element nucleosynthesis in a collapsar

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.

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arXiv2005

Possible evolutionary transition from rapidly rotating neutron stars to strange stars due to spin-down

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.

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arXiv2005

R-Process Nucleosynthesis in MHD Jet Explosions of Core-Collapse Supernovae

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 $β$-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.

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arXiv2001

Explosive Nucleosynthesis in Hypernovae

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.

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