arXiv subjects

Masaomi Ono

Publications and source records attributed to Masaomi Ono.

27 records · Page 2Linked to original sources

arXiv2019

Effects of the Metallicity on Li and B Production in Supernova Neutrino Process

The neutrino process ($ν$-process) for the production of 7Li and 11B in core-collapse supernovae (SNe) is extensively investigated. Initial abundances of s-nuclei and other physical conditions are derived from an updated calculation of the SN 1987A progenitor. The nuclear reaction network including neutrino reactions is constructed with the variable order Bader-Deuflhard integration method. We find that yields of 7Li and 11B significantly depend on the stellar metallicity while they are independent of the weak s-process during the stellar evolution. When the metallicity is high, there are more neutron absorbers, i.e., 56Fe, 14N (from initial CNO nuclei), and 54Fe, and the neutron abundance is small during the $ν$-process. Since 7Be is predominantly destroyed via 7Be(n,p)7Li, a change in the neutron abundance results in different 7Be yields. Then, the calculated yield ratio 7Li/11B=0.93 for the solar metallicity is larger than that for the SN 1987A 7Li/11B=0.80 by 16 % in the inverted mass hierarchy case. We analyze contributions of respective reactions as well as abundance evolution, and clarify the $ν$-process of 7Li and 11B.

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arXiv2019

From the supernova to the supernova remnant: the three-dimensional imprint of a thermonuclear explosion

Recent progress in the three-dimensional modeling of supernovae (SN) has shown the importance of asymmetries for the explosion. This calls for a reconsideration of the modeling of the subsequent phase, the supernova remnant (SNR), which has commonly relied on simplified ejecta models. In this paper we bridge SN and SNR studies by using the output of a SN simulation as the input of a SNR simulation carried on until 500~yr. We consider the case of a thermonuclear explosion of a carbon-oxygen white dwarf star as a model for a Type Ia SN; specifically we use the N100 delayed detonation model of Seitenzahl et al 2013. In order to analyze the morphology of the SNR, we locate the three discontinuities that delineate the shell of shocked matter: the forward shock, the contact discontinuity, and the reverse shock, and we decompose their radial variations as a function of angular scale and time. Assuming a uniform ambient medium, we find that the impact of the SN on the SNR may still be visible after hundreds of years. Previous 3D simulations aiming at reproducing Tycho's SNR, that started out from spherically symmetric initial conditions, failed to reproduce structures at the largest angular scales observed in X-rays. Our new simulations strongly suggest that the missing ingredient was the initial asymmetries from the SN itself. With this work we establish a way of assessing the viability of SN models based on the resulting morphology of the SNR.

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arXiv2019

Supernova Neutrino Process of Li and B Revisited

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.

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arXiv2015

Effects of Triple-$α$ and $^{12}\rm C(α,γ)^{16}O$ Reaction Rates on the Supernova Nucleosynthesis in a Massive Star of 25 $M_{\odot}$

We investigate effects of triple-$α$ and $^{12}\rm C(α,γ) ^{16}O$ 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-$α$ reaction rates and two $^{12}\rm C(α,γ)^{16}O$ 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-$α$ reaction rate and a rather higher value of the reaction rate within the upper limit for the experimental uncertainties is favorable for a $^{12}\rm C(α,γ)^{16}O$ rate.

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arXiv2013

Photospheric emission from stratified jets

We explore photospheric emissions from stratified two-component jets, wherein a highly relativistic spine outflow is surrounded by a wider and less relativistic sheath outflow. Thermal photons are injected in regions of high optical depth and propagated until they escape at the photosphere. Due to the presence of shear in velocity (Lorentz factor) at the boundary of the spine and sheath region, a fraction of the injected photons are accelerated via a Fermi-like acceleration mechanism such that a high energy power-law tail is formed in the resultant spectrum. We show, in particular, that if a velocity shear with a considerable variance in the bulk Lorentz factor is present, the high energy part of observed Gamma-ray Bursts (GRBs) photon spectrum can be explained by this photon acceleration mechanism. We also show that the accelerated photons may also account for the origin of the extra hard power-law component above the bump of the thermal-like peak seen in some peculiar bursts (e.g., GRB 090510, 090902B, 090926A). It is demonstrated that time-integrated spectra can also reproduce the low energy spectrum of GRBs consistently due to a multi-temperature effect when time evolution of the outflow is considered. Finally, we show that the empirical Ep-Lp relation can be explained by differences in the outflow properties of individual sources.

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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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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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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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