arXiv subjects

M. Hashimoto

Publications and source records attributed to M. Hashimoto.

84 records · Page 5Linked to original sources

arXiv2004

Nucleosynthesis inside Gamma-Ray Burst Accretion Disks

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.

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arXiv2002

P-Process Nucleosynthesis inside Supernova-Driven Supercritical Accretion Disks

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.

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arXiv2002

Topcolor Dynamics and The effective gluon-gluon-Higgs Operator

We discuss the production of the composite Higgs boson in topcolor models via the gluon fusion process. We consider the contribution of color-octet massive gauge bosons (colorons) strongly interacting with the top quark, in addition to nonstandard contributions of the top-Yukawa coupling and heavy colored fermions other than the top quark. In order to estimate the contribution of colorons, we derive the low-energy effective theory by eliminating colorons by using the equation of motion for colorons. We replace the composite operator (\bar{q}_L t_R)(\bar{t}_R q_L) in the effective theory by the composite Higgs operator. We then obtain the effective gluon-gluon-Higgs (ggH-) operator induced by colorons and find that its coefficient (A_{col}) is proportional to m_{dyn}^2/M^2, where M and m_{dyn} denote the coloron mass and the mass dynamically generated by colorons, respectively. The contribution of colorons A_{col} becomes comparable to the top-loop effect A_{top} for M=O(1TeV) and m_{dyn}=O(0.6TeV). Such a large dynamical mass can be realized in top-seesaw (TSS) models consistently with the experimental value of the top quark mass (m_t^{exp}), while the dynamical mass itself is adjusted to m_t^{exp} in topcolor assisted technicolor models (TC2). We find that the coloron contribution A_{col} can be sizable in a certain class of TSS models: the contribution of colorons (the top-loop) is dominant in the real (imaginary) part of the H->gg amplitude for the Higgs boson mass m_H of the order of 1 TeV. On the other hand, enhancement of the top-Yukawa coupling becomes important in TC2. We can observe signatures of the Higgs boson in TC2 with m_H \sim 200 GeV even at the Tevatron Run II as well as at the LHC. We estimate S/\sqrt{B}=3-6 for an integrated luminosity of 2 fb^{-1} and m_H=190 GeV at the Tevatron Run II.

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arXiv2002

Dynamical chiral symmetry breaking in gauge theories with extra dimensions

We investigate dynamical chiral symmetry breaking in vector-like gauge theories in $D$ dimensions with ($D-4$) compactified extra dimensions, based on the gap equation (Schwinger-Dyson equation) and the effective potential for the bulk gauge theories within the improved ladder approximation. The non-local gauge fixing method is adopted so as to keep the ladder approximation consistent with the Ward-Takahashi identities. Using the one-loop $\bar{\rm MS}$ gauge coupling of the truncated KK effective theory which has a nontrivial ultraviolet fixed point (UV-FP) $g_*$ for the (dimensionless) bulk gauge coupling ${\hat g}$, we find that there exists a critical number of flavors, $N_f^{\rm crit}$ ($\simeq 4.2, 1.8$ for $D=6, 8$ for SU(3) gauge theory): For $N_f > N_f^{\rm crit}$, the dynamical chiral symmetry breaking takes place not only in the ``strong-coupling phase'' (${\hat g} >g_*$) but also in the ``weak-coupling phase'' (${\hat g} <g_*$) when the cutoff is large enough. For $N_f < N_f^{\rm crit}$, on the other hand, only the strong-coupling phase is a broken phase and we can formally define a continuum (infinite cutoff) limit, so that the physics is insensitive to the cutoff in this case. We also perform a similar analysis using the one-loop ``effective gauge coupling''. We find the $N_f^{\rm crit}$ turns out to be a value similar to that of the $\bar{\rm MS}$ case, notwithstanding the enhancement of the coupling compared with that of the $\bar{\rm MS}$.

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arXiv1999

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 combinations of white dwarf and envelope masses which give the same peak temperature, the explosion is more violent for a lower white dwarf mass owing to its smaller gravitational potential. Comparison of the nucleosynthesis results with observations implies that at least two-third of the white dwarf masses for the observed ONeMg novae are $\simeq 1.1 M_\odot$, which are significantly lower than estimated by previous hydrodynamic studies but consistent with the observations of V1974 Cyg. Moreover, the envelope masses derived from the comparison are $\gtrsim 10^{-4} M_\odot$, which are in good agreement with the ejecta masses estimated from observations but significantly higher than in previous hydrodynamic studies. With such a low mass white dwarf and a high mass envelope, the nova can produce interesting amounts of $γ$-ray emitters $^7$Be, $^{22}$Na, and $^{26}$Al. We suggest that V1974 Cyg has produced $^{22}$Na as high as the upper limit derived from the COMPTEL survey. In addition, a non-negligible part of the Galactic $^{26}$Al may originate from ONeMg novae, if not the major contributors. Both the future INTEGRAL survey for these $γ$-ray emitters and abundance estimates derived from ultraviolet, optical, and near infrared spectroscopies will impose a severe constraint on the current nova models.

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arXiv1998

High Ratio of 44Ti/56Ni in Cas A and Axisymmetric Collapse-Driven Supernova Explosion

The large abundance ratio of $^{44}Ti/^{56}Ni$ in Cas A is puzzling. In fact, the ratio seems to be larger than the theoretical constraint derived by Woosley & Hoffman (1991). 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. (1997) calculated the explosive nucleosynthesis of axisymmetrically deformed collapse-driven supernova. They reported that the ratio of $^{44}Ti/^{56}Ni$ was enhanced by the stronger alpha-rich freezeout in the polar region. In this paper, we apply these results to Cas A and examine whether this effect can explain the large amount of $^{44}Ti$ and the large ratio of $^{44}Ti/^{56}Ni$. We demonstrate that the conventional spherically symmetric explosion model can not explain the $^{44}$Ti mass produced in Cas A if its lifetime is shorter than $\sim$ 80 years and the intervening space is transparent to the gamma-ray line from the decay of $^{44}$Ti. On the other hand, we show 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.

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arXiv1998

Initial Shock Waves for Explosive Nucleosynthesis in Type II Supernova

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

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arXiv1998

Dispersion of Inferred SNe Ia/SNe II Ratios for Current Models of Supernova Nucleosynthesis

We have estimated the dispersion of the inferred relative frequencies of Type-Ia and Type- II supernovae ($N_{Ia}/ N_{II}$) in our Galaxy by fitting the numerical results of supernova nucleosynthesis to the solar-system abundances. The ratio $N_{Ia}/ N_{II}$ is estimated to be 0.056--0.14, which is consistent with the observation, if the model of Woosley and Weaver (1995, WW95) is adopted for Type-II supernovae (SNe II). On the other hand, the upper limit of $N_{Ia}/ N_{II}$ becomes too large for the model of Hashimoto (1995, Ha95). However, Ha95 can fit the solar values better than WW95 as far as the abundant nuclei are concerned. These results mean that Ha95 can reproduce the main nuclei of the solar-system well and that WW95 can reproduce the solar values over a wide mass-number range. We also note that $N_{Ia}/ N_{II}$ tends to become smaller if the delayed detonation model is adopted for Type-Ia supernovae (SNe Ia). The dispersion of $N_{Ia}/ N_{II}$ obtained in the present investigation is larger than that concluded by Tsujimoto et al. (1995), which will have an influence on the estimate of the average life time of SNe Ia's progenitors and/or the star-formation rate history in our galaxy.

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arXiv1998

Gravitino Overproduction through Moduli Decay

We derive cosmological constraints on the masses of generic scalar fields which decay only through gravitationally suppressed interactions into unstable gravitinos and ordinary particles in the supersymmetric standard model. For the gravitino mass 100GeV-1TeV, the scalar masses should be larger than 100TeV to keep the success of big-bang nucleosynthesis if no late-time entropy production dilutes the gravitino density.

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arXiv1998

Axion Cosmology with its Scalar Superpartner

In supersymmetric theories, the Peccei-Quinn symmetry has a complex extension as a symmetry of the superpotential, so that the scalar potential always has an almost flat direction, the dilaton. We discuss how coherent oscillation of the dilaton affects axion cosmology. We stress that the dilaton decay, if its dominant mode is not into axions, releases large entropy at a late epoch of the Universe's evolution to dilute axion energy density and the upperbound of the decay constant is raised up to about $10^{15}$ GeV. The case of the M-theory axion is also discussed.

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arXiv1998

Nucleosynthesis Basics and Applications to Supernovae

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

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arXiv1997

Nucleosynthesis in Type II Supernovae

Presupernova evolution and explosive nucleosynthesis in massive stars for main-sequence masses from 13 $M_\odot$ to 70 $M_\odot$ are calculated. We examine the dependence of the supernova yields on the stellar mass, $^{12}C(α, γ) ^{16}O}$ rate, and explosion energy. The supernova yields integrated over the initial mass function are compared with the solar abundances.

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