T. Shigeyama - Academia.edu (original) (raw)
Papers by T. Shigeyama
Soryushiron Kenkyu Electronics, 1996
Meeting Abstracts of the Physical Society of Japan (Nihon Butsuri Gakkai koen gaiyoshu), 2016
The Astrophysical Journal, 2003
The star formation history of the globular cluster ω Centauri is investigated in the context of a... more The star formation history of the globular cluster ω Centauri is investigated in the context of an inhomogeneous chemical evolution model in which supernovae induce star formation. The proposed model explains recent observations for ω Cen stars, and divides star formation into three epochs. At the end of the first epoch, ∼ 70% of the gas was expelled by supernovae. AGB stars then supplied s-process elements to the remaining gas during the first interval of ∼ 300 Myr. This explains the observed sudden increase in Ba/Fe ratios in ω Cen stars at [Fe/H]∼ −1.6. Supernovae at the end of the second epoch were unable to expel the gas. Eventually, Type Ia supernovae initiated supernova-induced star formation, and remaining gas was stripped when the cluster passed through the newly formed disk of the Milky Way. The formation of ω Cen is also discussed in the framework of globular cluster formation triggered by cloud-cloud collisions. In this scenario, the relative velocity of clouds in the collision determines the later chemical evolution in the clusters. A head-on collision of proto-cluster clouds with a low relative velocity would have converted less than 1% of gas into stars and promoted the subsequent chemical evolution by supernova-driven star formation. This is consistent with present observed form of ω Cen. In contrast, the other Galactic globular clusters are expected to have formed from more intense head-on collisions, and the resultant clouds would have been too thin for supernovae to accumulate enough gas to form the next generation of stars. This explains the absence of chemical evolution in these other globular clusters.
Astronomy & Astrophysics, 2020
Context. Observations suggest that some massive stars experience violent and eruptive mass loss a... more Context. Observations suggest that some massive stars experience violent and eruptive mass loss associated with significant brightening that cannot be explained by hydrostatic stellar models. This event seemingly forms dense circumstellar matter (CSM). The mechanism of eruptive mass loss has not been fully explained. We focus on the fact that the timescale of nuclear burning gets shorter than the dynamical timescale of the envelope a few years before core collapse for some massive stars. Aims. To reveal the properties of the eruptive mass loss, we investigate its relation to the energy injection at the bottom of the envelope supplied by nuclear burning taking place inside the core. In this study, we do not specify the actual mechanism for transporting energy from the site of nuclear burning to the bottom of the envelope. Instead, we parameterize the amount of injected energy and the injection time and try to extract information on these parameters from comparisons with observations....
International Astronomical Union Colloquium, 1996
We summarize various explosion models of Type la supernovae and their nucleosynthesis features fo... more We summarize various explosion models of Type la supernovae and their nucleosynthesis features for both Chandrasekhar and sub-Chandrasekhar mass white dwarf models. These models provide different predictions of the photometric and spectroscopic variations among Type la supernovae, which are compared with observations. Some attempts to model the peculiar SNe 1991T and 1991bg are shown.
International Astronomical Union Colloquium, 1988
The X-ray spectrum observed by Ginga is characterized by a component below 10keV which decreases ... more The X-ray spectrum observed by Ginga is characterized by a component below 10keV which decreases with increasing photon energy, and a component above 10keV which is nearly flat. This unusual X-ray spectrum may be understood as follows; X-rays below 10keV is likely to be due to thermal emission coming from the shock-heated ejecta, and X-rays above 10keV to be due to γ-ray degradation inside the ejecta. If thermal emission due to the collision of the ejecta with circumstellar matter (CSM) is responsible for X-rays below 10keV, the epoch of the collision can be estimated to be ∼ 0.2yr after the explosion if ∼ 0.5yr is the time when the X-ray flux at ∼ 10keV reaches its maximum. The X-ray light curve then requires the inner radius of CSM to be ∼ 1×1016cm for an expansion velocity, Vex ≃2×109cm s−1.
International Astronomical Union Colloquium, 1988
Gamma-rays originating from radioactive decays of56Ni and56Co and hard X-rays due to Compton degr... more Gamma-rays originating from radioactive decays of56Ni and56Co and hard X-rays due to Compton degradation ofγ-rays have been predicted to emerge when the supernova becomes sufficiently thin. The X-ray detections by Ginga (Dotani et al. 1988) and Kvant (Sunyaev et al. 1988) and more recent report ofγ-ray detections by SMM (Matz et al. 1988) were much earlier than the theoretical predictions. (See Itoh et al. 1987 and references therein.)These observations would give important constraints on the distribution of the heavy elements and56Co in the ejecta. We adopted the hydrodynamical model 11E1Y6 (Nomoto et al. 1988) and carried out Monte Carlo simulation for photon transfer. A step-like distribution of56Co was assumed where the mass fraction of56Co in the layers atMr≤ 4.6M⊙, 4.6 − 6M⊙, 6 − 8M⊙, and 8 − 10M⊙areXCo= 0.0128, 0.0035, 0.0021, and 0.0011, respectively. Other heavy elements were distributed with mass fractions in proportion to56Co.
AIP Conference Proceedings, 1991
ABSTRACT
Origin of Matter and Evolution of Galaxies 2003, 2004
ABSTRACT
AIP Conference Proceedings, 2014
ABSTRACT By making use of high spatial resolution of Subaru 8.2m Telescope and IRCS Echelle spect... more ABSTRACT By making use of high spatial resolution of Subaru 8.2m Telescope and IRCS Echelle spectrograph, we obtained high-resolution near-infrared spectra of images A and B (AB ˜ 0''.5) of the gravitationally lensed QSO B1422+231 (z = 3.628) consisting of four known lensed images. We detected MgII and FeII absorption lines at z = 3.54, which show a large variance of column densities and velocities between the sightlines A and B with a projected separation of only 8.4 h70-1 pc at the redshift. The observed systematic variances between images A and B imply that the absorbing gas cloud is an expanding shell of a supernova remnant (SNR) as originally suggested by Rauch et al. for images A and C. Along with the Fe richness of the gas cloud, we conclude that the SNR is produced by a SNIa explosion.
Hydrodynamic evolution of supernova remnants in collided proto-cluster clouds is investigated in ... more Hydrodynamic evolution of supernova remnants in collided proto-cluster clouds is investigated in the frame work of a supernova-driven star formation scenario. It is found that the relative velocity of proto-clouds must be greater than a certain value, which is a function of the mass of each proto-cloud, to produce a stellar cluster with little dispersion of Fe/H ratios among the member stars. The metallicity distribution function for globular clusters in the Galactic halo is calculated from a simple model. This calculation shows that cloud-cloud collisions can reproduce the observed metallicity distribution function for globular clusters. Before cloud-cloud collisions, each cloud has been enriched with heavy elements according to a supernova-driven star formation scenario that reproduces the observed abundance distribution function for the Galactic halo field stars.
Soryushiron Kenkyu Electronics, 1996
Meeting Abstracts of the Physical Society of Japan (Nihon Butsuri Gakkai koen gaiyoshu), 2016
The Astrophysical Journal, 2003
The star formation history of the globular cluster ω Centauri is investigated in the context of a... more The star formation history of the globular cluster ω Centauri is investigated in the context of an inhomogeneous chemical evolution model in which supernovae induce star formation. The proposed model explains recent observations for ω Cen stars, and divides star formation into three epochs. At the end of the first epoch, ∼ 70% of the gas was expelled by supernovae. AGB stars then supplied s-process elements to the remaining gas during the first interval of ∼ 300 Myr. This explains the observed sudden increase in Ba/Fe ratios in ω Cen stars at [Fe/H]∼ −1.6. Supernovae at the end of the second epoch were unable to expel the gas. Eventually, Type Ia supernovae initiated supernova-induced star formation, and remaining gas was stripped when the cluster passed through the newly formed disk of the Milky Way. The formation of ω Cen is also discussed in the framework of globular cluster formation triggered by cloud-cloud collisions. In this scenario, the relative velocity of clouds in the collision determines the later chemical evolution in the clusters. A head-on collision of proto-cluster clouds with a low relative velocity would have converted less than 1% of gas into stars and promoted the subsequent chemical evolution by supernova-driven star formation. This is consistent with present observed form of ω Cen. In contrast, the other Galactic globular clusters are expected to have formed from more intense head-on collisions, and the resultant clouds would have been too thin for supernovae to accumulate enough gas to form the next generation of stars. This explains the absence of chemical evolution in these other globular clusters.
Astronomy & Astrophysics, 2020
Context. Observations suggest that some massive stars experience violent and eruptive mass loss a... more Context. Observations suggest that some massive stars experience violent and eruptive mass loss associated with significant brightening that cannot be explained by hydrostatic stellar models. This event seemingly forms dense circumstellar matter (CSM). The mechanism of eruptive mass loss has not been fully explained. We focus on the fact that the timescale of nuclear burning gets shorter than the dynamical timescale of the envelope a few years before core collapse for some massive stars. Aims. To reveal the properties of the eruptive mass loss, we investigate its relation to the energy injection at the bottom of the envelope supplied by nuclear burning taking place inside the core. In this study, we do not specify the actual mechanism for transporting energy from the site of nuclear burning to the bottom of the envelope. Instead, we parameterize the amount of injected energy and the injection time and try to extract information on these parameters from comparisons with observations....
International Astronomical Union Colloquium, 1996
We summarize various explosion models of Type la supernovae and their nucleosynthesis features fo... more We summarize various explosion models of Type la supernovae and their nucleosynthesis features for both Chandrasekhar and sub-Chandrasekhar mass white dwarf models. These models provide different predictions of the photometric and spectroscopic variations among Type la supernovae, which are compared with observations. Some attempts to model the peculiar SNe 1991T and 1991bg are shown.
International Astronomical Union Colloquium, 1988
The X-ray spectrum observed by Ginga is characterized by a component below 10keV which decreases ... more The X-ray spectrum observed by Ginga is characterized by a component below 10keV which decreases with increasing photon energy, and a component above 10keV which is nearly flat. This unusual X-ray spectrum may be understood as follows; X-rays below 10keV is likely to be due to thermal emission coming from the shock-heated ejecta, and X-rays above 10keV to be due to γ-ray degradation inside the ejecta. If thermal emission due to the collision of the ejecta with circumstellar matter (CSM) is responsible for X-rays below 10keV, the epoch of the collision can be estimated to be ∼ 0.2yr after the explosion if ∼ 0.5yr is the time when the X-ray flux at ∼ 10keV reaches its maximum. The X-ray light curve then requires the inner radius of CSM to be ∼ 1×1016cm for an expansion velocity, Vex ≃2×109cm s−1.
International Astronomical Union Colloquium, 1988
Gamma-rays originating from radioactive decays of56Ni and56Co and hard X-rays due to Compton degr... more Gamma-rays originating from radioactive decays of56Ni and56Co and hard X-rays due to Compton degradation ofγ-rays have been predicted to emerge when the supernova becomes sufficiently thin. The X-ray detections by Ginga (Dotani et al. 1988) and Kvant (Sunyaev et al. 1988) and more recent report ofγ-ray detections by SMM (Matz et al. 1988) were much earlier than the theoretical predictions. (See Itoh et al. 1987 and references therein.)These observations would give important constraints on the distribution of the heavy elements and56Co in the ejecta. We adopted the hydrodynamical model 11E1Y6 (Nomoto et al. 1988) and carried out Monte Carlo simulation for photon transfer. A step-like distribution of56Co was assumed where the mass fraction of56Co in the layers atMr≤ 4.6M⊙, 4.6 − 6M⊙, 6 − 8M⊙, and 8 − 10M⊙areXCo= 0.0128, 0.0035, 0.0021, and 0.0011, respectively. Other heavy elements were distributed with mass fractions in proportion to56Co.
AIP Conference Proceedings, 1991
ABSTRACT
Origin of Matter and Evolution of Galaxies 2003, 2004
ABSTRACT
AIP Conference Proceedings, 2014
ABSTRACT By making use of high spatial resolution of Subaru 8.2m Telescope and IRCS Echelle spect... more ABSTRACT By making use of high spatial resolution of Subaru 8.2m Telescope and IRCS Echelle spectrograph, we obtained high-resolution near-infrared spectra of images A and B (AB ˜ 0''.5) of the gravitationally lensed QSO B1422+231 (z = 3.628) consisting of four known lensed images. We detected MgII and FeII absorption lines at z = 3.54, which show a large variance of column densities and velocities between the sightlines A and B with a projected separation of only 8.4 h70-1 pc at the redshift. The observed systematic variances between images A and B imply that the absorbing gas cloud is an expanding shell of a supernova remnant (SNR) as originally suggested by Rauch et al. for images A and C. Along with the Fe richness of the gas cloud, we conclude that the SNR is produced by a SNIa explosion.
Hydrodynamic evolution of supernova remnants in collided proto-cluster clouds is investigated in ... more Hydrodynamic evolution of supernova remnants in collided proto-cluster clouds is investigated in the frame work of a supernova-driven star formation scenario. It is found that the relative velocity of proto-clouds must be greater than a certain value, which is a function of the mass of each proto-cloud, to produce a stellar cluster with little dispersion of Fe/H ratios among the member stars. The metallicity distribution function for globular clusters in the Galactic halo is calculated from a simple model. This calculation shows that cloud-cloud collisions can reproduce the observed metallicity distribution function for globular clusters. Before cloud-cloud collisions, each cloud has been enriched with heavy elements according to a supernova-driven star formation scenario that reproduces the observed abundance distribution function for the Galactic halo field stars.