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Ken'ichi Nomoto

Publications and source records attributed to Ken'ichi Nomoto.

At least 217 records · Page 12Linked to original sources
arXiv2001

The metamorphosis of SN1998bw

We present and discuss the photometric and spectroscopic evolution of the peculiar SN1998bw, associated with GRB980425, through an analysis of optical and near IR data collected at ESO-La Silla. The spectroscopic data, spanning the period from day -9 to day +376 (relative to B maximum), have shown that this SN was unprecedented, although somewhat similar to SN1997ef. Maximum expansion velocities as high as 3x10^4 km/s to some extent mask its resemblance to other Type Ic SNe. At intermediate phases, between photospheric and fully nebular, the expansion velocities (~10^4 km/s) remained exceptionally high compared to those of other recorded core-collapse SNe at a similar phase. The mild linear polarization detected at early epochs suggests the presence of asymmetry in the emitting material. The degree of asymmetry, however, cannot be decoded from these measurements alone. The HeI 1.083 mu and 2.058 mu lines are identified and He is suggested to lie in an outer region of the envelope. The temporal behavior of the fluxes and profiles of emission lines of MgI]4571A, [OI]6300,6364A and a feature ascribed to Fe are traced to stimulate future modeling work.

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arXiv2001

Cosmic Supernova Rate History and Type Ia Supernova Progenitors

Adopting a single degenerate scenario for Type Ia supernova progenitors with the metallicity effect, we make a prediction of the cosmic supernova rate history as a composite of the supernova rates in spiral and elliptical galaxies, and compare with the recent observational data up to z ~ 0.55.

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arXiv2000

Light Curve and Spectral Models for the Hypernova SN 1998bw associated with GRB980425

A refined model for the unusual Type Ic supernova 1998bw, discovered as the optical counterpart of GRB980425, is presented, and synthetic light curves and spectra are compared with the observations. The first 30 days of the light curve and the broad line features of the spectra can be reproduced with the hydrodynamical model of the explosion of a 14$M_\odot$ C+O star, the core of a star with initial mass 40$M_\odot$, assuming that the explosion was very energetic (kinetic energy $E_{\rm K} = 5 \times 10^{52}$ erg) and that 0.4$M_\odot$ of $^{56}$Ni were synthesized. At late times, however, the observed light curve tail declines more slowly than this energetic model, and is in better agreement with a less energetic ($E_{\rm K} = 7 \times 10^{51}$ erg) one. This shift to a less energetic model may imply that the inner part of the ejecta has higher density and lower velocities than the spherically symmetric model with $E_{\rm K} = 5 \times 10^{52}$ erg, so that $γ$-rays deposit more efficiently. We suggest that an aspherical explosion can produce such a structure of the ejecta. We also study detailed nucleosynthesis calculations for hyper-energetic supernova explosions and compare the yields with those of normal supernovae.

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arXiv2000

Can differences in the nickel abundance in Chandrasekhar mass models explain the relation between brightness and decline rate of normal Type Ia Supernovae?

The use of Type Ia supernovae as distance indicators relies on the determination of their brightness. This is not constant, but it can be calibrated using an observed relation between the brightness and the properties of the optical light curve (decline rate, width, shape), which indicates that brighter SNe have broader, slower light curves. However, the physical basis for this relation is not yet fully understood. Among possible causes are different masses of the progenitor white dwarfs or different opacities in Chandrasekhar-mass explosions. We parametrise the Chandrasekhar-mass models presented by Iwamoto et al (1999), which synthesize different amounts of Ni, and compute bolometric light curves and spectra at various epochs. Since opacity in SNe Ia is due mostly to spectral lines, it should depend on the mass of Fe-peak elements synthesized in the explosion, and on the temperature in the ejecta. Bolometric light curves computed using these prescriptions for the optical opacity reproduce the relation between brightness and decline rate. Furthermore, when spectra are calculated, the change in colour between maximum and two weeks later allows the observed relation between M_B(Max) and Dm_{15}(B) to be reproduced quite nicely. Spectra computed at various epochs compare well with corresponding spectra of spectroscopically normal SNeIa selected to cover a similar range of Dm_{15}(B) values.

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arXiv2000

A Spectroscopic Analysis of the Energetic Type Ic `Hypernova' SN 1997ef

The properties of the bright and energetic Type Ic SN 1997ef are investigated using a Monte Carlo spectrum synthesis code. Analysis of the earliest spectra is used to determine the time of outburst. The changing features of the spectrum and the light curve are used to probe the ejecta and to determine their composition, verifying the results of explosion calculations. Since synthetic spectra computed using our best explosion model CO100 are only moderately good reproductions of the observations, the inverse approach is adopted, and a density structure is derived by demanding that it gives the best possible fit to the observed spectrum at every epoch analysed. It is found that the density structure of model CO100 is adequate at intermediate velocities (5000--25000 km/s), but that a slower density decline ($ρ\propto r^{-4}$) is required to obtain the extensive line blending at high velocities (25000--50000 km/s). The `best fit' density distribution results in somewhat different parameters for the SN, namely an ejecta mass of 9.6$M_\odot$ and an explosion kinetic energy of 1.75 x 10^{52} erg. The modified density structure is used to compute a synthetic light curve, which is found to agree very well with the observed bolometric light curve around maximum. The amount of radioactive $^{56}$Ni produced by the SN is confirmed at 0.13$M_\odot$. In the context of an axisymmetric explosion, a somewhat smaller kinetic energy than that of SN 1998bw may have resulted from the non alignment of the symmetry axis of the SN and the line of sight. This might also explain the lack of evidence for a Gamma Ray Burst correlated with SN 1997ef.

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arXiv2000

A Model for the Quiescent Phase of the Recurrent Nova U Scorpii

A theoretical light curve is constructed for the quiescent phase of the recurrent nova U Scorpii in order to resolve the existing distance discrepancy between the outbursts (6 kpc) and the quiescences (14 kpc). Our U Sco model consists of a very massive white dwarf (WD), an accretion disk (ACDK) with a flaring-up rim, and a lobe-filling, slightly evolved, main-sequence star (MS). The model properly includes an accretion luminosity of the WD, a viscous luminosity of the ACDK, a reflection effect of the MS and the ACDK irradiated by the WD photosphere. The B light curve is well reproduced by a model of 1.37 M_sun WD + 1.5 M_sun MS (0.8---2.0 M_sun MS is acceptable) with an ACDK having a flaring-up rim, and the inclination angle i~80 degree. The calculated color is rather blue (B-V~0.0) for a suggested mass accretion rate of 2.5 x 10^{-7} M_sun yr^{-1}, thus indicating a large color excess of E(B-V)~0.56 with the observational color of B-V=0.56 in quiescence. Such a large color excess corresponds to an absorption of A_V~1.8 and A_B~2.3, which reduces the distance to 6-8 kpc. This is in good agreement with the distance estimation of 4-6 kpc for the latest outburst. Such a large intrinsic absorption is very consistent with the recently detected period change of U Sco, which is indicating a mass outflow of ~3 x 10^{-7} M_sun yr^{-1} through the outer Lagrangian points in quiescence.

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arXiv2000

The History of the Comic Supernova Rate Derived from the Evolution of the Host Galaxies

We make a prediction of the cosmic supernova rate history as a composite of the supernova rates in spiral and elliptical galaxies. We include the metallicity effect on the evolution of Type Ia supernova (SN Ia) progenitors, and construct detailed models for the evolutions of spiral and elliptical galaxies in clusters and field to meet the latest observational constraints. In the cluster environment, the synthesized cosmic star formation rate (SFR) has an excess at $z \gtsim 3$ corresponding to the early star burst in ellipticals and a shallower slope from the present to the peak at the redshift of $z \sim 1.4$ compared with Madau's plot. In the field environment, we assume that ellipticals form over a wide range of redshifts as $1 \ltsim z \ltsim 4$. The synthesized cosmic SFR has a broad peak around $z \sim 3$, which is in good agreement with the observed one. The resultant cosmic SFRs lead to the following predictions for the cosmic SN Ia rate: 1) The SN Ia rate in spirals has a break at $z \sim 2$ due to the low-metallicity inhibition of SNe Ia, regardless of clusters or field. 2) At high redshifts, the SN Ia rate has a strong peak around $z \sim 3$ in clusters, whereas in field much lower rate is expected, reflecting the difference in the formation epochs of ellipticals.

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arXiv2000

Type Ia Supernovae: Progenitors and Evolution with Redshift

Relatively uniform light curves and spectral evolution of Type Ia supernovae (SNe Ia) have led to the use of SNe Ia as a ``standard candle'' to determine cosmological parameters. Whether a statistically significant value of the cosmological constant can be obtained depends on whether the peak luminosities of SNe Ia are sufficiently free from the effects of cosmic and galactic evolutions. Here we first review the single degenerate scenario for the Chandrasekhar mass white dwarf (WD) models of SNe Ia. We identify the progenitor's evolution and population with two channels: (1) the WD+RG (red-giant) and (2) the WD+MS (near main-sequence He-rich star) channels. In these channels, the strong wind from accreting WDs plays a key role, which yields important age and metallicity effects on the evolution. We then address the questions whether the nature of SNe Ia depends systematically on environmental properties such as metallicity and age of the progenitor system and whether significant evolutionary effects exist. We suggest that the variation of the carbon mass fraction $X$(C) in the C+O WD (or the variation of the initial WD mass) causes the diversity of the brightness of SNe Ia. This model can explain the observed dependences of SNe Ia brightness on the galaxy types and the distance from the galactic center. Finally, applying the metallicity effect on the evolution of SN Ia progenitors, we make a prediction of the cosmic supernova rate history as a composite of the supernova rates in different types of galaxies.

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arXiv2000

The Role of Electron Captures in Chandrasekhar Mass Models for Type Ia Supernovae

The Chandrasekhar mass model for Type Ia Supernovae (SNe Ia) has received increasing support from recent comparisons of observations with light curve predictions and modeling of synthetic spectra. It explains SN Ia events via thermonuclear explosions of accreting white dwarfs in binary stellar systems, being caused by central carbon ignition when the white dwarf approaches the Chandrasekhar mass. As the electron gas in white dwarfs is degenerate, characterized by high Fermi energies for the high density regions in the center, electron capture on intermediate mass and Fe-group nuclei plays an important role in explosive burning. Electron capture affects the central electron fraction Y_e, which determines the composition of the ejecta from such explosions. Up to the present, astrophysical tabulations based on shell model matrix elements were only available for light nuclei in the sd-shell. Recently new Shell Model Monte Carlo (SMMC) and large-scale shell model diagonalization calculations have also been performed for pf-shell nuclei. These lead in general to a reduction of electron capture rates in comparison with previous, more phenomenological, approaches. Making use of these new shell model based rates, we present the first results for the composition of Fe-group nuclei produced in the central regions of SNe Ia and possible changes in the constraints on model parameters like ignition densities and burning front speeds.

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arXiv2000

Numerical Simulations of Globular Cluster Formation

We examine various physical processes associated with the formation of globular clusters by using the three-dimensional Smoothed Particle Hydrodynamics (SPH) code. Our code includes radiative cooling of gases, star formation, energy feedback from stars including stellar winds and supernovae, and chemical enrichment by stars. We assume that, in the collapsing galaxy, isothermal cold clouds form through thermal condensations and become proto-globular clouds. We calculate the size of proto-globular clouds by solving the linearized equations for perturbation. We compute the evolution of the inner region of the proto-cloud with our SPH code for various initial radius and initial composition of gases. When the initial gases contain no heavy elements, the evolution of proto-clouds sensitively depends on the initial radius. For a smaller initial radius, the initial star burst is so intense that the subsequent star formation occurs in the central regions to form a dense star cluster as massive as the globular cluster. When the initial gases contain some heavy elements, the metallicity of gases affects the evolution and the final stellar mass. If the initial radius of the proto-globular clouds was relatively large, the formation of a star cluster as massive as the globular clusters requires the initial metallicity as high as [Fe/H] $\geq -2$. The self-enrichment of heavy elements in the star cluster does not occur in all cases.

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arXiv1999

Evolution and Nucleosynthesis of Metal-Free Massive Stars

We calculate presupernova evolutions and supernova explosions of massive stars (M=13-25 Mo) for various metallicities. We find the following characteristic abundance patterns of nucleosynthesis in the metal-free (Pop III) stars. (1) The alpha-nuclei (from C to Zn) are more efficiently produced than other isotopes, and the abundance pattern of alpha-nuclei can be similar to the solar abundance. In particular, near solar ratios of alpha elements/Fe might be a signature of Pop III which could produce a large amount of Fe. (2) The abundance ratios of odd Z to even Z elements such as Na/Mg and Al/Mg become smaller for lower metallicity. However, these ratios almost saturate below Z <~ 10^{-5}, and [Na, Al/Mg] ~ - 1 for Pop III and low metal Pop II nucleosynthesis. This result is consistent with abundance pattern of metal poor stars, in which these ratios also saturate around -1. We suggest that these stars with the lowest [Na/Mg] or [Al/Mg] may contain the abundance pattern of Pop III nucleosynthesis. (3) Metal poor stars show interesting trends in the ratios of [Cr, Mn, Co/Fe]. We discuss that these trends are not explained by the differences in metallicity, but by the relative thickness between the complete and the incomplete Si burning layers. Large [Co/Fe] and small [Cr, Mn/Fe] values found in the observations are explained if mass cut is deep or if matter is ejected from complete Si burning layer in a form of a jet or bullets. (4) We also find that primary ^{14}N production occurs in the massive Pop III stars, because these stars have radiative H-rich envelopes so that the convective layer in the He-shell burning region can reach the H-rich region.

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arXiv1999

Radiation hydrodynamics of SN 1987A: I. Global analysis of the light curve for the first 4 months

The optical/UV light curves of SN 1987A are analyzed with the multi-energy group radiation hydrodynamics code STELLA. The calculated monochromatic and bolometric light curves are compared with observations shortly after shock breakout, during the early plateau, through the broad second maximum, and during the earliest phase of the radioactive tail. We have concentrated on a progenitor model calculated by Nomoto & Hashimoto and Saio, Nomoto, & Kato, which assumes that 14 solar masses of the stellar mass is ejected. Using this model, we have updated constraints on the explosion energy and the extent of mixing in the ejecta. In particular, we determine the most likely range of E/M (explosion energy over ejecta mass) and R_0 (radius of the progenitor). In general, our best models have energies in the range E = (1.1 +/- 0.3) x 10^{51} ergs, and the agreement is better than in earlier, flux-limited diffusion calculations for the same explosion energy. Our modeled B and V fluxes compare well with observations, while the flux in U undershoots after about 10 days by a factor of a few, presumably due to NLTE and line transfer effects. We also compare our results with IUE observations, and a very good quantitative agreement is found for the first days, and for one IUE band (2500-3000 A) as long as for 3 months. We point out that the V flux estimated by McNaught & Zoltowski should probably be revised to a lower value.

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arXiv1999

Type Ia Supernova Progenitors, Environmental Effects, and Cosmic Supernova Rates

Relatively uniform light curves and spectral evolution of Type Ia supernovae (SNe Ia) have led to the use of SNe Ia as a ``standard candle'' to determine cosmological parameters, such as the Hubble constant, the density parameter, and the cosmological constant. Whether a statistically significant value of the cosmological constant can be obtained depends on whether the peak luminosities of SNe Ia are sufficiently free from the effects of cosmic and galactic evolutions. Here we first review the single degenerate scenario for the Chandrasekhar mass white dwarf (WD) models of SNe Ia. We identify the progenitor's evolution and population with two channels: (1) the WD+RG (red-giant) and (2) the WD+MS (near main-sequence He-rich star) channels. In these channels, the strong wind from accreting white dwarfs plays a key role, which yields important age and metallicity effects on the evolution. We then address the questions whether the nature of SNe Ia depends systematically on environmental properties such as metallicity and age of the progenitor system and whether significant evolutionary effects exist. We suggest that the variation of the carbon mass fraction $X$(C) in the C+O WD (or the variation of the initial WD mass) causes the diversity of the brightness of SNe Ia. This model can explain the observed dependence of SNe Ia brighness on the galaxy types. Finally, applying the metallicity effect on the evolution of SN Ia progenitors, we make a prediction of the cosmic supernova rate history as a composite of the supernova rates in different types of galaxies.

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arXiv1999

A Wide Symbiotic Channel to Type Ia Supernovae

As a promising channel to Type Ia supernovae (SNe Ia), we have proposed a symbiotic binary system consisting of a white dwarf (WD) and a low mass red-giant (RG), where strong winds from the accreting WD play a key role to increase the WD mass to the Chandrasekhar mass limit. Here we propose two new evolutionary processes which make the symbiotic channel to SNe Ia much wider. (1) We first show that the WD + RG close binary can form from a wide binary even with such a large initial separation as $a_i \lesssim 40000 R_\odot$. Such a binary consists of an AGB star and a low mass main-sequence (MS) star, where the AGB star is undergoing superwind before becoming a WD. If the superwind at the end of AGB evolution is as fast as or slower than the orbital velocity, the wind outflowing from the system takes away the orbital angular momentum effectively. As a result the wide binary shrinks greatly to become a close binary. Therefore, the WD + RG binary can form from much wider binaries than our earlier estimate. (2) When the RG fills its inner critical Roche lobe, the WD undergoes rapid mass accretion and blows a strong optically thick wind. Our earlier analysis has shown that the mass transfer is stabilized by this wind only when the mass ratio of RG/WD is smaller than 1.15. Our new finding is that the WD wind can strip mass from the RG envelope, which could be efficient enough to stabilize the mass transfer even if the RG/WD mass ratio exceeds 1.15. With the above two new effects (1) and (2), the symbiotic channel can account for the inferred rate of SNe Ia in our Galaxy.

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arXiv1999

A New Evolutionary Path to Type Ia Supernovae: Helium-Rich Super-Soft X-Ray Source Channel

We have found a new evolutionary path to Type Ia supernovae (SNe Ia) which has been overlooked in previous work. In this scenario, a carbon-oxygen white dwarf (C+O WD) is originated, not from an asymptotic giant branch star with a C+O core, but from a red-giant star with a helium core of $\sim 0.8-2.0 M_\odot$. The helium star, which is formed after the first common envelope evolution, evolves to form a C+O WD of $\sim 0.8-1.1 M_\odot$ with transferring a part of the helium envelope onto the secondary main-sequence star. This new evolutionary path, together with the optically thick wind from mass-accreting white dwarf, provides a much wider channel to SNe Ia than previous scenarios. A part of the progenitor systems are identified as the luminous supersoft X-ray sources or the recurrent novae like U Sco, which are characterized by the accretion of helium-rich matter. The white dwarf accretes hydrogen-rich, helium-enhanced matter from a lobe-filling, slightly evolved companion at a critical rate and blows excess matter in the wind. The white dwarf grows in mass to the Chandrasekhar mass limit and explodes as an SN Ia. A theoretical estimate indicates that this channel contributes a considerable part of the inferred rate of SNe Ia in our Galaxy, i.e., the rate is about ten times larger than the previous theoretical estimates for white dwarfs with slightly evolved companions.

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arXiv1998

Dissipative Process as a Mechanism of Differentiating Internal Structures between Dwarf and Normal Elliptical Galaxies in a CDM Universe

We simulate the dynamical, chemical, and spectro-photometric evolution of dwarf and normal elliptical galaxies embedded in a dark matter halo, using a three-dimensional N-body/SPH simulation code. For the forming dwarf elliptical galaxies, supernova-driven winds propagating outwards from inside the system collide with the infalling gas and produce the super-shell in which stars are formed. The resulting stellar system forms a loosely bound virialized system due to the significant mass loss and has a large velocity dispersion and a large core. Consequently the surface brightness distribution shows an exponential profile and the color distribution shows a positive gradient such that the colors become redder away from the galaxy center in agreement with observations. On the other hand, for the normal elliptical galaxies with deep gravitational potential, the mass loss out of the system does not have a significant dynamical effect on the stellar system. The resulting surface brightness distribution has a large central concentration like de Vaucouleurs' $r^{1/4}$-profile and the color distribution shows a negative gradient as observed. Our simulation shows that different features between dwarf and normal elliptical galaxies stem from different cooling efficiencies for their respective protogalaxies in a standard CDM universe.

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arXiv1998

Nucleosynthesis in Type II supernovae and the abundances in metal-poor stars

We explore the effects on nucleosynthesis in Type II supernovae of various parameters (mass cut, neutron excess, explosion energy, progenitor mass) in order to explain the observed trends of the iron-peak element abundance ratios ([Cr/Fe], [Mn/Fe], [Co/Fe] and [Ni/Fe]) in halo stars as a function of metallicity for the range $ -4 \le$ [Fe/H] $\le -2.5$. [Cr/Fe] and [Mn/Fe] decrease with decreasing [Fe/H], while [Co/Fe] behaves the opposite way and increases. We show that such a behavior can be explained by a variation of mass cuts in Type II supernovae as a function of progenitor mass, which provides a changing mix of nucleosynthesis from an alpha-rich freeze-out of Si-burning and incomplete Si-burning. This explanation is consistent with the amount of ejected $^{56}$Ni determined from modeling the early light curves of individual supernovae. We also suggest that the ratio [H/Fe] of halo stars is mainly determined by the mass of interstellar hydrogen mixed with the ejecta of a single supernova which is larger for larger explosion energy and the larger Strömgren radius of the progenitor.

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