arXiv subjects

Ken'ichi Nomoto

Publications and source records attributed to Ken'ichi Nomoto.

At least 145 records · Page 8Linked to original sources

arXiv2010

Abundance stratification in Type Ia supernovae - III. The normal SN 2003du

The element abundance distributions in the ejecta of Type Ia supernova (SN) is studied by modelling a time series of optical spectra of SN 2003du until ~1 year after the explosion. Since SN 2003du is a very normal Type Ia SN both photometrically and spectroscopically, the abundance distribution derived for it can be considered as representative of normal Type Ia SNe. We find that the innermost layers are dominated by stable Fe-group elements, with a total mass of ~ 0.2 Msun, which are synthesized through electron capture. Above the core of stable elements there are thick 56Ni-rich layers. The total mass of 56Ni is 0.65 Msun. The Si- and S-rich layers are located above the 56Ni-rich layers. The dominant element in the outermost layers (M_r > 1.1 Msun, v > 13000 km/s) is O, with a small amount of Si. Little unburned C remains, with an upper limit of 0.016 Msun. The element distributions in the ejecta are moderately mixed, but not fully mixed as seen in three-dimensional deflagration models.

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arXiv2010

Fallback Supernovae: A Possible Origin of Peculiar Supernovae with Extremely Low Explosion Energies

We perform hydrodynamical calculations of core-collapse supernovae (SNe) with low explosion energies. These SNe do not have enough energy to eject the whole progenitor and most of the progenitor falls back to the central remnant. We show that such fallback SNe can have a variety of light curves (LCs) but their photospheric velocities can only have some limited values with lower limits. We also perform calculations of nucleosynthesis and LCs of several fallback SN model, and find that a fallback SN from the progenitor with a main-sequence mass of 13 Msun can account for the properties of the peculiar Type Ia supernova SN 2008ha. The kinetic energy and ejecta mass of the model are 1.2*10^{48} erg and 0.074 Msun, respectively, and the ejected 56Ni mass is 0.003 Msun. Thus, SN 2008ha can be a core-collapse SN with a large amount of fallback. We also suggest that SN 2008ha could have been accompanied with long gamma-ray bursts and long gamma-ray bursts without associated SNe may be accompanied with very faint SNe with significant amount of fallback which are similar to SN 2008ha.

astro-ph.HE↗

arXiv2010

A Core-Collapse Supernova Model for the Extremely Luminous Type Ic Supernova 2007bi: An Alternative to the Pair-Instability Supernova Model

We present a core-collapse supernova model for the extremely luminous Type Ic supernova 2007bi. By performing numerical calculations of hydrodynamics, nucleosynthesis, and radiation transport, we find that SN 2007bi is consistent with the core-collapse supernova explosion of a 43 Msun carbon and oxygen core obtained from the evolution of a progenitor star with a main sequence mass of 100 Msun and metallicity of Z = Zsun/200, from which its hydrogen and helium envelopes are artificially stripped. The ejecta mass and the ejecta kinetic energy of the models are 40 Msun and 3.6*10^{52} erg. The ejected 56Ni mass is as large as 6.1 Msun, which results from the explosive nucleosynthesis with large explosion energy. We also confirm that SN 2007bi is consistent with a pair-instability supernova model as has recently been claimed. We show that the earlier light curve data can discriminate between the models for such luminous supernovae.

astro-ph.HE↗

arXiv2010

Spectropolarimetry of Extremely Luminous Type Ia Supernova 2009dc: Nearly Spherical Explosion of Super-Chandrasekhar Mass White Dwarf

We present the first spectropolarimetric observations of a candidate of super-Chandrasekhar mass Type Ia supernova (SN): SN 2009dc. The observations were performed at 5.6 and 89.5 days after the B-band maximum. The data taken at the later epoch are used to determine the interstellar polarization. Continuum polarization is found to be small (<0.3 %), indicating that the explosion is nearly spherically symmetric. This fact suggests that a very aspherical explosion is not a likely scenario for SN 2009dc. Polarization at the Si II and Ca II lines clearly shows a loop in the Q-U plane, indicating a non-axisymmetric, clumpy distribution of intermediate-mass elements. The degree of line polarization at the Si and Ca line is moderate (0.5% +- 0.1% and 0.7% +- 0.1%, respectively), but it is higher than expected from the trend of other Type Ia SNe. This may suggest that there are thick enough, clumpy Si-rich layers above the thick 56Ni-rich layers (>~ 1.2 Msun). The observed spectropolarimetric properties, combined with the photometric and spectroscopic properties, suggest that the progenitor of SN 2009dc has a super-Chandrasekhar mass, and that the explosion geometry is globally spherically symmetric, with clumpy distribution of intermediate-mass elements.

astro-ph.CO↗

arXiv2010

Formation and Evolution of Dust in Type IIb Supernova with Application to the Cassiopeia A Supernova Remnant

We investigate the formation of dust grains in the ejecta of a SN IIb and their evolution in the shocked gas in the SNR by considering the uniform and power-law density structures for the CSM. Based on these calculations, we also simulate the time evolution of thermal emission from the shock-heated dust in the SNR and compare the results with the observations of Cas A SNR. We find that the total mass of dust formed in the SN IIb is as large as 0.167 M_sun but the average radius of dust is smaller than 0.01 mum and is significantly different from those in SNe II-P with the massive H-envelope. In the explosion with the small-mass H-envelope, the expanding He core undergoes little deceleration, so that the gas density in the He core is too low for large-sized grains to form. In addition, the low-mass H-envelope of the SN IIb leads to the early arrival of the reverse shock at the dust-forming region. If the CSM is more or less spherical, therefore, the newly formed grains would be completely destroyed in the relatively dense shocked gas for the CSM density of n_H > 0.1 cm^{-3}. However, the actual CSM is likely to be non-spherical, so that a part of grains could be ejected into the ISM without being shocked. We demonstrate that the time evolution of the SED by thermal dust emission is sensitive to the ambient gas density and structure that affects the passage of the reverse shock into the ejecta. Thus, the SED evolution well reflects the evolution of dust through erosion by sputtering and stochastic heating. For Cas A, we consider the CSM produced by the steady mass loss of ~8x10^{-5} M_sun/yr during the supergiant phase. Then we find the infrared SED of Cas A is reasonably reproduced by thermal emission from the newly formed dust of 0.08 M_sun, which consists of 0.008 M_sun shocked warm dust and 0.072 M_sun unshocked cold dust.

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arXiv2009

Nebular Spectra and Explosion Asymmetry of Type Ia Supernovae

The spectral signatures of asymmetry in Type Ia Supernova (SN Ia) explosions are investigated, using a sample of late-time nebular spectra. First, a kinematical model is constructed for SN Ia 2003hv, which can account for the main features in its optical, Near-Infrared (NIR), and Mid-Infrared (Mid-IR) late-time spectra. It is found that an asymmetric off-center model can explain the observed characteristics of SN 2003hv. This model includes a relatively high density, Fe-rich region which displays a large velocity off-set, and a relatively low density, extended 56Ni-rich region which is more spherically distributed. The high density region consists of the inner stable Fe-Ni region and outer 56Ni-rich region. Such a distribution may be the result of a delayed-detonation explosion, in which the first deflagration produces the global asymmetry in the innermost ejecta, while the subsequent detonation can lead to the bulk spherical symmetry. This configuration, if viewed from the direction of the off-set, can consistently explain the blueshift in some of the emission lines and virtually no observed shift in other lines in SN 2003hv. For this model, we then explore the effects of different viewing angles and the implications for SNe Ia in general. The model predicts that a variation of the central wavelength, depending on the viewing angle, should be seen in some lines (e.g., [Ni II]7378), while the strongest lines (e.g., [Fe III] blend at 4700A) will not show this effect. By examining optical nebular spectra of 12 SNe Ia, we have found that such a variation indeed exists. We suggest that the global asymmetry in the innermost ejecta, as likely imprint of the deflagration flame propagation, is a generic feature of SNe Ia (abridged).

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arXiv2009

The Role of Type Ia Supernovae in Chemical Evolution I: Lifetime of Type Ia Supernovae and Galactic Supernova Rates

We construct a new model of Type Ia Supernovae (SNe Ia), based on the single degenerate scenario, taking account of the metallicity dependences of the white dwarf (WD) wind and the mass-stripping effect on the binary companion star. Our model naturally predicts that the SN Ia lifetime distribution spans a range of 0.1-20 Gyr with the double peaks at ~0.1 and 1 Gyr. While the present SN Ia rate in elliptical galaxies can be reproduced with the old population of the red-giants+WD systems, the large SN Ia rate in radio galaxies could be explained with the young population of the main-sequence+WD systems. Because of the metallicity effect, i.e., because of the lack of winds from WDs in the binary systems, the SN Ia rate in the systems with [Fe/H]<-1, e.g., high-z spiral galaxies, is supposed to be very small. Our SN Ia model can give better reproduction of the [(alpha, Mn, Zn)/Fe]-[Fe/H] relations in the solar neighborhood than other models such as the double-degenerate scenario. The metallicity effect is more strongly required in the presence of the young population of SNe Ia. We also succeed in reproducing the galactic supernova rates with their dependence on the morphological type of galaxies, and the cosmic SN Ia rate history with a peak at z~1. At z>1, the predicted SN Ia rate decreases toward higher redshifts and SNe Ia will be observed only in the systems that have evolved with a short timescale of chemical enrichment. This suggests that the evolution effect in the supernova cosmology can be small.

astro-ph↗

arXiv2009

SN2003bg: a broad-lined Type IIb Supernova with Hydrogen

Models for the spectra and the light curve, in the photospheric as well as in the late nebular phase, are used to infer the properties of the very radio-bright, broad-lined type IIb Supernova 2003bg. Consistent fits to the light curve and the spectral evolution are obtained with an explosion that ejected ~ 4 M_sun of material with a kinetic energy of ~ 5 10^51 erg. A thin layer of hydrogen, comprising ~ 0.05 M_sun, is inferred to be present in the ejecta at the highest velocities (v >~ 9000 km/s), while a thicker helium layer, comprising ~ 1.25 M_sun, was ejected at velocities between 6500 and 9000 km/s. At lower velocities, heavier elements are present, including ~ 0.2 M_sun of 56Ni that shape the light curve and the late-time nebular spectra. These values suggest that the progenitor star had a mass of ~ 20-25 M_sun (comparable to, but maybe somewhat smaller than that of the progenitor of the XRF/SN 2008D). The rather broad-lined early spectra are the result of the presence of a small amount of material (~ 0.03 M_sun) at velocities > 0.1 c, which carries ~ 10 % of the explosion kinetic energy. No clear signatures of a highly aspherical explosion are detected.

astro-ph.HE↗

arXiv2009

Spectropolarimetry of the Unique Type Ib Supernova 2005bf: Larger Asymmetry Revealed by Later-Phase Data

We present an optical spectropolarimetric observation of the unique Type Ib supernova (SN) 2005bf at 8 days after the second maximum. The data, combined with the polarization spectrum taken at 6 days before the second maximum (Maund et al. 2007a), enable us to closely examine the intrinsic properties of the SN. The polarization percentage is smaller at the later epoch over a wide wavelength range, while the position angle is similar at the two epochs. We find that an assumption of complete depolarization of strong lines at the emission peak is not necessarily correct. The intrinsic polarization of the SN is larger, and thus, the ejecta of SN 2005bf would be more asymmetric than previously expected. The axis ratio of the photosphere projected on the sky deviates from unity by at least 20 %. If the position angle of interstellar polarization is aligned with the spiral arm of the host galaxy, the deviation is larger than 25 %. The line polarization at the He I, Ca II and Fe II lines is also smaller at the later epoch. The changes in the position angle across these lines, which were observed at the earlier epoch, are still marginally present at the later epoch. The similar polarimetric behavior suggests that the distributions of these ions are correlated. Properties of polarization, as well as the light curve and the spectra both at photospheric and nebular phases, can be explained by an aspherical, possibly unipolar explosion of a WN star in which the blob of 56Ni penetrates C+O core and stops within the He layer.

astro-ph.HE↗

arXiv2009

Nebular Phase Observations of the Type Ib Supernova 2008D/X-ray Transient 080109: Side-Viewed Bipolar Explosion

We present optical spectroscopic and photometric observations of supernova (SN) 2008D, associated with the luminous X-ray transient 080109, at >300 days after the explosion (nebular phases). We also give flux measurements of emission lines from the H II region at the site of the SN, and estimates of the local metallicity. The brightness of the SN at nebular phases is consistent with the prediction of the explosion models with an ejected 56Ni mass of 0.07 Msun, which explains the light curve at early phases. The [O I] line in the nebular spectrum shows a double-peaked profile while the [Ca II] line does not. The double-peaked [O I] profile strongly indicates that SN 2008D is an aspherical explosion. The profile can be explained by a torus-like distribution of oxygen viewed from near the plane of the torus. We suggest that SN 2008D is a side-viewed, bipolar explosion with a viewing angle of > 50^{\circ} from the polar direction.

astro-ph.HE↗

arXiv2009

Chemical Yields from Supernovae and Hypernovae

We review the final stages of stellar evolution, supernova properties, and chemical yields as a function of the progenitor's mass M. (1) 8 - 10 Ms stars are super-AGB stars when the O+Ne+Mg core collapses due to electron capture. These AGB-supernovae may constitute an SN 2008S-like sub-class of Type IIn supernovae. These stars produce little alpha-elements and Fe-peak elements, but are important sources of Zn and light p-nuclei. (2) 10 - 90 Ms stars undergo Fe-core collapse. Nucleosynthesis in aspherical explosions is important, as it can well reproduce the abundance patterns observed in extremely metal-poor stars. (3) 90 - 140 Ms stars undergo pulsational nuclear instabilities at various nuclear burning stages, including O and Si-burning. (4) 140 - 300 Ms stars become pair-instability supernovae, if the mass loss is small enough. (5) Stars more massive than 300 Ms undergo core-collapse to form intermediate mass black holes.

astro-ph.SR↗

arXiv2009

The broad line type Ic supernova SN 2007ru: Adding to the diversity of type Ic supernovae

Photometric and spectral evolution of the Type Ic supernova SN 2007ru until around 210 days after maximum are presented. The spectra show broad spectral features due to very high expansion velocity, normally seen in hypernovae. The photospheric velocity is higher than other normal Type Ic supernovae. It is lower than SN 1998bw at $\sim$ 8 days after the explosion, but is comparable at later epochs. The light curve evolution of SN 2007ru indicates a fast rise time of 8$\pm$3 days to $B$ band maximum and post-maximum decline more rapid than other broad-line Type Ic supernovae. With an absolute $V$ magnitude of -19.06, SN 2007ru is comparable in brightness with SN 1998bw and lies at the brighter end of the observed Type Ic supernovae. The ejected mass of \Nifs is estimated to be $\sim0.4\Msun$. The fast rise and decline of the light curve and the high expansion velocity suggest that SN 2007ru is an explosion with a high kinetic energy/ejecta mass ratio ($E_{\rm K}/M_{\rm {ej}}$). This adds to the diversity of Type Ic supernovae. Although the early phase spectra are most similar to those of broad-line SN 2003jd, the [OI] line profile in the nebular spectrum of SN 2007ru shows the singly-peaked profile, in contrast to the doubly-peaked profile in SN 2003jd. The singly-peaked profile, together with the high luminosity and the high expansion velocity, may suggest that SN 2007ru could be an aspherical explosion viewed from the polar direction. Estimated oxygen abundance 12 + log(O/H) of $\sim$8.8 indicates that SN 2007ru occurred in a region with nearly solar metallicity.

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arXiv2009

Dust in Supernovae; Formation and Evolution

Core--collapsed supernovae (CCSNe) have been considered to be one of sources of dust in the universe. What kind and how much mass of dust are formed in the ejecta and are injected into the interstellar medium (ISM) depend on the type of CCSNe, through the difference in the thickness (mass) of outer envelope. In this review, after summarizing the existing results of observations on dust formation in CCSNe, we investigate formation of dust in the ejecta and its evolution in the supernova remnants (SNRs) of Type II--P and Type IIb SNe. Then, the time evolution of thermal emission from dust in the SNR of Type IIb SN is demonstrated and compared with the observation of Cas A. We find that the total dust mass formed in the ejecta does not so much depend on the type; $\sim 0.3-0.7 M_{\odot}$ in Type II--P SNe and $\sim 0.13 M_{\odot}$ in Type IIb SN. However the size of dust sensitively depends on the type, being affected by the difference in the gas density in the ejecta: the dust mass is dominated by grains with radii larger than 0.03 $μ$m in Type II-P, and less than 0.006 $μ$m in Type IIb, which decides the fate of dust in the SNR. The surviving dust mass is $\sim 0.04-0.2 M_{\odot}$ in the SNRs of Type II--P SNe for the ambient hydrogen density of $n_{\rm H}=10.0-1.0$ cm$^{-3}$, while almost all dust grains are destroyed in the SNR of Type IIb. The spectral energy distribution (SED) of thermal emission from dust in SNR well reflects the evolution of dust grains in SNR through erosion by sputtering and stochastic heating. The observed SED of Cas A SNR is reasonably reproduced by the model of dust formation and evolution for Type IIb SN.

astro-ph.GA↗

arXiv2009

Evolution of Very Massive Population III Stars with Mass Accretion from Pre-Main Sequence to Collapse

We calculate the evolution of zero-metallicity Population III (Pop III) stars whose mass grows from the initial mass of $\sim 1M_{\odot}$ by accreting the surrounding gases. Our calculations cover a whole evolutionary stages from the pre-main sequence, via various nuclear burning stages, through the final core collapse or pair-creation instability phases. We adopt the following stellar mass-dependent accretion rates which are derived from cosmological simulations of early structure formation based on the low mass dark matter halos at redshifts $z \sim 20$: (1) the accretion rates for the first generation (Pop III.1) stars and (2) the rates for zero-metallicity but the second generation (Pop III.2) stars which are affected by radiation from the Pop III.1 stars. For comparison, we also study the evolution with the mass-dependent accretion rates which are affected by radiatibe feedback. We show that the final mass of Pop III.1 stars can be as large as $\sim 1000M_{\odot}$, beyond the mass range ($140 - 300M_{\odot}$) for the pair-instability supernovae. Such massive stars undergo core-collapse to form intermediate-mass black holes, which may be the seeds for merger trees to supermassive black holes. On the other hand, Pop III.2 stars become less massive ($\lsim 40 - 60M_{\odot}$), being in the mass range of ordinary iron core-collapse stars. Such stars explode and eject heavy elements to contribute to chemical enrichment of the early universe as observed in the abundance patterns of extremely metal-poor stars in the Galactic halo.

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arXiv2009

Type II Supernovae as Probes of Cosmology

- Constraining the cosmological parameters and understanding Dark Energy have tremendous implications for the nature of the Universe and its physical laws. - The pervasive limit of systematic uncertainties reached by cosmography based on Cepheids and Type Ia supernovae (SNe Ia) warrants a search for complementary approaches. - Type II SNe have been shown to offer such a path. Their distances can be well constrained by luminosity-based or geometric methods. Competing, complementary, and concerted efforts are underway, to explore and exploit those objects that are extremely well matched to next generation facilities. Spectroscopic follow-up will be enabled by space- based and 20-40 meter class telescopes. - Some systematic uncertainties of Type II SNe, such as reddening by dust and metallicity effects, are bound to be different from those of SNe Ia. Their stellar progenitors are known, promising better leverage on cosmic evolution. In addition, their rate - which closely tracks the ongoing star formation rate - is expected to rise significantly with look- back time, ensuring an adequate supply of distant examples. - These data will competitively constrain the dark energy equation of state, allow the determination of the Hubble constant to 5%, and promote our understanding of the processes involved in the last dramatic phases of massive stellar evolution.

astro-ph.CO↗

arXiv2009

Type Ib Supernova 2008D associated with the Luminous X-ray Transient 080109: An Energetic Explosion of a Massive Helium Star

We present a theoretical model for supernova (SN) 2008D associated with the luminous X-ray transient 080109. The bolometric light curve and optical spectra of the SN are modelled based on the progenitor models and the explosion models obtained from hydrodynamic/nucleosynthetic calculations. We find that SN 2008D is a more energetic explosion than normal core-collapse supernovae, with an ejecta mass of Mej = 5.3 +- 1.0 Msun and a kinetic energy of E = 6.0 +- 2.5 x 10^{51} erg. The progenitor star of the SN has a 6-8 Msun He core with essentially no H envelope (< 5 x 10^{-4} Msun) prior to the explosion. The main-sequence mass of the progenitor is estimated to be Mms =20-25 Msun, with additional systematic uncertainties due to convection, mass loss, rotation, and binary effects. These properties are intermediate between those of normal SNe and hypernovae associated with gamma-ray bursts. The mass of the central remnant is estimated as 1.6 - 1.8 Msun, which is near the boundary between neutron star and black hole formation.

astro-ph↗

arXiv2009

The Extremely Luminous Supernova 2006gy at Late Phase: Detection of Optical Emission from Supernova

We performed optical spectroscopy and photometry of SN 2006gy at late time, ~400 days after the explosion, with the Subaru/FOCAS in a good seeing condition. We found that the SN faded by ~3 mag from ~200 to ~400 days after the explosion (i.e., by ~5 mag from peak to ~400 days) in R band. The overall light curve is marginally consistent with the 56Ni heating model, although the flattening around 200 days suggests the optical flux declined more steeply between ~200 and ~400 days. The late time spectrum was quite peculiar among all types of SNe. It showed many intermediate width (~2000 km/s FWHM) emission lines, e.g., [Fe II], [Ca II], and Ca II. The absence of the broad [O I] 6300, 6364 line and weakness of [Fe II] and [Ca II] lines compared with Ca II IR triplet would be explained by a moderately high electron density in the line emitting region. This high density assumption seems to be consistent with the large amount of ejecta and low expansion velocity of SN 2006gy. The H-alpha line luminosity was as small as ~1x10^39 erg/s, being comparable with those of normal Type II SNe at similar epochs. Our observation indicates that the strong CSM interaction had almost finished by ~400 days. If the late time optical flux is purely powered by radioactive decay, at least M_Ni ~ 3 M_sun should be produced at the SN explosion. In the late phase spectrum, there were several unusual emission lines at 7400--8800 AA and some of them might be due to Ti or Ni synthesized at the explosion. (abridged)

astro-ph.SR↗

arXiv2009

Empirical Abundance Scaling Laws and Implications for the Gamma-Process in Core-Collapse Supernovae

Analyzing the solar system abundances, we have found two empirical abundance scaling laws concerning the p- and s-nuclei with the same atomic number. The first scaling is s/p ratios are almost constant over a wide range of the atomic number, where the p-nculei are lighter than the s-nuclei by two or four neutrons. The second scaling is p/p ratios are almost constant, where the second $p$-nuclei are lighter than the first p-nucleus by two neutrons. These scalings are a piece of evidence that most p-nuclei are dominantly synthesized by the gamma-process in supernova explosions. The scalings lead to a novel concept of "universality of gamma-process" that the s/p and p/p ratios of nuclei produced by individual gamma-processes are almost constant, respectively. We have calculated the ratios by gamma-process based on core-collapse supernova explosion models under various astrophysical conditions and found that the scalings hold for materials produced by individual gamma-processes independent of the astrophysical conditions assumed. The universality originates from three mechanisms: the shifts of the gamma-process layers to keep their peak temperature, the weak s-process in pre-supernovae, and the independence of the s/p ratios of the nuclear reactions. The results further suggest an extended universality that the s/p ratios in the gamma-process layers are not only constant but also centered on a specific value of 3. With this specific value and the first scaling, we estimate that the ratios of $s$-process abundance contributions from the AGB stars to the massive stars are almost 6.7 for the $s$-nuclei of A > 90. We find that large enhancements of s/p ratios for Ce, Er, and W are a piece of evidence that the weak s-process actually occurred before SNe.

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