arXiv subjects

Ken'ichi Nomoto

Publications and source records attributed to Ken'ichi Nomoto.

At least 181 records · Page 10Linked to original sources

arXiv2007

Hypernovae and their Gamma-Ray Bursts Connection

The connection between long Gamma Ray Bursts (GRBs) and Supernovae (SNe), have been established through the well observed cases of GRB980425/SN 1998bw, GRB030329/SN 2003dh and GRB031203/SN 2003lw. These events can be explained as the prompt collapse to a black hole (BH) of the core of a massive star (M ~ 40 Msun) that had lost its outer hydrogen and helium envelopes. All these SNe exhibited strong oxygen lines, and their energies were much larger than those of typical SNe, thus these SNe are called Hypernovae (HNe). The case of SN 2006aj/GRB060218 appears different: the GRB was weak and soft (an X-Ray Flash, XRF); the SN is dimmer and has very weak oxygen lines. The explosion energy of SN 2006aj was smaller, as was the ejected mass. In our model, the progenitor star had a smaller mass than other GRB/SNe (M ~ 20 Msun), suggesting that a neutron star (NS) rather than a black hole was formed. If the nascent neutron star was strongly magnetized (a so-called magnetar) and rapidly spinning, it may launch a weak GRB or an XRF. The final fate of 20-30 Msun stars show interesting variety, as seen in the very peculiar Type Ib/c SN 2005bf. This mass range corresponds to the NS to BH transition. We also compare the nucleosynthesis feature of HNe with the metal-poor stars and suggest the Hypernova-First Star connection.

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arXiv2007

Evolution of Dust in Primordial Supernova Remnants: Can Dust Grains Formed in the Ejecta Survive and be Injected into the Early Interstellar Medium?

We investigate the evolution of dust that formed at Population III supernova (SN) explosions and its processing through the collisions with the reverse shocks resulting from the interaction of the SN ejecta with the ambient medium. In particular, we investigate the transport of the shocked dust within the SN remnant (SNR), and its effect on the chemical composition, the size distribution, and the total mass of dust surviving in SNR. We find that the evolution of the reverse shock, and hence its effect on the processing of the dust depends on the thickness of the envelope retained by the progenitor star. Furthermore, the transport and survival of the dust grains depend on their initial radius, a_{ini}, and composition: For Type II SNRs expanding into the interstellar medium (ISM) with a density of n_{H,0}=1 cm^{-3}, small grains with a_{ini} < ~ 0.05 micron are completely destroyed by sputtering in the postshock flow, while grains with a_{ini}= 0.05--0.2 micron are trapped into the dense shell behind the forward shock. Very large grains of a_{ini} > ~ 0.2 micron are ejected into the ISM without decreasing their sizes significantly. We find that the total mass fraction of dust that is destroyed by the reverse shock ranges from 0.2 to 1.0, depending on the energy of the explosion and the density of the ambient ISM. The results of our calculations have significant impact on the abundance pattern of subsequent generation of stars that form in the dense shell of primordial SNRs.

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arXiv2007

Thermal Stability of White Dwarfs Accreting Hydrogen-rich Matter and Progenitors of Type Ia Supernovae

We revisit the properties of white dwarfs accreting hydrogen-rich matter by constructing steady-state models, in which hydrogen shell burning consumes hydrogen at the same rate as the white dwarf accretes it. We obtain such steady-state models for various accretion rates and white dwarf masses. We confirm that these steady models are thermally stable only when the accretion rate is higher than \sim 10^{-7} M_sun/yr. We show that recent models of ``quiescent burning'' in the ``surface hydrogen burning'' at a much wider range of accretion rates results from the too large zone mass in the outer part of the models; hydrogen burning must occur in a much thinner layer. A comparison of the positions on the HR diagram suggests that most of the luminous supersoft X-ray sources are white dwarfs accreting matter at rates high enough that the hydrogen burning shell is thermally stable. Implications on the progenitors of Type Ia supernovae are discussed.

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arXiv2007

SN 2006aj Associated with XRF 060218 At Late Phases: Nucleosynthesis-Signature of A Neutron Star-Driven Explosion

Optical spectroscopy and photometry of SN 2006aj have been performed with the Subaru telescope at t > 200 days after GRB060218, the X-ray Flash with which it was associated. Strong nebular emission-lines with an expansion velocity of v ~ 7,300 km/s were detected. The peaked but relatively broad [OI]6300,6363 suggests the existence of ~ 2 Msun of materials in which ~1.3 Msun is oxygen. The core might be produced by a mildly asymmetric explosion. The spectra are unique among SNe Ic in (1) the absence of [CaII]7291,7324 emission, and (2) a strong emission feature at ~ 7400A, which requires ~ 0.05 Msun of newly-synthesized 58Ni. Such a large amount of stable neutron-rich Ni strongly indicates the formation of a neutron star. The progenitor and the explosion energy are constrained to 18 Msun < Mms < 22 Msun and E ~ (1 - 3) 10^{51} erg, respectively.

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arXiv2007

The Connection between Gamma-Ray Bursts and Extremely Metal-Poor Stars: Black Hole-forming Supernovae with Relativistic Jets

Long-duration gamma-ray bursts (GRBs) are thought to be connected to luminous and energetic supernovae (SNe), called hypernovae (HNe), resulting from the black-hole (BH) forming collapse of massive stars. For recent nearby GRBs~060505 and 060614, however, the expected SNe have not been detected. The upper limits to the SN brightness are about 100 times fainter than GRB-associated HNe (GRB-HNe), corresponding to the upper limits to the ejected $^{56}$Ni masses of $M({\rm ^{56}Ni})\sim 10^{-3}M_\odot$. SNe with a small amount of $^{56}$Ni ejection are observed as faint Type II SNe. HNe and faint SNe are thought to be responsible for the formation of extremely metal-poor (EMP) stars. In this Letter, a relativistic jet-induced BH forming explosion of the 40 $M_\odot$ star is investigated and hydrodynamic and nucleosynthetic models are presented. These models can explain both GRB-HNe and GRBs without bright SNe in a unified manner. Their connection to EMP stars is also discussed. We suggest that GRBs without bright SNe are likely to synthesize $\Mni\sim 10^{-4}$ to $10^{-3}M_\odot$ or $\sim 10^{-6}M_\odot$.

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arXiv2007

Supernova Nucleosynthesis in Population III 13 -- 50 $M_{\odot}$ Stars and Abundance Patterns of Extremely Metal-Poor Stars

We perform hydrodynamical and nucleosynthesis calculations of core-collapse supernovae (SNe) and hypernovae (HNe) of Population (Pop) III stars. We provide new yields for the main-sequence mass of $M_{\rm MS}=13-50$ $M_{\odot}$ and the explosion energy of $E=1-40\times10^{51}$ ergs to apply for chemical evolution studies. Our HN yields based on the mixing-fallback model of explosions reproduce the observed abundance patterns of extremely metal-poor (EMP) stars ($-4< {\rm [Fe/H]}< -3$), while those of very metal-poor (VMP) stars ($-3<{\rm [Fe/H]} < -2$) are reproduced by the normal SN yields integrated over the Salpeter initial mass function. Moreover, the observed trends of abundance ratios [X/Fe] against [Fe/H] with small dispersions for the EMP stars can be reproduced as a sequence resulting from the various combination of $M_{\rm MS}$ and $E$. This is because we adopt the empirical relation that a larger amount of Fe is ejected by more massive HNe. Our results imply that the observed trends with small dispersions do not necessarily mean the rapid homogeneous mixing in the early galactic halo at [Fe/H] $< -3$, but can be reproduced by the ``inhomogeneous'' chemical evolution model. In addition, we examine how the modifications of the distributions of the electron mole fraction $Y_{\rm e}$ and the density in the presupernova models improve the agreement with observations. In this connection, we discuss possible contributions of nucleosynthesis in the neutrino-driven wind and the accretion disk.

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arXiv2007

Spectroscopic Studies of Extremely Metal-Poor Stars with the Subaru High Dispersion Spectrograph. IV. The alpha-Element-Enhanced Metal-Poor Star BS16934-002

A detailed elemental abundance analysis has been carried out for the very metal-poor ([Fe/H]=-2.7) star BS16934-002, which was identified in our previous work as a star exhibiting large overabundances of Mg and Sc. A comparison of the abundance pattern of this star with that of the well-studied metal-poor star HD122563 indicates excesses of O, Na, Mg, Al, and Sc in BS16934-002. Of particular interest, no excess of C or N is found in this object, in contrast to CS22949-037 and CS29498-043, two previously known carbon-rich, extremely metal-poor stars with excesses of the alpha elements. No established nucleosynthesis model exists that explains the observed abundance pattern of BS16934-002. A supernova model, including mixing and fallback, assuming severe mass loss before explosion, is discussed as a candidate progenitor of BS16934-002.

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arXiv2006

A neutron-star-driven X-ray flash associated with supernova SN 2006aj

Supernovae connected with long-duration gamma-ray bursts (GRBs) are hyper-energetic explosion resulting from the collapse of very massive stars (about 40Mo, where Mo is the mass of the Sun) stripped of their outher hydrogen and helium envelopes. A very massive progenitor, collapsing to a black hole, was thought to be a requirement for the launch of a GRB. Here we report the results of modelling the spectra and light curve of SN 2006aj, which demonstrate that the supernova had a much smaller explosion energy and ejected much less mass than the other GRB-sueprnovae, suggesting that it was produced by a star whose initial mass was only about 20Mo. A star of this mass is expected to form a neutron star rather than a black hole when its core collapses. The smaller explosion energy of SN 2006aj is matched by the weakness and softness of GRB 060218 (an X-ray flash), and the weakness of the radio flux of the supernova. Our results indicate that the supernova-GRB connection extends to a much broader range of stellar masses than previously thought, possibly involving different physical mechanisms: a `collapsar' for the more massive stars collapsing to a black hole, and magnetic activity of the nascent neutron star for the less massive stars.

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arXiv2006

Chemical Abundances in the Secondary Star of the Black Hole Binary V4641 Sagittarii (SAX J1819.3-2525)

We report on detailed spectroscopic studies performed for the secondary star in the black hole binary (micro-quasar) V4641 Sgr in order to examine its surface chemical composition and to see if its surface shows any signature of pollution by ejecta from a supernova explosion. High-resolution spectra of V4641 Sgr observed in the quiescent state in the blue-visual region are compared with those of the two bright well-studied B9 stars (14 Cyg and $ν$ Cap) observed with the same instrument. The effective temperature of V4641 Sgr (10500 $\pm$ 200 K) is estimated from the strengths of He~{\sc i} lines, while its rotational velocity, $\it v$ sin $\it i$ (95 $\pm$ 10 km s${}^{-1}$), is estimated from the profile of the Mg~{\sc ii} line at 4481 Å. We obtain abundances of 10 elements and find definite over-abundances of N (by 0.8 dex or more) and Na (by 0.8 dex) in V4641 Sgr. From line-by-line comparisons of eight other elements (C, O, Mg, Al, Si, Ti, Cr, and Fe) between V4641 Sgr and the two reference stars, we conclude that there is no apparent difference in the abundances of these elements between V4641 Sgr and the two normal late B-type stars, which have been reported to have solar abundances. An evolutionary model of a massive close binary system has been constructed to explain the abundances observed in V4641 Sgr. The model suggests that the progenitor of the black hole forming supernova was as massive as ~ 35 Msun on the main-sequence and, after becoming a ~ 10 Msun He star, underwent "dark" explosion which ejected only N and Na-rich outer layer of the He star without radioactive $^{56}$Ni.

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arXiv2006

Nucleosynthesis Yields of Core-Collapse Supernovae and Hypernovae, and Galactic Chemical Evolution

We present new nucleosynthesis yields as functions of the stellar mass, metallicity, and explosion energy (corresponding to normal supernovae and Hypernovae). We apply the results to the chemical evolution of the solar neighborhood. Our new yields are based on the new developments in the observational/theoretical studies of supernovae (SNe) and extremely metal-poor (EMP) stars in the halo, which have provided excellent opportunities to test the explosion models and their nucleosynthesis. We use the light curve and spectra fitting of individual SN to estimate the mass of the progenitor, explosion energy, and produced 56Ni mass. Comparison with the abundance patterns of EMP stars has made it possible to determine the model parameters of core-collapse SNe, such as mixing-fallback parameters. More specifically, we take into account the two distinct new classes of massive SNe: 1) very energetic Hypernovae, whose kinetic energy (KE) is more than 10 times the KE of normal core-collapse SNe, and 2) very faint and low energy SNe (Faint SNe). These two new classes of SNe are likely to be ``black-hole-forming'' SNe with rotating or non-rotating black holes. Nucleosynthesis in Hypernovae is characterized by larger abundance ratios (Zn,Co,V,Ti)/Fe and smaller (Mn,Cr)/Fe than normal SNe, which can explain the observed trends of these ratios in EMP stars. Nucleosynthesis in Faint SNe is characterized by a large amount of fall-back, which explains the abundance pattern of the most Fe-poor stars. These comparisons suggest that black-hole-forming SNe made important contributions to the early Galactic (and cosmic) chemical evolution.

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arXiv2006

Models for the Type Ic Hypernova SN 2003lw associated with GRB 031203

The Gamma-Ray Burst 031203 at a redshift z=0.1055 revealed a highly reddened Type Ic Supernova, SN 2003lw, in its afterglow light. This is the third well established case of a link between a long-duration GRB and a type Ic SN. The SN light curve is obtained subtracting the galaxy contribution and is modelled together with two spectra at near-maximum epochs. A red VLT grism 150I spectrum of the SN near peak is used to extend the spectral coverage, and in particular to constrain the uncertain reddening, the most likely value for which is E_{G+H}(B-V) about 1.07 +/- 0.05. Accounting for reddening, SN 2003lw is about 0.3 mag brighter than the prototypical GRB-SN 1998bw. Light curve models yield a 56Ni mass of about 0.55 solar mass. The optimal explosion model is somewhat more massive (ejecta mass about 13 solar mass) and energetic (kinetic energy about 6 times 10^52 erg) than the model for SN 1998bw, implying a massive progenitor (40 - 50 solar mass). The mass at high velocity is not very large (1.4 solar mass above 30000 km/s, but only 0.1 solar mass above 60000 km/s), but is sufficient to cause the observed broad lines. The similarity of SNe 2003lw and 1998bw and the weakness of their related GRBs, GRB031203 and GRB980425, suggest that both GRBs may be normal events viewed slightly off-axis or a weaker but possibly more frequent type of GRB.

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arXiv2006

Progenitors of Type Ia Supernovae: Circumstellar Interaction, Rotation, and Steady Hydrogen Burning

Among the important issues in identifying the progenitor system of Type Ia supernovae (SNe Ia), we focus mostly on circumstellar interaction in SN 2002ic, and give brief discussion on the controversial issues of the effects of rotation in merging double degenerates and steady hydrogen shell burning in accreting white dwarfs. SN 2002ic is a unique supernova which shows the typical spectral features of SNe Ia near maximum light, but also apparent hydrogen features that have been absent in SNe Ia. Based on the hydrodynamical models of circumstellar interaction in SN Ia, we suggest that circumstellar medium is aspherical (or highly clumpy) and contains ~1.3 Msun. Possible progenitor systems of SN 2002ic are discussed.

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arXiv2006

Yields of Population III Supernovae and the Abundance Patterns of Extremely Metal-Poor Stars

The abundance patterns of extremely metal-poor (EMP) stars provide us with important information on nucleosynthesis in supernovae (SNe) formed in a Pop III or EMP environment, and thus on the nature of the first stars in the Universe. We review nucleosynthesis yields of various types of those SNe, focusing on core-collapse (black-hole-forming) SNe with various progenitor masses, explosion energies (including Hypernovae), and asphericity. We discuss the implications of the observed trends in the abundance ratios among iron-peak elements, and the large C/Fe ratio observed in certain EMP stars with particular attention to recently discovered hyper metal-poor (HMP) stars. We show that the abundance pattern of the HMP stars with [Fe/H] < -5 and other EMP stars are in good accord with those of black-hole-forming supernovae, but not pair-instability supernovae. This suggests that black-hole-forming supernovae made important contributions to the early Galactic (and cosmic) chemical evolution. Finally we discuss the nature of First (Pop III) Stars.

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arXiv2006

3D Models for High Velocity Features in Type Ia Supernovae

Spectral synthesis in 3-dimensional (3D) space for the earliest spectra of Type Ia supernovae (SNe Ia) is presented. In particular, the high velocity absorption features that are commonly seen at the earliest epochs ($\sim 10$ days before maximum light) are investigated by means of a 3D Monte Carlo spectral synthesis code. The increasing number of early spectra available allows statistical study of the geometry of the ejecta. The observed diversity in strength of the high velocity features (HVFs) can be explained in terms of a ``covering factor'', which represents the fraction of the projected photosphere that is concealed by high velocity material. Various geometrical models involving high velocity material with a clumpy structure or a thick torus can naturally account for the observed statistics of HVFs. HVFs may be formed by a combination of density and abundance enhancements. Such enhancements may be produced in the explosion itself or may be the result of interaction with circumstellar material or an accretion disk. Models with 1 or 2 blobs, as well as a thin torus or disk-like enhancement are unlikely as a standard situation.

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arXiv2006

The Optical/Near-Infrared Light Curves of SN 2002ap for the First 1.5 Years after Discovery

Late-time BVRIJHK photometry of the peculiar Type Ic SN 2002ap, taken between 2002 June 12 and 2003 August 29 with the MAGNUM telescope, is presented. The light curve decline rate is derived in each band and the color evolution is studied through comparison with nebular spectra and with SN 1998bw. Using the photometry, the OIR bolometric light curve is built, extending from before light maximum to day 580 after explosion. The light curve has a late-time shape strikingly similar to that of the hypernova SN 1998bw. The decline rate changes from 0.018 mag/day between day 130 and 230 to 0.014 mag/day between day 270 and 580. To reproduce the late-time light curve, a dense core must be added to the 1-D hypernova model that best fits the early-time observations, bringing the ejecta mass from 2.5 Msun to 3 Msun without much change in the kinetic energy, which is 4 times 10^51 ergs. This is similar to the case of other hypernovae and suggests asymmetry. A large H-band bump developed in the spectral energy distribution after about day 300, probably caused by strong [Si I] 1.646 micron and 1.608 micron emissions. The near-infrared flux contribution increased simultaneously from <30% to >50% at day 580. The near-infrared light curves were compared with those of other Type Ib/c supernovae, among which SN 1983I seems similar to SN 2002ap both in the near-infrared and in the optical.

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arXiv2005

Pre-Maximum Spectropolarimetry of the Type Ia SN 2004dt

We report observations of SN 2004dt obtained with the Very Large Telescope of the European Southern Observatory on August 13.30, 2004 when the supernova was more than a week before optical maximum. SN 2004dt showed strong lines of \ion{O}{1}, \ion{Mg}{2}, \ion{Si}{2}, and \ion{Ca}{2} with typical velocities of absorption minimum around 17,000 \kms. The line profiles show material moving at velocities as high as 25,000 \kms in these lines. The observations also reveal absorption lines from \ion{S}{2} and \ion{Si}{3} with a velocity of only 11,000 \kms. The highest velocity in the \ion{S}{2} features can be traced no higher than 15,000 \kms, much lower than those of O, Mg, Si, and Ca. SN 2004dt has a polarization spectrum unlike any previously observed. The variation of the polarization across some \ion{Si}{2} lines approaches 2%, making SN 2004dt the most highly polarized SN Ia ever observed. In contrast, the strong line of O I at 777.4 nm shows little or no polarization signature. The degree of polarization points to a richly-structured partially burned silicon layer with substantial departure from spherical symmetry. A geometry that would account for the observations is one in which the distribution of oxygen is essentially spherically symmetric, but with bubbles of intermediate-mass elements with significant opacity within the oxygen substrate.

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arXiv2005

Precursors and Main-bursts of Gamma Ray Bursts in a Hypernova Scenario

We investigate a "hypernova" model for gamma-ray bursts (GRBs), i.e., massive C+O star model with relativistic jets. In this model, non-thermal precursors can be produced by the "first" relativistic shell ejected from the star. Main GRBs are produced behind the "first"-shell by the collisions of several relativistic shells. They become visible to distant observers after the colliding region becomes optically thin. We examine six selected conditions using relativistic hydrodynamical simulations and simple analyses. Interestingly, our simulations show that sub-relativistic $(v \sim 0.8c)$ jets from the central engine is sufficient to produce highly-relativistic $(Γ> 100)$ shells. We find that the relativistic shells from such a star can reproduce observed GRBs with certain conditions. Two conditions are especially important. One is the sufficiently long duration of the central engine $ \gsim 100$ sec. The other is the existence of a dense-shell somewhere behind the "first"-shell. Under these conditions, both the existence and non-existence of precursors, and long delay between precursors and main GRBs can be explained.

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arXiv2005

The first chemical enrichment in the universe and the formation of hyper metal-poor stars

The recent discovery of a hyper metal-poor (HMP) star, whose metallicity Fe/H is smaller than 1/100,000 of the solar ratio, together with one earlier HMP star, has raised a challenging question if these HMP stars are the actual first generation, low mass stars in the Universe. We argue that these HMP stars are the second generation stars being formed from gases which were chemically enriched by the first generation supernovae. The key to this solution is the very unusual abundance patterns of these HMP stars with important similarities and differences. We can reproduce these abundance features with the core-collapse ``faint'' supernova models which undergo extensive matter mixing and fallback during the explosion.

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