arXiv subjects

H. Umeda

Publications and source records attributed to H. Umeda.

32 records · Page 2Linked to original sources

arXiv2003

Type Ia Supernovae: Progenitors and Diversities

A key question for supernova cosmology is whether the peak luminosities of Type Ia supernovae (SNe Ia) are sufficiently free from the effects of cosmic and galactic evolution. To answer this question, we review the currently popular scenario of SN Ia progenitors, i.e., the single degenerate scenario for the Chandrasekhar mass white dwarf (WD) models. We identify the progenitor's evolution with two channels: (1) the WD+RG (red-giant) and (2) the WD+MS (near main-sequence He-rich star) channels. The strong wind from accreting WDs plays a key role, which yields important age and metallicity effects on the evolution. 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 SN Ia brightness. This model can explain the observed dependence of SNe Ia brightness on the galaxy types. We then predict how SN Ia brightness evolves along the redshift (with changing metallicity and age) for elliptical and spiral galaxies. Such evolutionary effects along the redshift can be corrected as has been made for local SNe Ia. We also touch on several related issues: (1) the abundance pattern of stars in dwarf spheroidal galaxies in relation to the metallicity effect on SNe Ia, (2) effects of angular momentum brought into the WD in relation to the diversities and the fate of double degenerates, and (3) possible presence of helium in the peculiar SN Ia 2000cx in relation to the sub-Chandrasekhar mass model.

astro-ph↗

arXiv2003

Hypernovae and Other Black-Hole-Forming Supernovae

During the last few years, a number of exceptional core-collapse supernovae (SNe) have been discovered. Their kinetic energy of the explosions are larger by more than an order of magnitude than the typical values for this type of SNe, so that these SNe have been called `Hypernovae'. We first describe how the basic properties of hypernovae can be derived from observations and modeling. These hypernovae seem to come from rather massive stars, thus forming black holes. On the other hand, there are some examples of massive SNe with only a small kinetic energy. We suggest that stars with non-rotating black holes are likely to collapse "quietly" ejecting a small amount of heavy elements (Faint supernovae). In contrast, stars with rotating black holes are likely to give rise to very energetic supernovae (Hypernovae). We present distinct nucleosynthesis features of these two types of "black-hole-forming" supernovae. Hypernova nucleosynthesis is characterized by larger abundance ratios (Zn,Co,V,Ti)/Fe and smaller (Mn,Cr)/Fe. Nucleosynthesis in Faint supernovae is characterized by a large amount of fall-back. We show that the abundance pattern of the most Fe deficient star, HE0107-5240, and other extremely metal-poor carbon-rich 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.

astro-ph↗

arXiv2003

On The Spectrum and Spectropolarimetry of Type Ic Hypernova SN 2003dh/GRB 030329

Spectroscopic and spectropolarimetric observations of SN 2003dh/GRB 030329 obtained in 2003 May using the Subaru 8.2 m telescope are presented. The properties of the SN are investigated through a comparison with spectra of the Type Ic hypernovae SNe 1997ef and 1998bw. (Hypernovae being a tentatively defined class of SNe with very broad absorption features: these features suggest a large velocity of the ejected material and possibly a large explosion kinetic energy.) Comparison with spectra of other hypernovae shows that the spectrum of SN 2003dh obtained on 2003 May 8 and 9, i.e., 34-35 rest-frame days after the GRB (for z=0.1685), are similar to those of SN 1997ef obtained ~34-42 days after the fiducial time of explosion of that SN. The match with SN 1998bw spectra is not as good (at rest 7300-8000 A, but again spectra obtained ~33-43 days after GRB 980425 are preferred. This indicates that the SN may have intermediate properties between SNe 1997ef and 1998bw. Based on the analogy with the other hypernovae, the time of explosion of SN 2003dh is then constrained to be between -8 and +2 days of the GRB. The Si and O P-Cygni lines of SN 2003dh seem comparable to those of SN 1997ef, which suggests that the ejected mass in SN 2003dh may match that in SN 1997ef. Polarization was marginally detected at optical wavelengths. This is consistent with measurements of the late afterglow, implying that it mostly originated in the interstellar medium of the host galaxy.

astro-ph↗

arXiv2003

Nucleosynthesis in Black-Hole-Forming Supernovae and Abundance Patterns of Extremely Metal-Poor Stars

Stars more massive than $\sim$ 20 - 25 \ms form a black hole at the end of their evolution. Stars with non-rotating black holes are likely to collapse "quietly" ejecting a small amount of heavy elements (Faint supernovae). In contrast, stars with rotating black holes are likely to give rise to very energetic supernovae (Hypernovae). We present distinct nucleosynthesis features of these two types of "black-hole-forming" supernovae. Nucleosynthesis in Hypernovae is characterized by larger abundance ratios (Zn,Co,V,Ti)/Fe and smaller (Mn,Cr)/Fe than normal supernovae, which can explain the observed trend of these ratios in extremely metal-poor stars. Nucleosynthesis in Faint supernovae is characterized by a large amount of fall-back. We show that the abundance pattern of the recently discovered most Fe-poor star, HE0107-5240, and other extremely metal-poor carbon-rich 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.

astro-ph↗

arXiv2003

The Optical/Near-Infrared Light Curves of SN 2002ap for the First 140 Days after Discovery

Supernova (SN) 2002ap in M74 was observed in the $UBVRIJHK$ bands for the first 40 days following its discovery (2002 January 29) until it disappeared because of solar conjunction, and then in June after it reappeared. The magnitudes and dates of peak brightness in each band were determined. While the rate of increase of the brightness before the peak is almost independent of wavelength, the subsequent rate of decrease becomes smaller with wavelength from the $U$ to the $R$ band, and is constant at wavelengths beyond $I$. The photometric evolution is faster than in the well-known ``hypernovae'' SNe~1998bw and 1997ef, indicating that SN 2002ap ejected less mass. The bolometric light curve of SN 2002ap for the full period of observations was constructed. The absolute magnitude is found to be much fainter than that of SN 1998bw, but is similar to that of SN 1997ef, which lies at the faint end of the hypernova population. The bolometric light curve at the early epochs was best reproduced with the explosion of a C+O star that ejects $2.5~M_\sun$ with kinetic energy $E_{\rm K}=4\times 10^{51}~{\rm ergs}$. A comparison of the predicted brightness of SN 2002ap with that observed after solar conjunction may imply that $γ$-ray deposition at the later epochs was more efficient than in the model. This may be due to an asymmetric explosion.

astro-ph↗

arXiv2002

The Type Ic Hypernova SN 2002ap

Photometric and spectroscopic data of the energetic Type Ic supernova (SN) 2002ap are presented, and the properties of the SN are investigated through models of its spectral evolution and its light curve. The SN is spectroscopically similar to the "hypernova" SN 1997ef. However, its kinetic energy [$\sim (4-10) \times 10^{51}$ erg] and the mass ejected (2.5-5 $M_{\odot}$) are smaller, resulting in a faster-evolving light curve. The SN synthesized $\sim 0.07 M_{\odot}$ of $^{56}$Ni, and its peak luminosity was similar to that of normal SNe. Brightness alone should not be used to define a hypernova, whose defining character, namely very broad spectral features, is the result of a high kinetic energy. The likely main-sequence mass of the progenitor star was 20-25 $M_{\odot}$, which is also lower than that of both hypernovae SNe 1997ef and 1998bw. SN 2002ap appears to lie at the low-energy and low-mass end of the hypernova sequence as it is known so far. Observations of the nebular spectrum, which is expected to dominate by summer 2002, are necessary to confirm these values.

astro-ph↗

arXiv2001

Gamma-Ray Signatures of Supernovae and Hypernovae

We review the characteristics of nucleosynthesis and radioactivities in 'Hypernovae', i.e., supernovae with very large explosion energies ($ \gsim 10^{52} $ ergs) and their $γ$-ray line signatures. We also discuss the $^{44}$Ti line $γ$-rays from SN1987A and the detectability with INTEGRAL. Signatures of hypernova nucleosynthesis are seen in the large [(Ti, Zn)/Fe] ratios in very metal poor stars. Radioactivities in hypernovae compared to those of ordinary core-collapse supernovae show the following characteristics: 1) The complete Si burning region is more extended, so that the ejected mass of $^{56}$Ni can be much larger. 2) Si-burning takes place in higher entropy and more $α$-rich environment. Thus the $^{44}$Ti abundance relative to $^{56}$Ni is much larger. In aspherical explosions, $^{44}$Ti is even more abundant and ejected with velocities as high as $\sim$ 15,000 km s$^{-1}$, which could be observed in $γ$-ray line profiles. 3) The abundance of $^{26}$Al is not so sensitive to the explosion energy, while the $^{60}$Fe abundance is enhanced by a factor of $\sim$ 3.

astro-ph↗

arXiv2001

Hypernova Nucleosynthesis and Galactic Chemical Evolution

We study nucleosynthesis in 'hypernovae', i.e., supernovae with very large explosion energies ($ \gsim 10^{52} $ ergs) for both spherical and aspherical explosions. The hypernova yields compared to those of ordinary core-collapse supernovae show the following characteristics: 1) Complete Si-burning takes place in more extended region, so that the mass ratio between the complete and incomplete Si burning regions is generally larger in hypernovae than normal supernovae. As a result, higher energy explosions tend to produce larger [(Zn, Co)/Fe], small [(Mn, Cr)/Fe], and larger [Fe/O], which could explain the trend observed in very metal-poor stars. 2) Si-burning takes place in lower density regions, so that the effects of $α$-rich freezeout is enhanced. Thus $^{44}$Ca, $^{48}$Ti, and $^{64}$Zn are produced more abundantly than in normal supernovae. The large [(Ti, Zn)/Fe] ratios observed in very metal poor stars strongly suggest a significant contribution of hypernovae. 3) Oxygen burning also takes place in more extended regions for the larger explosion energy. Then a larger amount of Si, S, Ar, and Ca ("Si") are synthesized, which makes the "Si"/O ratio larger. The abundance pattern of the starburst galaxy M82 may be attributed to hypernova explosions. Asphericity in the explosions strengthens the nucleosynthesis properties of hypernovae except for "Si"/O. We thus suggest that hypernovae make important contribution to the early Galactic (and cosmic) chemical evolution.

astro-ph↗

arXiv2000

Impacts of the Detection of Cassiopeia A Point Source

Very recently the Chandra First Light Observation discovered a point-like source in Cassiopeia A (Cas A) supernova remnant. This detection was subsequently confirmed by the analyses of the archival data from both ROSAT and Einstein observations. Here we compare the results from these observations with the scenarios involving both black holes (BH) and neutron stars (NS). If this point source is a BH we offer as a promising model a disk-corona type model with low accretion rate where a soft photon source at ~ 0.1 keV is Comptonized by higher energy electrons in the corona. If it is a NS the dominant radiation observed by Chandra most likely originates from smaller, hotter regions of the stellar surface, but we argue that it is still worthwhile to compare the cooler component from the rest of the surface with cooling theories. We emphasize that the detection of this point source itself should potentially provide enormous impacts on the theories of supernova explosion, progenitor scenario, compact remnant formation, accretion to compact objects, and neutron star thermal evolution.

astro-ph↗

arXiv2000

Nucleosynthesis in Chandrasekhar Mass Models for Type Ia Supernovae and Constraints on Progenitor Systems and Burning-Front Propagation

We calculate explosive nucleosynthesis in Chandrasekhar mass models for Type Ia Supernovae(SNe Ia) to obtain new constraints on the rate of matter accretion onto the progenitor white dwarf and on the ignition density of central carbon deflagration. The calculated abundance of the Fe-group neutron-rich nuclei is highly sensitive to the electron captures taking place in the central layers. The yields obtained from a slow central deflagration, and from a fast deflagration or delayed detonation in the outer layers, are combined and put to comparison with solar isotopic abundances. We found that (1) to avoid too large ratios of $^{54}$Cr/$^{56}$Fe and $^{50}$Ti/$^{56}$Fe, the ignition density should be as low as \ltsim 2 \e9 \gmc, and that (2) to avoid the overproduction of $^{58}$Ni and $^{54}$Fe, either the flame speed should not exceed a few % of the sound speed in the central low $Y_e$ layers, or the progenitor star has to be metal-poor compared with solar. Such low central densities can be realized by a rapid accretion as fast as $\dot M$ \gtsim 1 $\times$ 10$^{-7}$M$_\odot$ yr$^{-1}$. In order to reproduce the solar abundance of $^{48}$Ca, one also needs progenitor systems that undergo ignition at higher densities. We also found that the total amount of $^{56}$Ni, the Si-Ca/Fe ratio, and the abundance of elements like Mn and Cr (incomplete Si-burning ashes), depend on the density of the deflagration-detonation transition in delayed detonations. Our nucleosynthesis results favor transition densities slightly below 2.2$\times 10^7$ g cm$^{-3}$.

astro-ph↗

arXiv1999

Influence of the Stellar Population on Type Ia Supernovae: Consequences for the Determination of Omega

The influence of the metallicity at the main sequence on the chemical structure of the exploding white dwarf, the nucleosynthesis during the explosion and the light curves of an individual Type Ia supernovae have been studied. Detailed calculations of the stellar evolution, the explosion, and light curves of delayed detonation models are presented. Detailed stellar evolution calculations with a main sequence mass of 7 M_o have been performed to test the influence of the metallicity Z on the structure of the progenitor. A change of Z influences the central helium burning and, consequently, the size of the C/O core which becomes a C/O white dwarf and its C/O ratio. Subsequently, the white dwarf may grow to the Chandrasekhar mass and explode as a Type Ia supernovae. Consequently, the C/O structure of the exploding white dwarf depends on Z. Since C and O are the fuel for the thermonuclear explosion, Z indirectly changes the energetics of the explosion. In our example, changing Z from Pop I to Pop II causes a change in the zero point of the maximum brightness/decline relation by about $0.1^m$ and a change in the rise time by about 1 day. Combined with previous studies, We provide a relation between a change of the rise time in SNeIa and the off-set in the brightness/decline relation. We find that dM = dt where dM is the offset in the relation and dt is the change of the rise time in days. This relation provides a way to detect and test for evolutionary effects, and to estimate the possible consequences for the determination of the cosmological constant. However, due to uncertainties in the time evolution of the properties of the progenitors, we cannot predict whether evolution becomes important at z=0.5, 1 or 2. This question has to be answered by observations.

astro-ph↗

arXiv1999

The Origin of Diversity of Type Ia Supernovae and Environmental Effects

Observations suggest that the properties of Type Ia supernovae (SNe Ia) may depend on environmental characteristics, such as morphology, metallicity, and age of host galaxies. The influence of these environmental properties on the resulting SNe Ia is studied in this paper. First it is shown that the carbon mass fraction X(C) in the C+O white dwarf SN Ia progenitors tends to be smaller for lower metallicity and older the binary system age. It is then suggested that the variation of X(C) causes the diversity in the brightness of SNe Ia: a smaller X(C) leads to a dimmer SN Ia. Further studies of the propagation of the turbulent flame are necessary to confirm this relation. Our model for the SN Ia progenitors then predicts that when the progenitors belong to an older population or to a low metallicity environment, the number of bright SNe Ia is reduced, so that the variation in brightness among the SNe Ia is also smaller. Thus our model can explain why the mean SN Ia brightness and its dispersion depend on the morphology of the host galaxies and on the distance of the SN from the center of the galaxy. It is further predicted that at higher redshift (z >~ 1) both the the mean brightness of SNe Ia and its variation should be smaller in spiral galaxies than in elliptical galaxies. These variations are within the range observed in nearby SNe Ia. In so far as the variation in X(C) is the most important cause for the diversity among SNe Ia, the light curve shape method currently used to determine the absolute magnitude of SNe Ia can be applied also to high redshift SNe Ia.

astro-ph↗

arXiv1998

A `Hypernova' model for SN 1998bw associated with gamma-ray burst of 25 April 1998

The discovery of the peculiar supernova (SN) 1998bw and its possible association with the gamma-ray burst (GRB) 980425$^{1,2,3}$ provide new clues to the understanding of the explosion mechanism of very massive stars and to the origin of some classes of gamma-ray bursts. Its spectra indicate that SN~1998bw is a type Ic supernova$^{3,4}$, but its peak luminosity is unusually high compared with typical type Ic supernovae$^3$. Here we report our findings that the optical spectra and the light curve of SN 1998bw can be well reproduced by an extremely energetic explosion of a massive carbon+oxygen (C+O) star. The kinetic energy is as large as $\sim 2-5 \times 10^{52}$ ergs, more than ten times the previously known energy of supernovae. For this reason, the explosion may be called a `hypernova'. Such a C+O star is the stripped core of a very massive star that has lost its H and He envelopes. The extremely large energy, suggesting the existence of a new mechanism of massive star explosion, can cause a relativistic shock that may be linked to the gamma-ray burst.

astro-ph↗

arXiv1998

Discovery of the peculiar supernova 1998bw in the error box of GRB980425

The discovery of X-ray, optical and radio afterglows of gamma-ray bursts (GRBs) and the measurements of the distances to some of them have established that these events come from Gpc distances and are the most powerful photon emitters known in the Universe, with peak luminosities up to 10^52 erg/s. We here report the discovery of an optical transient, in the BeppoSAX Wide Field Camera error box of GRB980425, which occurred within about a day of the gamma-ray burst. Its optical light curve, spectrum and location in a spiral arm of the galaxy ESO 184-G82, at a redshift z = 0.0085, show that the transient is a very luminous type Ic supernova, SN1998bw. The peculiar nature of SN1998bw is emphasized by its extraordinary radio properties which require that the radio emitter expand at relativistical speed. Since SN1998bw is very different from all previously observed afterglows of GRBs, our discovery raises the possibility that very different mechanisms may give rise to GRBs, which differ little in their gamma-ray properties.

astro-ph↗