Takuya Ohkubo

Galactic Chemical Evolution: Carbon through Zinc

We calculate the evolution of heavy-element abundances from C to Zn in the solar neighborhood, adopting our new nucleosynthesis yields. Our yields are calculated for wide ranges of metallicity (Z = 0-Z☉) and the explosion energy (normal supernovae and hypernovae), based on the light-curve and spectra fitting of individual supernovae. The elemental abundance ratios are in good agreement with observations. Among the α-elements, O, Mg, Si, S, and Ca show a plateau at [Fe/H] -1, while Ti is underabundant overall. The observed abundance of Zn ([Zn/Fe] ~ 0) can be explained only by the high-energy explosion models, as it requires a large contribution of hypernovae. The observed decrease in the odd-Z elements (Na, Al, and Cu) toward low [Fe/H] is reproduced by the metallicity effect on nucleosynthesis. The iron-peak elements (Cr, Mn, Co, and Ni) are consistent with the observed mean values at -2.5 [Fe/H] -1, and the observed trend at the lower metallicity can be explained by the energy effect. We also show the abundance ratios and the metallicity distribution functions of the Galactic bulge, halo, and thick disk. Our results suggest that the formation timescale of the thick disk is ~1-3 Gyr.

Core-Collapse Very Massive Stars: Evolution, Explosion, and Nucleosynthesis of Population III 500-1000 M☉ Stars

We calculate evolution, collapse, explosion, and nucleosynthesis of Population III very massive stars with 500 and 1000 M☉. Presupernova evolution is calculated in spherical symmetry. Collapse and explosion are calculated by a two-dimensional code, based on the bipolar jet models. We compare the results of nucleosynthesis with the abundance patterns of intracluster matter, hot gases in M82, and extremely metal-poor stars in the Galactic halo. It was found that both 500 and 1000 M☉ models enter the region of pair instability but continue to undergo core collapse. In the presupernova stage, silicon-burning regions occupy a large fraction, more than 20% of the total mass. For moderately aspherical explosions, the patterns of nucleosynthesis match the observational data of both the intracluster medium and M82. Our results suggest that explosions of Population III core-collapse very massive stars contribute significantly to the chemical evolution of gases in clusters of galaxies. For Galactic halo stars our [O/Fe] ratios are smaller than the observational abundances. However, our proposed scenario is naturally consistent with this outcome. The final black hole masses are ~230 and ~500 M☉ for the 500 and 1000 M☉ models, respectively. This result may support the view that Population III very massive stars are responsible for the origin of intermediate-mass black holes, which were recently reported to be discovered.

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 ~1 M ☉ by accreting the surrounding gases. Our calculations cover 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 two different sets of stellar mass accretion rates as our fiducial models. One is derived from a cosmological simulation of the first generation (PopIII.1) stars, and the other is derived from a simulation of the second generation stars that are affected by radiation from PopIII.1 stars. The latter represents one case of PopIII.2 stars. We also adopt additional models that include radiative feedback effects. We show that the final mass of Pop III.1 stars can be as large as ~1000 M ☉, beyond the mass range (140-300 M ☉) 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 (40-60 M ☉), 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. In view of the large range of possible accretion rates, further studies are important to see if these fiducial models are actually the cases.

The Asymmetric Explosion of Type Ia Supernovae as Seen from Near-Infrared Observations

We present near-infrared spectra of late-phase (>200 days) Type Ia supernovae (SNe Ia) taken at the Subaru Telescope. The [Fe II] line of SN 2003hv shows a clear flat-topped feature, while that of SN 2005W shows a less prominent flatness. In addition, a large shift in their line center, varying from -3000 to 1000 km s-1 with respect to the host galaxies, is seen. Such a shift suggests the occurrence of an off-center, nonspherical explosion in the central region and provides important, new constraints on the explosion models of SNe Ia.

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 progenitors 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

Early Black Hole formation by accretion of gas and dark matter

X

Nucleosynthesis in hypernovae and extremely metal-poor stars

(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.

Pop III hypernova nucleosynthesis and abundance trends in extremely metal-poor halo stars

Recent discovery of luminous quasars at z > 6 has posed a severe challenge to the theory of structure formation of the universe. These quasars are thought to be powered by supermassive black holes (SMBHs). However no consensus is yet to be reached as to the origin and early formation mechanism of massive SMBHs. We propose a model in which intermediate-mass black holes (IMBHs) with mass of ~ 104M⊙ are formed in early dark matter halos. We carry out detailed stellar evolution calculations for the first generation stars including annihilation energy of dark matter (DM) particles. We show that very massive stars, as massive as 104M⊙, can be formed in an early DM halo. Such stars are extremely bright with Log L/L⊙ 8.2. They will gravitationally collapse to form IMBHs. These black holes could have seeded the formation of early SMBHs.

Evolution of Core‐Collapse Very Massive Population III Stars

Abstract Hypernovae are the core-collapse supernovae with very large explosion energies (⪆ 10 52 ergs). Nucleosynthesis in hypernovae show the following characteristics: 1) Higher energy explosions tend to produce larger [(Zn, Co, V)/Fe] and smaller [(Mn, Cr)/Fe], which can explain the trend observed in very metal-poor stars. 2) Because of enhanced α-rich freezeout, 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 takes place in more extended regions in hypernovae, which makes the Si/O ratio larger. We thus suggest that hypernovae make important contribution to the early Galactic (and cosmic) chemical evolution. We then discuss the evolutionary origin of the recently discovered most Fe deficient star, HE0107-5240. We show that the abundance pattern of HE0107-5240 and other extremely metal-poor (EMP) stars are in good accord with those of supernovae which originate from ∼ 20–130 M ⊙ stars and form ∼ 3–10 M ⊙ black holes.

Hypernovae : Their properties and gamma-ray burst connection

Abstract We calculate evolution and nucleosynthesis in massive Pop III stars with M = 13 ∼ 270 M ⊙ and compare the results with abundances of very metal-poor halo stars. The observed abundances can be explained by the energetic core-collapse supernovae with M ⪅ 130 M ⊙ (“hypernovae”) but not by pair-instability supernovae (PISNe) with M ∼ 140–270 M ⊙ . This result constrains the IMF for the Pop III and very metal-poor Pop II stars.