Showing papers on "Hypernova published in 1997"

Extremely Metal-poor Stars. IV. The Carbon-rich Objects

Abstract: Abundances are presented for some 20 elements in three extremely metal-poor, carbon-rich stars. All have [Fe/H] < -2.5. Based on their high proper motions and spectroscopic gravities two of them have relatively low luminosity: LP 706-7 is on or near the main sequence, while LP 625-44 is a subgiant. The third is the giant CS 22892-052, discovered to be r-processes enriched by Sneden et al. (1994). All three stars have large C and N overabundances, and large enhancements of the heavy neutron-capture elements— ~ 1-2 dex. In contrast to the r-process signature observed in CS 22892-052, however, both LP 625-44 and LP 706-7 are clearly s-process enriched, suggesting that they may be progenitors of the well-known halo CH giants, the peculiarities of which are believed to result from mass transfer across a binary system containing an asymptotic giant branch star. In both LP 625-44 and LP 706-7 the distribution of s-process elements is heavily weighted toward higher atomic number. [hs/ls] ~ 1.5, considerably larger than the values 0.5 found in the s-process enhanced near-main-sequence stars of the disk populations. This implies a much higher neutron exposure per seed nucleus in the Population II objects and identifies 13C(α, n)16O as the neutron source. For LP 625-44 radial velocity and Li abundance data are consistent with the binary hypothesis. LP 706-7, however, remains something of an enigma in these respects: it shows no clear evidence for velocity variations, and its Li abundance lies precisely on the Spite Plateau. We estimate that the probability of this occurring in the Ba/CH class of objects is, roughly, 1%. In metal-poor stars the incidence of carbon enrichment appears to increase toward the lowest metallicities, and below [Fe/H] = -2.5 supersolar values of [C/Fe] are not uncommon. Comparison of the available observational material with the Galactic chemical enrichment model of Timmes et al. (1995) shows that the model produces too little carbon. While the difference may result in the sensitivity of carbon production to modeling uncertainties such as the treatment of convection, we also discuss the possible role of a class of carbon producing zero heavy element supernovae and of massive "hypernovae" discussed by Woosley & Weaver (1982, 1995) in explaining this result. The carbon problem is also implicit in the suggestion that the r-process signature seen in CS 22892-052 results from normal supernovae enrichment—where then does its large carbon overabundance originate? The models one might invoke to produce carbon overabundances leave black hole remnants in which the layers containing the seed nuclei for the r-process are not available for ejection.

The Collapse of Neutron Stars in High-Mass Binaries as the Energy Source for the Gamma-Ray Bursts

Abstract: The energy source has remained to be the great mystery in understanding of the gamma-ray bursts (GRBs) if the events are placed at cosmological distances as indicated by a number of recent observations. The currently popular models include (1)the merger of two neutron stars or a neutron star and a black hole binary and (2)the hypernova scenario of the collapse of a massive member in a close binary. Since a neutron star will inevitably collapse into a black hole if its mass exceeds the limit $M_{max}\approx3M_{\odot}$, releasing a total binding gravitational energy of $\sim10^{54}$ erg, we explore semi-empirically the possibility of attributing the energy source of GRB to the accretion- induced collapse of a neutron star (AICNS) in a massive X-ray binary system consisting of a neutron star and a type O/B companion. This happens because a significant mass flow of $\sim10^{-3}$--$10^{-4}M_{\odot}$ yr$^{-1}$ may be transferred onto the neutron star through the Roche-lobe overflow and primarily during the spiral-in phase when it plunges into the envelope of the companion, which may eventually lead to the AICNS before the neutron star merges with the core of the companion. In this scenario, a ``dirty'' fireball with a moderate amount of beaming is naturally expected because of the nonuniformity of the stellar matter surrounding the explosion inside the companion, and a small fraction ($\sim0.1%$) of the energy is sufficient to create the observed GRBs. In addition, the bulk of the ejecting matter of the companion star with a relatively slow expansion rate may act as the afterglow. Assuming a non-evolutionary model for galaxies, we estimate that the birthrate of the AICNS events is about 2 per day within a volume to redshift $z=1$ for an $\Omega_0=1$ universe, consistent with the reported GRB rate.

Gamma-Ray Bursts as Hypernovae

Abstract: The energetics of optical and radio afterglows following BeppoSAX and BATSE gamma-ray bursts (GRBs) suggests that gamma-ray emission is not narrowly collimated, but a moderate beaming is possible, so the total energy of a GRB may be in the range 10^{50} - 10^{51} erg. All attempts to generate a fireball powered by neutrino-antineutrino annihilation have failed so far, and a rapid rotation combined with a magnetic field of $ \sim 10^{15} $ gauss gains popularity. In this paper a hypernova scenario is described: a collapse of a massive member in a close binary system, similar to the `failed' Type Ib supernova model proposed by Woosley (1993). The collapse may lead to explosion, with energy transmitted from the rapidly spinning hot neutron core or black hole to the envelope by a strong magnetic field, as in a supernova model proposed by Ostriker and Gunn (1971). However, because of a large mass and rapid rotation of the core the explosion of a hypernova may release up to 10^{54} erg of kinetic energy, creating a `dirty' fireball. In this scenario a moderately non-spherical explosion may accelerate a very small fraction of matter to a very large Lorentz factor, and this may give rise to a gamma-ray burst and its afterglow, just like in a conventional fireball model. However, the highest velocity ejecta from a hypernova are followed with matter which expands less rapidly but carries the bulk of kinetic energy, providing a long term power source for the afterglow. If the afterglows remain luminous for a very long time then the proposed hypernova scenario may provide an explanation. Pre-hypernovae, being massive stars, are likely to be located in the young, star forming regions.

GRBs as Hypernovae

Abstract: A standard fireball/afterglow model of a gamma-ray burst relates the event to a merging neutron star binary, or a neutron star - black hole binary, which places the events far away from star forming regions, and is thought to have an energy of ~ 10^51 erg. A hypernova, the death of a massive and rapidly spinning star, may release ~ 10^54 erg of kinetic energy by tapping the rotational energy of a Kerr black hole formed in the core collapse. Only a small fraction of all energy is in the debris ejected with the largest Lorentz factors, those giving rise to the GRB itself, but all energy is available to power the afterglow for a long time. In this scenario GRBs should be found in star forming regions, the optical afterglows may be obscured by dust, and the early thermal emission of the massive ejecta may give rise to X-ray precursors, as observed by Ginga. The optical and X-ray afterglows of GRBs 970228, 97508, 97828 provide some evidence that these bursts were located in galaxies, most likely in dwarf galaxies, in or near star forming regions.

Towards Solution of the Gamma-ray Burst Mystery II. "Failed Type I supernova/hypernova" in high-redshift starburst galaxies

Abstract: We study the scenario where the gamma-ray bursts (GRBs) arise from core-collapse of very massive stars in star-forming regions in the starburst galaxies at high redshift The bimodial structure of the gamma-ray bursts point to their association with the two redshift peaks observed in the recent study of high-redshift galaxies; one of these regions involves massive star-forming galaxies of normal type at z~3, the other is related to the star formation in dwarf galaxies around z ~1 The cosmological time-dilation observed in GRBs is consistent with this picture and the time-dilation observed in high-redshift Type I supernovae The details of the GRB characteristics, the nature of the starburst galaxies, and a failed-TypeI supernova/hypernova model of GRBs are discussed in several papers In this paper II we discuss the details of star-formation in starburst galaxies both in the local universe and at high redshift, and we study the association of these star-forming regions with GRBs The details of the failed-supernova/hypernova model of GRBs will be discussed in the next article (paper III) According to this model, the collapse of a massive star produces a dense torus of circumstellar material as well as a relativistic shock-wave accompanied by ejecta, which generate shells by impact with the material around the collapsed core, which in turn cause high-energy gamma-ray emission and the observed afterglow