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Search Results: 1 - 10 of 462038 matches for " A. Heger "
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B[e] supergiants: What is their evolutionary status?
N. Langer,A. Heger
Physics , 1997,
Abstract: In this paper, we investigate the evolutionary status of B[e]~stars from the point of view of stellar evolution theory. We try to answer to the question of how massive hot supergiants --- i.e. evolved stars --- can be capable of producing a circumstellar disk. We find and discuss three possibilities: very massive evolved main sequence stars close to critical rotation due to their proximity to their Eddington-limit, blue supergiants which have just left the red supergiant branch, and single star merger remnants of a close binary system. While the latter process seems to be required to understand the properties of the spectroscopic binary R4 in the LMC, the other two scenarios may be capable of explaining the distribution of the B[e] stars in the HR~diagram. The three scenarios make different predictions about the duration of the B[e]~phase, the time integrated disk mass and the stellar properties during the B[e]~phase, which may ultimately allow to distinguish them observationally.
Presupernova Evolution of Rotating Massive Stars II: -- Evolution of the Surface Properties
A. Heger,N. Langer
Physics , 2000, DOI: 10.1086/317239
Abstract: We investigate the evolution of the surface properties of models for rotating massive stars, i.e., their luminosities, effective temperatures, surface rotational velocities, and surface abundances of all isotopes, from the zero age main sequence to the supernova stage. Our results are based on the grid of stellar models by Heger, Langer, & Woosley, which covers solar metallicity stars in the initial mass range 8 - 25 solar masses. Results are parameterized by initial mass, initial rotational velocity and major uncertainties in the treatment of the rotational mixing inside massive stars. Rotationally induced mixing processes widen the main sequence and increase the core hydrogen burning lifetime, similar to the effects of convective overshooting. It can also significantly increase the luminosity during and after core hydrogen burning, and strongly affects the evolution of the effective temperature. Our models predict surface rotational velocities for various evolutionary stages, in particular for blue supergiants, red supergiants, and for the immediate presupernova stage. We discuss the changes of the surface abundances due to rotationally induced mixing for main sequence and post main sequence stars. We single out two characteristics by which the effect of rotational mixing can be distinguished from that of massive close binary mass transfer, the only alternative process leading to non-standard chemical surface abundances in massive stars. A comparison with observed abundance anomalies in various types of massive stars supports the concept of rotational mixing in massive stars and indicates that it is responsible for most of the observed abundance anomalies.
Multi-Zone Models of Superbursts from Accreting Neutron Stars
L. Keek,A. Heger
Physics , 2011, DOI: 10.1088/0004-637X/743/2/189
Abstract: Superbursts are rare and energetic thermonuclear carbon flashes observed to occur on accreting neutron stars. We create the first multi-zone models of series of superbursts using a stellar evolution code. We self-consistently build up the fuel layer at different rates, spanning the entire range of observed mass accretion rates for superbursters. For all models light curves are presented. They generally exhibit a shock breakout, a precursor burst due to shock heating, and a two-component power-law decay. Shock heating alone is sufficient for a bright precursor, that follows the shock breakout on a short dynamical time scale due to the fall-back of expanded layers. Models at the highest accretion rates, however, lack a shock breakout, precursor, and the first power law decay component. The ashes of the superburst that form the outer crust are predominantly composed of iron, but a superburst leaves a silicon-rich layer behind in which the next one ignites. Comparing the model light curves to an observed superburst from 4U 1636-53, we find for our accretion composition the best agreement with a model at three times the observed accretion rate. We study the dependence on crustal heating of observables such as the recurrence time and the decay time scale. It remains difficult, however, to constrain crustal heating, if there is no good match with the observed accretion rate, as we see for 4U 1636-53.
Carbon production on accreting neutron stars in a new regime of stable nuclear burning
L. Keek,A. Heger
Physics , 2015, DOI: 10.1093/mnrasl/slv167
Abstract: Accreting neutron stars exhibit Type I X-ray bursts from both frequent hydrogen/helium flashes as well as rare carbon flashes. The latter (superbursts) ignite in the ashes of the former. Hydrogen/helium bursts, however, are thought to produce insufficient carbon to power superbursts. Stable burning could create the required carbon, but this was predicted to only occur at much larger accretion rates than where superbursts are observed. We present models of a new steady-state regime of stable hydrogen and helium burning that produces pure carbon ashes. Hot CNO burning of hydrogen heats the neutron star envelope and causes helium to burn before the conditions of a helium flash are reached. This takes place when the mass accretion rate is around 10% of the Eddington limit: close to the rate where most superbursts occur. We find that increased heating at the base of the envelope sustains steady-state burning by steepening the temperature profile, which increases the amount of helium that burns before a runaway can ensue.
On the Progenitors of Collapsars
A. Heger,S. E. Woosley
Physics , 2002, DOI: 10.1063/1.1579341
Abstract: We study the evolution of stars that may be the progenitors of common (long-soft) GRBs. Bare rotating helium stars, presumed to have lost their envelopes due to winds or companions, are followed from central helium ignition to iron core collapse. Including realistic estimates of angular momentum transport (Heger, Langer, & Woosley 2000) by non-magnetic processes and mass loss, one is still able to create a collapsed object at the end with sufficient angular momentum to form a centrifugally supported disk, i.e., to drive a collapsar engine. However, inclusion of current estimates of magnetic torques (Spruit 2002) results in too little angular momentum for collapsars.
Supernovae, Gamma-Ray Bursts, and Stellar Rotation
S. E. Woosley,A. Heger
Physics , 2003,
Abstract: One of the most dramatic possible consequences of stellar rotation is its influence on stellar death, particularly of massive stars. If the angular momentum of the iron core when it collapses is such as to produce a neutron star with a period of 5 ms or less, rotation will have important consequences for the supernova explosion mechanism. Still shorter periods, corresponding to a neutron star rotating at break up, are required for the progenitors of gamma-ray bursts (GRBs). Current stellar models, while providing an excess of angular momentum to pulsars, still fall short of what is needed to make GRBs. The possibility of slowing young neutron stars in ordinary supernovae by a combination of neutrino-powered winds and the propeller mechanism is discussed. The fall back of slowly moving ejecta during the first day of the supernova may be critical. GRBs, on the other hand, probably require stellar mergers for their production and perhaps less efficient mass loss and magnetic torques than estimated thus far.
The Nucleosynthetic Signature of Population III
A. Heger,S. E. Woosley
Physics , 2001, DOI: 10.1086/338487
Abstract: Growing evidence suggests that the first generation of stars may have been quite massive (~100-300 M_sun). Could these stars have left a distinct nucleosynthetic signature? We explore the nucleosynthesis of helium cores in the mass range M_He=64 to 133 Msun, corresponding to main-sequence star masses of approximately 140 to 260 M_sun. Above M_He=133 M_sun, without rotation and using current reaction rates, a black hole is formed and no nucleosynthesis is ejected. For lighter helium core masses, ~40 to 63 M_sun, violent pulsations occur, induced by the pair instability and accompanied by supernova-like mass ejection, but the star eventually produces a large iron core in hydrostatic equilibrium. It is likely that this core, too, collapses to a black hole, thus cleanly separating the heavy element nucleosynthesis of pair instability supernovae from those of other masses, both above and below. Indeed, black hole formation is a likely outcome for all Population III stars with main sequence masses between about 25 M_sun and 140 M_sun (M_He = 9 to 63 M_sun) as well as those above 260 M_sun. Nucleosynthesis in pair-instability supernovae varies greatly with the mass of the helium core which determines the maximum temperature reached during the bounce. At the upper range of exploding core masses, a maximum of 57 M_sun of Ni56 is produced making these the most energetic and the brightest thermonuclear explosions in the universe. Integrating over a distribution of masses, we find that pair instability supernovae produce a roughly solar distribution of nuclei having even nuclear charge, but are remarkably deficient in producing elements with odd nuclear charge. Also, essentially no elements heavier than zinc are produced due to a lack of s- and r-processes.
Core-Collapse Simulations of Rotating Stars
C. L. Fryer,A. Heger
Physics , 1999, DOI: 10.1086/309446
Abstract: We present the results from a series of two-dimensional core-collapse simulations using a rotating progenitor star. We find that the convection in these simulations is less vigorous because a) rotation weakens the core bounce which seeds the neutrino-driven convection and b) the angular momentum profile in the rotating core stabilizes against convection. The limited convection leads to explosions which occur later and are weaker than the explosions produced from the collapse of non-rotating cores. However, because the convection is constrained to the polar regions, when the explosion occurs, it is stronger along the polar axis. This asymmetric explosion can explain the polarization measurements of core-collapse supernovae. These asymmetries also provide a natural mechanism to mix the products of nucleosynthesis out into the helium and hydrogen layers of the star. We also discuss the role the collapse of these rotating stars play on the generation of magnetic fields and neutron star kicks. Given a range of progenitor rotation periods, we predict a range of supernova energies for the same progenitor mass. The critical mass for black hole formation also depends upon the rotation speed of the progenitor.
Nebular spectra of pair-instability supernovae
A. Jerkstrand,S. J. Smartt,A. Heger
Physics , 2015, DOI: 10.1093/mnras/stv2369
Abstract: If very massive stars (M >~ 100 Msun) can form and avoid too strong mass loss during their evolution, they are predicted to explode as pair-instability supernovae (PISNe). One critical test for candidate events is whether their nucleosynthesis yields and internal ejecta structure, being revealed through nebular-phase spectra at t >~ 1 yr, match those of model predictions. Here we compute theoretical spectra based on model PISN ejecta at 1-3 years post-explosion to allow quantitative comparison with observations. The high column densities of PISNe lead to complete gamma-ray trapping for t >~ 2 years which, combined with fulfilled conditions of steady state, leads to bolometric supernova luminosities matching the 56Co decay. Most of the gamma-rays are absorbed by the deep-lying iron and silicon/sulphur layers. The ionization balance shows a predominantly neutral gas state, which leads to emission lines of Fe I, Si I, and S I. For low-mass PISNe the metal core expands slowly enough to produce a forest of distinct lines, whereas high-mass PISNe expand faster and produce more featureless spectra. Line blocking is complete below ~5000 A for several years, and the model spectra are red. The strongest line is typically [Ca II] 7291,7323, one of few lines from ionized species. We compare our models with proposed PISN candidates SN 2007bi and PTF12dam, finding discrepancies for several key observables and thus no support for a PISN interpretation. We discuss distinct spectral features predicted by the models, and the possibility of detecting pair-instability explosions among non-superluminous supernovae.
Presupernova Evolution of Rotating Massive Stars I: Numerical Method and Evolution of the Internal Stellar Structure
A. Heger,N. Langer,S. E. Woosley
Physics , 1999, DOI: 10.1086/308158
Abstract: The evolution of rotating stars with zero-age main sequence (ZAMS) masses in the range 8 to 25 M_sun is followed through all stages of stable evolution. The initial angular momentum is chosen such that the star's equatorial rotational velocity on the ZAMS ranges from zero to ~ 70 % of break-up. Redistribution of angular momentum and chemical species are then followed as a consequence of rotationally induced circulation and instablities. The effects of the centrifugal force on the stellar structure are included. Uncertain mixing efficiencies are gauged by observations. We find, as noted in previous work, that rotation increases the helium core masses and enriches the stellar envelopes with products of hydrogen burning. We determine, for the first time, the angular momentum distribution in typical presupernova stars along with their detailed chemical structure. Angular momentum loss due to (non-magnetic) stellar winds and the redistribution of angular momentum during core hydrogen burning are of crucial importance for the specific angular momentum of the core. Neglecting magnetic fields, we find angular momentum transport from the core to the envelope to be unimportant after core helium burning. We obtain specific angular momenta for the iron core and overlaying material of 1E16...1E17 erg s. These values are insensitive to the initial angular momentum. They are small enough to avoid triaxial deformations of the iron core before it collapses, but could lead to neutron stars which rotate close to break-up. They are also in the range required for the collapsar model of gamma-ray bursts. The apparent discrepancy with the measured rotation rates of young pulsars is discussed.
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