The final evolution of stars in the mass range 70-140 M is explored. Depending upon their mass loss history and rotation rates, these stars will end their lives as pulsational pair-instability ...supernovae (PPISN) producing a great variety of observational transients with total durations ranging from weeks to millennia and luminosities from 1041 to over 1044 erg s−1. No nonrotating model radiates more than 5 × 10 50 erg of light or has a kinetic energy exceeding 5 × 10 51 erg, but greater energies are possible, in principle, in magnetar-powered explosions, which are explored. Many events resemble SNe Ibn, SNe Icn, and SNe IIn, and some potential observational counterparts are mentioned. Some PPISN can exist in a dormant state for extended periods, producing explosions millennia after their first violent pulse. These dormant supernovae contain bright Wolf-Rayet stars, possibly embedded in bright X-ray and radio sources. The relevance of PPISN to supernova impostors like Eta Carinae, to superluminous supernovae, and to sources of gravitational radiation is discussed. No black holes between 52 and 133 M are expected from stellar evolution in close binaries.
The evolution of helium stars with initial masses in the range 1.6-120 is studied, including the effects of mass loss by winds. These stars are assumed to form in binary systems when their expanding ...hydrogenic envelopes are promptly lost just after helium ignition. Significant differences are found with single-star evolution, chiefly because the helium core loses mass during helium burning rather than gaining it from hydrogen shell burning. Consequently, presupernova stars for a given initial mass function have considerably smaller mass when they die and will be easier to explode. Even accounting for this difference, the helium stars with mass loss develop more centrally condensed cores that should explode more easily than their single-star counterparts. The production of low-mass black holes may be diminished. Helium stars with initial masses below 3.2 experience significant radius expansion after helium depletion, reaching blue supergiant proportions. This could trigger additional mass exchange or affect the light curve of the supernova. The most common black hole mass produced in binaries is estimated to be about 9 . A new maximum mass for black holes derived from pulsational pair-instability supernovae is derived, 46 , and a new potential gap at 10-12 is noted. Models pertinent to SN 2014ft are presented, and a library of presupernova models is generated.
The extremely luminous supernova SN 2006gy (ref. 1) challenges the traditional view that the collapse of a stellar core is the only mechanism by which a massive star makes a supernova, because it ...seems too luminous by more than a factor of ten. Here we report that the brightest supernovae in the modern Universe arise from collisions between shells of matter ejected by massive stars that undergo an interior instability arising from the production of electron-positron pairs. This 'pair instability' leads to explosive burning that is insufficient to unbind the star, but ejects many solar masses of the envelope. After the first explosion, the remaining core contracts and searches for a stable burning state. When the next explosion occurs, several solar masses of material are again ejected, which collide with the earlier ejecta. This collision can radiate 1050 erg of light, about a factor of ten more than an ordinary supernova. Our model is in good agreement with the observed light curve for SN 2006gy and also shows that some massive stars can produce more than one supernova-like outburst.
THE PROGENITOR OF GW150914 Woosley, S. E.
Astrophysical journal. Letters,
06/2016, Letnik:
824, Številka:
1
Journal Article
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ABSTRACT The spectacular detection of gravitational waves (GWs) from GW150914 and its reported association with a gamma-ray burst (GRB) offer new insights into the evolution of massive stars. Here, ...it is shown that no single star of any mass and credible metallicity is likely to produce the observed GW signal. Stars with helium cores in the mass range 35-133 M encounter the pair instability and either explode or pulse until the core mass is less than 45 M , smaller than the combined mass of the observed black holes. The rotation of more massive helium cores is either braked by interaction with a slowly rotating hydrogen envelope, if one is present, or by mass loss, if one is not. The very short interval between the GW signal and the observed onset of the putative GRB in GW150914 is also too short to have come from a single star. A more probable model for making the gravitational radiation is the delayed merger of two black holes made by 70 and 90 M stars in a binary system. The more massive component was a pulsational-pair instability supernova before making the first black hole.
For carbon-oxygen white dwarfs accreting hydrogen or helium at rates in the range ~(1-10) X 10--8 M yr--1, a variety of explosive outcomes is possible well before the star reaches the Chandrasekhar ...mass. These outcomes are surveyed for a range of white dwarf masses (0.7-1.1 M ), accretion rates ((1-7) X 10--8 M yr--1), and initial white dwarf temperatures (0.01 and 1 L ). The results are particularly sensitive to the convection that goes on during the last few minutes before the explosion. Unless this convection maintains a shallow temperature gradient and unless the density is sufficiently high, the accreted helium does not detonate. Below a critical helium ignition density, which we estimate to be (5-10) X 105 g cm--3, either helium novae or helium deflagrations result. The hydrodynamics, nucleosynthesis, light curves, and spectra of a representative sample of detonating and deflagrating models are explored. Some can be quite faint indeed, powered at peak for a few days by the decay of 48Cr and 48V. Only the hottest, most massive white dwarfs considered with the smallest helium layers, show reasonable agreement with the light curves and spectra of common Type Ia supernovae (SNe Ia). For the other models, especially those involving lighter white dwarfs, the helium shell mass exceeds 0.05 M and the mass of the 56Ni that is synthesized exceeds 0.01 M . These explosions do not look like ordinary SNe Ia or any other frequently observed transient.
Abstract
Stellar evolution theory predicts a “gap” in the black hole birth function caused by the pair instability. Many presupernova stars that have a core mass below some limiting value,
M
low
, ...after all pulsational activity is finished, collapse to black holes, while more massive ones, up to some limiting value,
M
high
, explode, promptly and completely, as pair-instability supernovae. Previous work has suggested
M
low
≈ 50
M
⊙
and
M
high
≈ 130
M
⊙
. These calculations have been challenged by recent LIGO observations that show many black holes merging with individual masses
M
low
≳ 65
M
⊙
. Here we explore four factors affecting the theoretical estimates for the boundaries of this mass gap: nuclear reaction rates, evolution in detached binaries, rotation, and hyper-Eddington accretion after black hole birth. Current uncertainties in reaction rates by themselves allow
M
low
to rise to 64
M
⊙
and
M
high
as large as 161
M
⊙
. Rapid rotation could further increase
M
low
to ∼70
M
⊙
, depending on the treatment of magnetic torques. Evolution in detached binaries and super-Eddington accretion can, with great uncertainty, increase
M
low
still further. Dimensionless Kerr parameters close to unity are allowed for the more massive black holes produced in close binaries, though they are generally smaller.
Supernova iPTF14hls maintained a bright, variable luminosity for more than 600 days, while lines of hydrogen and iron in its spectrum had different speeds but showed little evolution. Here, several ...varieties of models are explored for iPTF14hls-like events. They are based upon circumstellar medium (CSM) interaction in an ordinary supernova, pulsational pair-instability supernovae (PPISN), and magnetar formation. Each is able to explain the enduring emission and brightness of iPTF14hls but each has shortcomings when confronted with other observed characteristics. The PPISN model can, in some cases, produce a presupernova transient like the one observed at the site of iPTF14hls in 1954. It also offers a clear path to providing the necessary half solar mass of material at ∼5 × 1016 cm for CSM interaction to work and it can give an irregular light curve without invoking additional assumptions. It explains the 4000 km s−1 seen in the iron lines but without additional energy input it strains to explain the nearly constant 8000 km s−1 velocity seen in H . Magnetar models can also explain most of the observed features but they give a smooth light curve and may be difficult to reconcile with the observation of slow-moving hydrogen at late times. The various models predict different spectral characteristics and a remnant that, today, could be a black hole, magnetar, or even a star. Further observations and calculations of radiation transport will narrow the range of possibilities.
Following an initial explosion that might be launched either by magnetic interactions or neutrinos, a rotating magnetar radiating according to the classic dipole formula could power a very luminous ...supernova. While some {sup 56}Ni might be produced in the initial explosion, the peak of the light curve in a Type I supernova would not be directly related to its mass. In fact, the peak luminosity would be most sensitive to the dipole field strength of the magnetar. The tail of the light curve could resemble radioactive decay for some time but, assuming complete trapping of the pulsar emission, would eventually be brighter. Depending on the initial explosion energy, both high and moderate velocities could accompany a very luminous light curve.
The density structure surrounding the iron core of a massive star when it dies is known to have a major effect on whether or not the star explodes. Here we repeat previous surveys of presupernova ...evolution with some important corrections to code physics and four to 10 times better mass resolution in each star. The number of presupernova masses considered is also much larger. Over 4000 models are calculated in the range from 12 to 60 M with varying mass loss rates. The core structure is not greatly affected by the increased spatial resolution. The qualitative patterns of compactness measures and their extrema are the same, but with the increased number of models, the scatter seen in previous studies is replaced by several localized branches. More physics-based analyses by Ertl et al. and Müller et al. show these branches with less scatter than the single-parameter characterization of O'Connor & Ott. These branches are particularly apparent for stars in the mass ranges 14-19 and 22-24 M . The multivalued solutions are a consequence of interference between several carbon- and oxygen-burning shells during the late stages of evolution. For a relevant range of masses, whether a star explodes or not may reflect the small, almost random differences in its late evolution more than its initial mass. The large number of models allows statistically meaningful statements about the radius, luminosity, and effective temperatures of presupernova stars, their core structures, and their remnant mass distributions.
A survey of Type II supernovae explosion models has been carried out to determine how their light curves and spectra vary with their mass, metallicity, and explosion energy. The presupernova models ...are taken from a recent survey of massive stellar evolution at solar metallicity supplemented by new calculations at subsolar metallicity. Explosions are simulated by the motion of a piston near the edge of the iron core and the resulting light curves and spectra are calculated using full multi-wavelength radiation transport. Formulae are developed that describe approximately how the model observables (light curve luminosity and duration) scale with the progenitor mass, explosion energy, and radioactive nucleosynthesis. Comparison with observational data shows that the explosion energy of typical supernovae (as measured by kinetic energy at infinity) varies by nearly an order of magnitude-from 0.5 to 4.0 X 1051 ergs, with a typical value of ~0.9 X 1051 ergs. Despite the large variation, the models exhibit a tight relationship between luminosity and expansion velocity, similar to that previously employed empirically to make SNe IIP standardized candles. This relation is explained by the simple behavior of hydrogen recombination in the supernova envelope, but we find a sensitivity to progenitor metallicity and mass that could lead to systematic errors. Additional correlations between light curve luminosity, duration, and color might enable the use of SNe IIP to obtain distances accurate to ~20% using only photometric data.
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