Models of pair-instability supernovae (PISNe) predict a gap in black hole (BH) masses between ∼45
M
⊙
and 120
M
⊙
, which is referred to as the upper BH mass-gap. With the advent of ...gravitational-wave astrophysics, it has become possible to test this prediction, and there is an important associated effort to understand which theoretical uncertainties modify the boundaries of this gap. In this work we study the impact of rotation on the hydrodynamics of PISNe, which leave no compact remnant, as well as the evolution of pulsational-PISNe (PPISNe), which undergo thermonuclear eruptions before forming a compact object. We perform simulations of nonrotating and rapidly rotating stripped helium stars in a metal-poor environment (
Z
⊙
/50) in order to resolve the lower edge of the upper mass-gap. We find that the outcome of our simulations is dependent on the efficiency of angular momentum transport: models that include efficient coupling through the Spruit-Tayler dynamo shift the lower edge of the mass-gap upward by ∼4%, while simulations that do not include this effect shift it upward by ∼15%. From this, we expect that the lower edge of the upper mass-gap is dependent on BH spin, which can be tested as the number of observed BH mergers increases. Moreover, we show that stars undergoing PPISNe have extended envelopes (
R
∼ 10 − 1000
R
⊙
) at iron-core collapse, making them promising progenitors for ultra-long gamma-ray bursts.
With recent advances in gravitational-wave astronomy, the direct detection of gravitational waves from the merger of two stellar-mass compact objects has become a realistic prospect. Evolutionary ...scenarios towards mergers of various double compact objects generally invoke so-called common-envelope evolution, which is poorly understood and leads to large uncertainties in the predicted merger rates. Here we explore, as an alternative, the scenario of massive overcontact binary (MOB) evolution, which involves two very massive stars in a very tight binary that remain fully mixed as a result of their tidally induced high spin. While many of these systems merge early on, we find many MOBs that swap mass several times, but survive as a close binary until the stars collapse. The simplicity of the MOB scenario allows us to use the efficient public stellar-evolution code MESA to explore it systematically by means of detailed numerical calculations. We find that, at low metallicity, MOBs produce double-black-hole (BH+BH) systems that will merge within a Hubble time with mass-ratios close to one, in two mass ranges, about 25...60 M⊙ and ≳130M⊙, with pair-instability supernovae (PISNe) being produced at intermediate masses. Our models are also able to reproduce counterparts of various stages in the MOB scenario in the local Universe, providing direct support for the scenario. We map the initial binary parameter space that produces BH+BH mergers, determine the expected chirp mass distribution, merger times, and expected Kerr parameters, and predict event rates. We find typically one BH+BH merger event for ~1000 core-collapse supernovae for Z ≲ Z⊙/ 10 . The advanced LIGO (aLIGO) detection rate is more uncertain and depends on the cosmic metallicity evolution. From deriving upper and lower limits from a local and a global approximation for the metallicity distribution of massive stars, we estimate aLIGO detection rates (at the aLIGO design limit) of ~19−550 yr-1 for BH-BH mergers below the PISN gap and of ~2.1−370 yr-1 above the PISN gap. Even with conservative assumptions, we find that aLIGO will probably soon detect BH+BH mergers from the MOB scenario. These could be the dominant source for aLIGO detections.
ABSTRACT We investigate observational properties of accretion-induced collapse (AIC) of white dwarfs (WDs) in radio frequencies. If AIC is triggered by accretion from a companion star, a dense ...circumstellar medium can be formed around the progenitor system. Then, the ejecta from AIC collide with the dense circumstellar medium, creating a strong shock. The strong shock can produce synchrotron emission that can be observed in radio frequencies. Even if AIC occurs as a result of WD mergers, we argue that AIC may cause fast radio bursts (FRBs) if a certain condition is satisfied. If AIC forms neutron stars (NSs) that are so massive that rotation is required to support themselves (i.e., supramassive NSs), the supramassive NSs may immediately lose their rotational energy by the r-mode instability and collapse to black holes. If the collapsing supramassive NSs are strongly magnetized, they may emit FRBs, as previously proposed. The AIC radio transients from single-degenerate systems may be detected in future radio transient surveys like the Very Large Array Sky Survey or the Square Kilometer Array transient survey. Because AIC has been proposed as a source of gravitational waves (GWs), GWs from AIC may be accompanied by radio-bright transients that can be used to confirm the AIC origin of observed GWs.
ABSTRACT
Supernova (SN) properties in radio strongly depend on their circumstellar environment and they are an important probe to investigate the mass-loss of SN progenitors. Recently, core-collapse ...SN observations in radio have been assembled and the rise time and peak luminosity distribution of core-collapse SNe at 8.4 GHz has been estimated. In this paper, we constrain the mass-loss prescriptions for red supergiants (RSGs) by using the rise time and peak luminosity distribution of Type II SNe in radio. We take the de Jager and van Loon mass-loss rates for RSGs, calculate the rise time and peak luminosity distribution based on them, and compare the results with the observed distribution. We found that the de Jager mass-loss rate explains the widely spread radio rise time and peak luminosity distribution of Type II SNe well, while the van Loon mass-loss rate predicts a relatively narrow range for the rise time and peak luminosity. We conclude that the mass-loss prescriptions of RSGs should have strong dependence on the luminosity as in the de Jager mass-loss rate to reproduce the widely spread distribution of the rise time and peak luminosity in radio observed in Type II SNe.
Explosions of Thorne–Żytkow objects Moriya, Takashi J
Monthly Notices of the Royal Astronomical Society Letters,
03/2018, Letnik:
475, Številka:
1
Journal Article
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Odprti dostop
Abstract
We propose that massive Thorne–Żytkow objects can explode. A Thorne–Żytkow object is a theoretically predicted star that has a neutron core. When nuclear reactions supporting a massive ...Thorne–Żytkow object terminate, a strong accretion occurs towards the central neutron core. The accretion rate is large enough to sustain a super-Eddington accretion towards the neutron core. The neutron core may collapse to a black hole after a while. A strong large-scale outflow or a jet can be launched from the super-Eddington accretion disc and the collapsing Thorne–Żytkow object can be turned into an explosion. The ejecta have about 10 M⊙ but the explosion energy depends on when the accretion is suppressed. We presume that the explosion energy could be as low as ∼1047 erg and such a low-energy explosion could be observed like a failed supernova. The maximum possible explosion energy is ∼1052 erg and such a high-energy explosion could be observed as an energetic Type II supernova or a superluminous supernova. Explosions of Thorne–Żytkow objects may provide a new path to spread lithium and other heavy elements produced through the irp process such as molybdenum in the Universe.
ABSTRACT
Some interaction-powered supernovae have long rise times of more than 100 d. We show that such long rise times are naturally expected if cirumstellar matters (CSM) have a flat density ...structure (s ≲ 1.5, where ρCSM∝r−s). In such cases, bolometric luminosities from the CSM interaction keep increasing as long as the CSM interacts with the outer layers of the SN ejecta. Thus, the rise time is determined by the dynamical time-scale in which the reverse shock propagates the outer layers of the SN ejecta, not by the time-scales in which photons diffuse in the CSM as often considered. Interaction-powered supernovae with very long rise times can be an important probe of extensive non-steady mass-loss in massive stars.
ABSTRACT
I investigate the possibility that a recently reported radio transient in M81, VTC J095517.5 + 690813, was caused by the accretion-induced collapse of a white dwarf. It became bright in the ...radio, but no associated optical transient was discovered. An accretion-induced collapse is predicted to be radio-bright but optically faint, satisfying the observed properties. I compare the predicted radio emission from an accretion-induced collapse with that of VTC J095517.5 + 690813 and show that this object can be reasonably explained by an accretion-induced collapse. Although it is difficult to firmly conclude that VTC J095517.5 + 690813 is an accretion-induced collapse, my results show that radio-bright transients without an optical counterpart could be related to stellar deaths.
We perform a series of two-dimensional radiation-hydrodynamic simulations of the collision between supernova ejecta and circumstellar media (CSMs). The hydrodynamic interaction of a fast flow and the ...surrounding media efficiently dissipates the kinetic energy of the fast flow and is considered as a dominant energy source for a specific class of core-collapse supernovae. Despite some observational evidence for aspherical ejecta and/or CSM structure, multidimensional effects in the ejecta-CSM interaction are relatively unexplored. Our numerical simulations equipped with an adaptive mesh refinement technique successfully reproduce hydrodynamic instabilities developing around the ejecta-CSM interface. We also investigate effects of disklike CSMs on the dynamical evolution of supernova ejecta and bolometric light curves. We find that emission powered by ejecta-disk interaction exhibits significant viewing angle dependence. For a line of sight close to the symmetry axis, the observer directly sees the supernova ejecta, leading to a short brightening timescale. For an observer seeing the emission through the CSM disk, thermal photons diffuse throughout the CSM, and thus the light curve is severely smeared out.
The energy liberated by fallback accretion has been suggested as a possible engine to power hydrogen-poor superluminous supernovae (SLSNe). We systematically investigate this model using the Bayesian ...light curve (LC) fitting code MOSFiT (Modular Open Source Fitter for Transients), fitting the LCs of 37 hydrogen-poor SLSNe assuming a fallback accretion central engine. We find that this model can yield good fits to their LCs, with a fit quality that rivals the popular magnetar engine models. Examining our derived parameters for the fallback model, we estimate the total energy requirements from the accretion disk to be 0.002-0.7 c2. If we adopt a typical conversion efficiency ∼10−3, the required mass to accrete is thus 2-700 . Many SLSNe, therefore, require an unrealistic accretion mass, and so only a fraction of these events could be powered by fallback accretion unless the true efficiency is much greater than our fiducial value. The SLSNe that require the smallest amounts of fallback mass are still fallback accretion-powered supernova candidates, but they are difficult to distinguish solely by their LC properties.
The detection of gravitational waves from the binary black hole (BH) merger GW150914 may enlighten our understanding of ultra-luminous X-ray sources (ULXs), as BHs of masses >30 M⊙ can reach ...luminosities >4 × 1039 erg s-1 without exceeding their Eddington luminosities. It is then important to study variations of evolutionary channels for merging BHs, which might instead form accreting BHs and become ULXs. It was recently shown that very massive binaries with mass ratios close to unity and tight orbits can undergo efficient rotational mixing and evolve chemically homogeneously, resulting in a compact BH binary. We study similar systems by computing ~120 000 detailed binary models with the MESA code covering a wide range of masses, orbital periods, mass ratios, and metallicities. For initial mass ratios q ≡ M2/M1 ≃ 0.1−0.4, primaries with masses above 40 M⊙ can evolve chemically homogeneously, remaining compact and forming a BH without experiencing Roche-lobe overflow. The secondary then expands and transfers mass to the BH, initiating a ULX phase. At a given metallicity this channel is expected to produce the most massive accreting stellar BHs and the brightest ULXs. We predict that ~1 out of 104 massive stars evolves this way, and that in the local universe 0.13 ULXs per M⊙ yr-1 of star formation rate are observable, with a strong preference for low metallicities. An additional channel is still required to explain the less luminous ULXs and the full population of high-mass X-ray binaries. At metallicities log Z> −3, BH masses in ULXs are limited to 60 M⊙, due to the occurrence of pair-instability supernovae which leave no remnant, resulting in an X-ray luminosity cut-off for accreting BHs. At lower metallicities, very massive stars can avoid exploding as pair-instability supernovae and instead form BHs with masses above 130 M⊙, producing a gap in the ULX luminosity distribution. After the ULX phase, neutron star BH binaries that merge in less than a Hubble time are produced with a low formation rate <0.2 Gpc-3 yr-1. We expect that upcoming X-ray observatories will test these predictions, which together with additional gravitational wave detections will provide strict constraints on the origin of the most massive BHs that can be produced by stars.
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