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Register a new userOn the population III binary black hole mergers beyond the pair-instability mass gap
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We perform a binary population synthesis calculation incorporating very massive population (Pop.) III stars up to 1500 $M_odot$, and investigate the nature of binary black hole (BBH) mergers. Above the pair-instability mass gap, we find that the typical primary black hole (BH) mass is 135-340 $M_odot$. The maximum primary BH mass is as massive as 686 $M_odot$. The BBHs with both of their components above the mass gap have low effective inspiral spin $sim$ 0. So far, no conclusive BBH merger beyond the mass gap has been detected, and the upper limit on the merger rate density is obtained. If the initial mass function (IMF) of Pop. III stars is simply expressed as $xi_m(m) propto m^{-alpha}$ (single power law), we find that $alpha gtrsim 2.8$ is needed in order for the merger rate density not to exceed the upper limit. In the future, the gravitational wave detectors such as Einstein telescope and Pre-DECIGO will observe BBH mergers at high redshift. We suggest that we may be able to impose a stringent limit on the Pop. III IMF by comparing the merger rate density obtained from future observations with that derived theoretically.
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During the first three observing runs of the Advanced gravitational-wave detector network, the LIGO/Virgo collaboration detected several black hole binary (BHBH) mergers. As the population of detected BHBH mergers grows, it will become possible to constrain different channels for their formation. Here we consider the chemically homogeneous evolution (CHE) channel in close binaries, by performing population synthesis simulations that combine realistic binary models with detailed cosmological calculations of the chemical and star-formation history of the Universe. This allows us to constrain population properties, as well as cosmological and aLIGO/aVirgo detection rates of BHBH mergers formed through this pathway. We predict a BHBH merger rate at redshift zero of $5.8 , textrm{Gpc}^{-3} textrm{yr}^{-1}$ through the CHE channel, to be compared with aLIGO/aVirgos measured rate of ${53.2}_{-28.2}^{+55.8} , text{Gpc}^{-3} text{yr}^{-1}$, and find that eventual merger systems have BH masses in the range $17 - 43 , textrm{M}_{odot}$ below the pair-instability supernova (PISN) gap, and $>124 , textrm{M}_{odot}$ above the PISN gap. We investigate effects of momentum kicks during black hole formation, and calculate cosmological and magnitude limited PISN rates. We also study the effects of high-redshift deviations in the star formation rate. We find that momentum kicks tend to increase delay times of BHBH systems, and our magnitude limited PISN rate estimates indicate that current deep surveys should be able to detect such events. Lastly, we find that our cosmological merger rate estimates change by at most $sim 8%$ for mild deviations of the star formation rate in the early Universe, and by up to $sim 40%$ for extreme deviations.
Using the Binary Population and Spectral Synthesis code BPASS, we have calculated the rates, timescales and mass distributions for binary black hole mergers as a function of metallicity. We consider these in the context of the recently reported 1st LIGO event detection. We find that the event has a very low probability of arising from a stellar population with initial metallicity mass fraction above Z=0.010 (Z>0.5Zsun). Binary black hole merger events with the reported masses are most likely in populations below 0.008 (Z<0.4Zsun). Events of this kind can occur at all stellar population ages from ~3 Myr up to the age of the universe, but constitute only 0.1 to 0.4 per cent of binary BH mergers between metallicities of Z=0.001 to 0.008. However at metallicity Z=0.0001, 26 per cent of binary BH mergers would be expected to have the reported masses. At this metallicity the progenitor merger times can be close to ~10Gyr and rotationally-mixed stars evolving through quasi-homogeneous evolution, due to mass transfer in a binary, dominate the rate. The masses inferred for the black holes in the binary progenitor of GW,150914 are amongst the most massive expected at anything but the lowest metallicities in our models. We discuss the implications of our analysis for the electromagnetic follow-up of future LIGO event detections.
Models of pair-instability supernovae (PISNe) predict a gap in black hole (BH) masses between $sim 45M_odot-120M_odot$, 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 what 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 non-rotating and rapidly-rotating stripped helium stars in a metal poor environment $(Z_odot/50)$ 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, with models that include efficient coupling through the Spruit-Tayler dynamo shifting the lower edge of the mass-gap upwards by $sim 4%$, while simulations that do not include this effect shift it upwards by $sim 15%$. From this, we expect the lower edge of the upper mass-gap to be 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 ($Rsim 10-1000~R_odot$) at iron-core collapse, making them promising progenitors for ultra-long gamma-ray bursts.
We performed population synthesis simulations of Population III binary stars with Maxwellian kick velocity distribution when MGCOs (Mass Gap Compact Objects with mass 2--5$,M_{odot}$) are formed. We found that for eight kick velocity dispersion models of $sigma_{rm k}=0$--$500$ km/s, the mean mass of black hole (BH)-MGCO binary is $sim (30 ,M_odot,,2.6 ,M_odot)$. In numerical data of our simulations, we found the existence of BH-MGCO binary with mass $(22.9 ,M_odot,,2.5 ,M_odot)$ which looks like GW190814.
Numerical studies of primordial star formation suggest that the first stars in the universe may have been very massive. Stellar models indicate that non-rotating Population III stars with initial masses of 140-260 Msun die as highly energetic pair-instability supernovae. We present new two-dimensional simulations of primordial pair-instability supernovae done with the CASTRO code. Our simulations begin at earlier times than previous multidimensional models, at the onset of core collapse, to capture any dynamical instabilities that may be seeded by collapse and explosive burning. Such instabilities could enhance explosive yields by mixing hot ash with fuel, thereby accelerating nuclear burning, and affect the spectra of the supernova by dredging up heavy elements from greater depths in the star at early times. Our grid of models includes both blue supergiants and red supergiants over the range in progenitor mass expected for these events. We find that fluid instabilities driven by oxygen and helium burning arise at the upper and lower boundaries of the oxygen shell $sim$ 20 - 100 seconds after core bounce. Instabilities driven by burning freeze out after the SN shock exits the helium core. As the shock later propagates through the hydrogen envelope, a strong reverse shock forms that drives the growth of Rayleigh--Taylor instabilities. In red supergiant progenitors, the amplitudes of these instabilities are sufficient to mix the supernova ejecta.
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