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Register a new userExploring the Lower Mass Gap and Unequal Mass Regime in Compact Binary Evolution
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On August 14, 2019, the LIGO and Virgo detectors observed GW190814, a gravitational-wave signal originating from the merger of a $simeq 23 M_odot$ black hole with a $simeq 2.6 M_odot$ compact object. GW190814s compact-binary source is atypical both in its highly asymmetric masses and in its lower-mass component lying between the heaviest known neutron star and lightest known black hole in a compact-object binary. If formed through isolated binary evolution, the mass of the secondary is indicative of its mass at birth. We examine the formation of such systems through isolated binary evolution across a suite of assumptions encapsulating many physical uncertainties in massive-star binary evolution. We update how mass loss is implemented for the neutronization process during the collapse of the proto-compact object to eliminate artificial gaps in the mass spectrum at the transition between neutron stars and black holes. We find it challenging for population modeling to match the empirical rate of GW190814-like systems whilst simultaneously being consistent with the rates of other compact binary populations inferred by gravitational-wave observations. Nonetheless, the formation of GW190814-like systems at any measurable rate requires a supernova engine model that acts on longer timescales such that the proto-compact object can undergo substantial accretion immediately prior to explosion, hinting that if GW190814 is the result of massive-star binary evolution, the mass gap between neutron stars and black holes may be narrower or nonexistent.
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The LIGO and Virgo detectors have recently directly observed gravitational waves from several mergers of pairs of stellar-mass black holes, as well as from one merging pair of neutron stars. These observations raise the hope that compact object mergers could be used as a probe of stellar and binary evolution, and perhaps of stellar dynamics. This colloquium-style article summarizes the existing observations, describes theoretical predictions for formation channels of merging stellar-mass black-hole binaries along with their rates and observable properties, and presents some of the prospects for gravitational-wave astronomy.
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.
In Einsteins general relativity, gravity is mediated by a massless spin-2 metric field, and its extension to include a mass for the graviton has profound implication for gravitation and cosmology. In 2002, Finn and Sutton used the gravitational-wave (GW) back-reaction in binary pulsars, and provided the first bound on the mass of graviton. Here we provide an improved analysis using 9 well-timed binary pulsars with a phenomenological treatment. First, individual mass bounds from each pulsar are obtained in the frequentist approach with the help of an ordering principle. The best upper limit on the graviton mass, $m_{g}<3.5times10^{-20} , {rm eV}/c^{2}$ (90% C.L.), comes from the Hulse-Taylor pulsar PSR B1913+16. Then, we combine individual pulsars using the Bayesian theorem, and get $m_{g}<5.2times10^{-21} , {rm eV}/c^{2}$ (90% C.L.) with a uniform prior for $ln m_g$. This limit improves the Finn-Sutton limit by a factor of more than 10. Though it is not as tight as those from GWs and the Solar System, it provides an independent and complementary bound from a dynamic regime.
The threshold mass for prompt collapse in binary neutron star mergers was empirically found to depend on the stellar properties of the maximum-mass non-rotating neutron star model. Here we present a semi-analytic derivation of this empirical relation which suggests that it is rather insensitive to thermal effects, to deviations from axisymmetry and to the exact rotation law in merger remnants. We utilize axisymmetric, cold equilibrium models with differential rotation and determine the threshold mass for collapse from the comparison between an empirical relation that describes the angular momentum in the remnant for a given total binary mass and the sequence of rotating equilibrium models at the threshold to collapse (the latter assumed to be near the turning point of fixed-angular-momentum sequences). In spite of the various simplifying assumptions, the empirical relation for prompt collapse is reproduced with good accuracy, which demonstrates its robustness. We discuss implications of our methodology and results for understanding other empirical relations satisfied by neutron-star merger remnants that have been discovered by numerical simulations and that play a key role in constraining the high-density equation of state through gravitational-wave observations.
We aim to study the progenitor properties and expected rates of the two lowest-mass binary black hole (BH) mergers, GW 151226 and GW 170608, detected within the first two Advanced LIGO-Virgo runs, in the context of the isolated binary-evolution scenario. We use the public MESA code, which we adapted to include BH formation and unstable mass transfer developed during a common-envelope (CE) phase. Using more than 60000 binary simulations, we explore a wide parameter space for initial stellar masses, separations, metallicities, and mass-transfer efficiencies. We obtain the expected distributions for the chirp mass, mass ratio, and merger time delay by accounting for the initial stellar binary distributions. Our simulations show that, while the progenitors we obtain are compatible over the entire range of explored metallicities, they show a strong dependence on the initial masses of the stars, according to stellar winds. All the progenitors follow a similar evolutionary path, starting from binaries with initial separations in the $30-200~R_odot$ range, experiencing a stable mass transfer interaction before the formation of the first BH, and a second unstable mass-transfer episode leading to a CE ejection that occurs either when the secondary star crosses the Hertzsprung gap or when it is burning He in its core. The CE phase plays a fundamental role in the considered low-mass range: only progenitors experiencing such an unstable mass-transfer phase are able to merge in less than a Hubble time. We find integrated merger-rate densities in the range $0.2-5.0~{rm yr}^{-1}~{rm Gpc}^{-3}$ in the local Universe for the highest mass-transfer efficiencies explored. The highest rate densities lead to detection rates of $1.2-3.3~{rm yr}^{-1}$, being compatible with the observed rates. A high CE-efficiency scenario with $alpha_{rm CE}=2.0$ is favored when comparing with observations. ABRIDGED.
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