Authors: Emmanouil Zapartas, M. Renzo, Tassos Fragos, Aaron Dotter, Jeff Andrews, Simone S. Bavera, Scotty Coughlin, Devina Misra, Konstantinos Kovlakas, Jaime Román-Garza, Juan Gabriel Serra-Pérez, Ying Qin, Kyle A. Rocha, Nam Hai Tran
Abstract: Stripped-envelope supernovae (Type IIb, Ib, Ic) showing little or no hydrogen are one of the main classes of explosions of massive stars. Their origin and the evolution of their progenitors are not fully understood as yet. Very massive single stars stripped by their own winds (≳25 - 30M⊙ at solar metallicity) are considered viable progenitors of these events. However, recent 1D core-collapse simulations show that some massive stars may collapse directly onto black holes after a failed explosion, with weak or no visible transient. In this letter, we estimate the effect of direct collapse onto a black hole on the rates of stripped-envelope supernovae that arise from single stars. For this, we compute single star MESA models at solar metallicity and map their final state to their core-collapse outcome following prescriptions commonly used in population synthesis. According to our models, no single stars that have lost their entire hydrogen-rich envelope are able to explode, and only a fraction of progenitors with a thin hydrogen envelope left (IIb progenitor candidates) do, unless we invoke increased wind mass-loss rates. This result increases the existing tension between the single-star scenario for stripped-envelope supernovae and their observed rates and properties. At face value, our results point towards an even higher contribution of binary progenitors for stripped-envelope supernovae. Alternatively, they may suggest inconsistencies in the common practice of mapping different stellar models to core-collapse outcomes and/or higher overall mass loss in massive stars.
Authors: Jaime Román-Garza, Simone S. Bavera, Tassos Fragos, Emmanouil Zapartas, Devina Misra, Jeff Andrews, Scotty Coughlin, Aaron Dotter, Konstantinos Kovlakas, Juan Gabriel Serra-Pérez, Ying Qin, Kyle A. Rocha, Nam Hai Tran
Abstract: Recent detailed 1D core-collapse simulations have brought new insights on the final fate of massive stars, which are in contrast to commonly used parametric prescriptions. In this work, we explore the implications of these results to the formation of coalescing black-hole (BH) - neutron-star (NS) binaries, such as the candidate event GW190426_152155 reported in GWTC-2. Furthermore, we investigate the effects of natal kicks and the NS's radius on the synthesis of such systems and potential electromagnetic counterparts linked to them. Synthetic models based on detailed core-collapse simulations result in an increased merger detection rate of BH-NS systems (~2.3 yr-1), 5 to 10 times larger than the predictions of "standard" parametric prescriptions. This is primarily due to the formation of low-mass BH via direct collapse, and hence no natal kicks, favored by the detailed simulations. The fraction of observed systems that will produce an electromagnetic counterpart, with the detailed supernova engine, ranges from 2-25%, depending on uncertainties in the NS equation of state. Notably, in most merging systems with electromagnetic counterparts, the NS is the first-born compact object, as long as the NS's radius is ≲12km. Furthermore, core-collapse models that predict the formation of low-mass BHs with negligible natal kicks increase the detection rate of GW190426_152155-like events to ~0.6 yr-1; with an associated probability of electromagnetic counterpart ≤ 10% for all supernova engines. However, increasing the production of direct-collapse low-mass BHs also increases the synthesis of binary BHs, over-predicting their measured local merger density rate. In all cases, models based on detailed core-collapse simulation predict a ratio of BH-NSs to binary BHs merger rate density that is at least twice as high as other prescriptions.
Authors: Simone S. Bavera, Tassos Fragos, Michael Zevin, Christopher P. L. Berry, Pablo Marchant, Jeff J. Andrews, Scott Coughlin, Aaron Dotter, Konstantinos Kovlakas, Devina Misra, Juan G. Serra-Pérez, Ying Qin, Kyle A. Rocha, Jaime Román-Garza, Nam H. Tran, Emmanouil Zapartas
Abstract: We study the impact of mass-transfer physics on the observable properties of binary black hole populations formed through isolated binary evolution. We investigate the impact of mass-accretion efficiency onto compact objects and common-envelope efficiency on the observed distributions of χeff, Μchirp and q. We find that low common envelope efficiency translates to tighter orbits post common envelope and therefore more tidally spun up second-born black holes. However, these systems have short merger timescales and are only marginally detectable by current gravitational-waves detectors as they form and merge at high redshifts (z ~ 2), outside current detector horizons. Assuming Eddington-limited accretion efficiency and that the first-born black hole is formed with a negligible spin, we find that all non-zero χeff systems in the detectable population can come only from the common envelope channel as the stable mass-transfer channel cannot shrink the orbits enough for efficient tidal spin-up to take place. We find the local rate density (z ≃ 0.01) for the common envelope channel is in the range ~ 17 − 113 Gpc−3 yr−1 considering a range of αCE ∈ [0.2,5.0] while for the stable mass transfer channel the rate density is ~ 25 Gpc−3 yr>−1. The latter drops by two orders of magnitude if the mass accretion onto the black hole is not Eddington limited because conservative mass transfer does not shrink the orbit as efficiently as non-conservative mass transfer does. Finally, using GWTC-2 events, we constrain the lower bound of branching fraction from other formation channels in the detected population to be ~ 0.2. Assuming all remaining events to be formed through either stable mass transfer or common envelope channels, we find moderate to strong evidence in favour of models with inefficient common envelopes.