We propose a new simple formalism to predict the orbital separations after common-envelope phases with massive star donors. We focus on the fact that massive red supergiants tend to have a sizeable ...radiative layer between the dense helium core and the convective envelope. Our formalism treats the common-envelope phase in two stages: dynamical in-spiral through the outer convective envelope and thermal timescale mass transfer from the radiative intershell. With fiducial choices of parameters, the new formalism typically predicts much wider separations compared to the classical energy formalism. Moreover, our formalism predicts that final separations strongly depend on the donor evolutionary stage and companion mass. Our formalism provides a physically-motivated alternative option for population synthesis studies to treat common-envelope evolution. This treatment will impact on predictions for massive-star binaries, including gravitational-wave sources, X-ray binaries and stripped-envelope supernovae.
We perform 3D hydrodynamical simulations of new-born neutron stars (NSs) colliding with main-sequence binary companions after supernova explosions. Based on those hydrodynamical models, we construct ...a semi-analytical formula that describes the drag force inside stars with steep density gradients. We then compute the outcome of NS--companion collisions over a wide range of parameters using the semi-analytical formula. Depending on the direction and magnitude of the natal kick, we find that the collision may lead to various outcomes. For relatively fast kicks and high impact parameters, the NS may penetrate the companion star envelope without merging. By allowing the NS to plunge through companions, the companion can be accelerated to have runaway velocities up to \(\sim10\) per cent above the theoretical upper limit considered in classical binary disruption scenarios. The NS can capture and carry away up to a few per cent of the companion envelope as it escapes, which may form pulsar planets or cause outflows through accretion to heat the ejecta from inside and power the supernova light curve. For lower impact parameters, the neutron star will directly merge with the companion and form a Thorne-Żytkow object. In intermediate cases, the NS penetrates the companion envelope several times before merging, possibly causing multiple bumps in the supernova light curve like in SN2015bn and SN2019stc.
We explore the effect of anisotropic wind driving on the properties of accretion onto black holes in close binaries. We specifically focus on line-driven winds, which are common in high-mass X-ray ...binaries. In close binary systems, the tidal force from the companion star can modify the wind structure in two different ways. One is the reduction of wind terminal velocity due to the weaker effective surface gravity. The other is the reduction in mass flux due to gravity darkening. We incorporate these effects into the so-called CAK theory in a simple way and investigate the wind flow around the accretor on the orbital scale. We find that a focused accretion stream is naturally formed when the Roche lobe filling factor is \(\gtrsim0.8\)-0.9, analogous to that of wind Roche lobe overflow, but only when the velocity reduction is taken into account. The formation of a stream is necessary to bring in sufficient angular momentum to form an accretion disc around the black hole. Gravity darkening effects reduce the amount of accreted angular momentum, but not enough to prevent the formation of a disc. Based on these results, we expect there to be a discrete step in the observability of high-mass X-ray binaries depending on whether the donor Roche lobe filling factor is below or above \(\sim\)0.8-0.9.
We explore the evolution of massive stars (>8 solar masses) with 1-D models and present analytical fits to the masses and binding energies of the convective portions of their envelopes. These fits ...are given as functions of total mass, metallicity, and surface temperature (used as a proxy for evolutionary phase). They enable the application of the two-stage common envelope formalism (Hirai & Mandel 2022) in rapid binary population synthesis frameworks. We estimate that the degree of orbital hardening following common-envelope ejection spans 6 orders of magnitude and is a very strong function of the accretor mass, and, to a lesser extent, donor evolutionary phase.
This letter complements a formation scenario of the progenitor of the supernova iPTF13bvn proposed in Hirai (2017). Although the scenario was successful in reproducing various observational features ...of the explosion and pre-explosion photometry by assuming that the progenitor had a relatively large black hole companion, it lacked an explanation for the origin of the black hole itself. We now explore the possible evolutionary paths towards this binary with a relatively large black hole companion. We found that the black hole was probably produced by a very massive star that experienced common envelope evolution. According to our MESA stellar models, the primary mass should have been 70M to reproduce the required remnant mass and final separation. This indicates that iPTF13bvn was likely a rare case and normal type Ib supernovae originate from different paths.
The formation scenario of the progenitor of iPTF13bvn has been revisited. iPTF13bvn is unique in the sense that a corresponding pre-supernova image has been identified. This has enabled us to ...strongly constrain the nature of its progenitor. From the pre-supernova image, light curve and the latest observations, it is currently widely accepted that the progenitor of iPTF13bvn was in a binary system. The fast decline in the light curve suggests a progenitor mass of \(\sim3.5M_\odot\), and the upper limit on the remaining companion is \(\sim20M_\odot\). Recent works suggest that binary evolution models involving common envelope episodes can satisfy all the observational constraints. We have examined the common envelope scenario based on latest knowledges on common envelope evolution. We have found that the common envelope scenario for the progenitor of iPTF13bvn seems not to be suitable since it can not explain the pre-supernova radius. We also propose an alternative model with a black hole companion. Stellar evolution calculations with a large black hole companion were carried out and succeeded in satisfying all observational constraints. However, more studies should be carried out to explain the origin of the large black hole companion.
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