We propose a common-envelope jets supernova (CEJSN) scenario for the fast-rising blue optical transient AT2018cow. In a CEJSN a neutron star (NS) spirals-in inside the extended envelope of a massive giant star and enters the core. The NS accretes mass from the core through an accretion disc and launches jets. These jets explode the core and the envelope. In the specific polar CEJSN scenario that we propose here the jets clear the polar regions of the giant star before the NS enters the core. The jets that the NS launches after it enters the core expand almost freely along the polar directions that contain a small amount of mass. This, we suggest, explains the fast rise to maximum and the fast ejecta observed at early times of the enigmatic transient AT2018cow. The slower later time ejecta is the more massive equatorial outflow. We roughly estimate the accretion phase onto the NS during the explosion phase to last for a time of ≈ 10 3 s, during which the average mass accretion rate is ≈ 10 −4 M s −1 . We outline the possible diversity of CEJSNe by listing five other scenarios in addition to the polar CEJSN scenario.

Context. Classical Wolf-Rayet (WR) stars are massive, hydrogen depleted, post main-sequence stars that exhibit emission-line dominated spectra. For a given metallicity Z, stars exceeding a certain initial mass M WR single (Z) can reach the WR phase through intrinsic mass-loss or eruptions (single-star channel). In principle, stars of lower masses can reach the WR phase via stripping through binary interactions (binary channel). Because winds become weaker at low Z, it is commonly assumed that the binary channel dominates the formation of WR stars in environments with low metallicity such as the Small and Large Magellanic Clouds (SMC, LMC). However, the reported WR binary fractions of 30 − 40% in the SMC (Z = 0.002) and LMC (Z = 0.006) are comparable to that of the Galaxy (Z = 0.014), and no evidence for the dominance of the binary channel at low Z could be identified observationally. Here, we explain this apparent contradiction by considering the minimum initial mass M WR spec (Z) needed for the stripped product to appear as a WR star. Aims. By constraining M WR spec (Z) and M WR single (Z), we estimate the importance of binaries in forming WR stars as a function of Z. Methods. We calibrate M WR spec (Z) using the lowest-luminosity WR stars in the Magellanic Clouds and the Galaxy. A range of M WR single values are explored using various evolution codes. We estimate the additional contribution of the binary channel by considering the interval [M WR spec (Z), M WR single (Z)], which characterizes the initial-mass range in which the binary channel can form additional WR stars. Results. The WR-phenomenon ceases below luminosities of log L≈4. 9, 5.25, and 5.6 [L ] in the Galaxy, the LMC, and the SMC, respectively, which translates to minimum He-star masses of 7.5, 11, 17 M and minimum initial masses of M WR spec = 18, 23, 37 M . Stripped stars with lower initial masses in the respective galaxies would tend to not appear as WR stars. The minimum mass necessary for self-stripping, M WR single (Z), is strongly model dependent, but lies in the range 20 − 30, 30 − 60, and 40 M for the Galaxy, LMC, and SMC, respectively. We find that that the additional contribution of the binary channel is a non-trivial and model-dependent function of Z that cannot be conclusively claimed to be monotonically increasing with decreasing Z. Conclusions. The WR spectral appearance arises from the presence of strong winds. Therefore, both M WR spec and M WR single increase with decreasing metallicity. Considering this, we show that one should not a-priori expect that binary interactions become increasingly important in forming WR stars at low Z, or that the WR binary fraction grows with decreasing Z.

We find that convective regions of collapsing massive stellar cores possess sufficient stochastic angular momentum to form intermittent accretion disks around the newly born neutron star (NS) or black hole (BH), as required by the jittering-jets model for core-collapse supernova (CCSN) explosions. To reach this conclusion we derive an approximate expression for stochastic specific angular momentum in convection layers of stars, and using the mixing-length theory apply it to four stellar models at core-collapse epoch. In all models, evolved using the stellar evolution code MESA, the convective helium layer has sufficient angular momentum to form an accretion disk. The mass available for disk formation around the NS or BH is 0.1 − 1.2M ⊙ ; stochastic accretion of this mass can form intermittent accretion disks that launch jets powerful enough to explode the star according to the jittering-jets model. Our results imply that even if no explosion occurs after accretion of the inner ∼ 2 − 5M ⊙ of the core onto the NS or BH (the mass depends on the stellar model), accretion of outer layers of the core will eventually lead to an energetic supernova explosion.

We propose a common envelope jets supernova scenario for the enigmatic supernova iPTF14hls where a neutron star that spirals-in inside the envelope of a massive giant star accretes mass and launches jets that power the ejection of the circumstellar shell and a few weeks later the explosion itself. To account for the kinetic energy of the circumstellar gas and the explosion, the neutron star should accrete a mass of ≈ 0.3M ⊙ . The tens×M ⊙ of circumstellar gas that accounts for some absorption lines is ejected while the neutron star orbits for about one to several weeks inside the envelope of the giant star. In the last hours of the interaction the neutron star merges with the core, accretes mass, and launches jets that eject the core and the inner envelope to form the explosion itself and the medium where the supernova photosphere resides. The remaining neutron star accretes fallback gas and further powers the supernova. We attribute the 1954 pre-explosion outburst to an eccentric orbit and temporary mass accretion by the neutron star at periastron passage prior to the onset of the common envelope phase.

We suggest that the energetic radiation from core-collapse super-energetic supernovae (SESNe) is due to a long lasting accretion process onto the newly born neutron star (NS), resulting from an inefficient operation of the jet-feedback mechanism. The jets that are launched by the accreting NS or black hole (BH) maintain their axis due to a rapidly rotating pre-collapse core, and do not manage to eject core material from near the equatorial plane. The jets are able to eject material from the core along the polar directions, and reduce the gravity near the equatorial plane. The equatorial gas expands, and part of it falls back over a timescale of minutes to days to prolong the jets-launching episode. According to the model for SESNe proposed in the present paper, the principal parameter that distinguishes between the different cases of CCSN explosions, such as between normal CCSNe and SESNe, is the efficiency of the jet-feedback mechanism. This efficiency in turn depends on the pre-collapse core mass, envelope mass, core convection, and most of all on the angular momentum profile in the core. One prediction of the inefficient jet-feedback mechanism for SESNe is the formation of a slow equatorial outflow in the explosion. Typical velocity and mass of this outflow are estimated to be v eq ≈ 1000 km s −1 and M eq 1M ⊙ , respectively, though quantitative values will have to be checked in future hydrodynamic simulations.

We find that applying a theoretical wind mass-loss rate from Monte Carlo radiative transfer models for hydrogen-deficient stars results in significantly more leftover hydrogen following stable mass transfer through Roche lobe overflow than when we use an extrapolation of an empirical fit for Galactic Wolf-Rayet stars, for which a negligible amount of hydrogen remains in a large set of binary stellar evolution computations. These findings have implications for modelling progenitors of Type Ib and Type IIb supernovae. Most importantly, our study stresses the sensitivity of the stellar evolution models to the assumed mass-loss rates and the need to develop a better theoretical understanding of stellar winds.

We propose a new type of repeating transient outburst initiated by a neutron star (NS) entering the envelope of an evolved massive star, accreting envelope material and subsequently launching jets which interact with their surroundings. This interaction is the result of either a rapid expansion of the primary star due to an instability in its core near the end of its nuclear evolution, or due to a dynamical process which rapidly brings the NS into the primary star. The ejecta can reach velocities of ≈ 10 4 km s −1 despite not being a supernova, and might explain such velocities in the 2011 outburst of the luminous blue variable progenitor of SN 2009ip. The typical transient duration and kinetic energy are weeks to months, and up to ≈ 10 51 erg, respectively. The interaction of a NS with a giant envelope might be a phase in the evolution of the progenitors of most NS-NS binary systems that later undergo a merger event. If the NS spirals in all the way to the core of the primary star and brings about its complete disruption we term this a 'common envelope jets supernova' (CEJSN), which is a possible explanation for the peculiar supernova iPTF14hls. For a limited interaction of the NS with the envelope we get a less luminous transient, which we term a CEJSN impostor.

Jets from supermassive black holes in the centres of galaxy clusters are a potential candidate for moderating gas cooling and subsequent star formation through depositing energy in the intra-cluster gas. In this work, we simulate the jet-intra-cluster medium interaction using the moving-mesh magnetohydrodynamics code Arepo. Our model injects supersonic, low density, collimated and magnetised outflows in cluster centres, which are then stopped by the surrounding gas, thermalise and inflate low-density cavities filled with cosmic-rays. We perform high-resolution, non-radiative simulations of the lobe creation, expansion and disruption, and find that its dynamical evolution is in qualitative agreement with simulations of idealised low-density cavities that are dominated by a large-scale Rayleigh-Taylor instability. The buoyant rising of the lobe does not create energetically significant small-scale chaotic motion in a volume-filling fashion, but rather a systematic upward motion in the wake of the lobe and a corresponding back-flow perpendicular to it. We find that, overall, 50 per cent of the injected energy ends up in material which is not part of the lobe, and about 25 per cent remains in the inner 100 kpc. We conclude that jet-inflated, buoyantly rising cavities drive systematic gas motions which play an important role in heating the central regions, while mixing of lobe material is subdominant. Encouragingly, the main mechanisms responsible for this energy deposition can be modelled already at resolutions within reach in future, high-resolution cosmological simulations of galaxy clusters.