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Abstract Common envelope (CE) evolution is an outstanding open problem in stellar evolution, critical to the formation of compact binaries including gravitational-wave sources. In the “classical” isolated binary evolution scenario for double compact objects, the CE is usually the second mass transfer phase. Thus, the donor star of the CE is the product of a previous binary interaction, often stable Roche lobe overflow (RLOF). Because of the accretion of mass during the first RLOF, the main-sequence core of the accretor star grows and is “rejuvenated.” This modifies the core-envelope boundary region and decreases significantly the envelope binding energy for the remaining evolution. Comparing accretor stars from self-consistent binary models to stars evolved as single, we demonstrate that the rejuvenation can lower the energy required to eject a CE by ∼42%–96% for both black hole and neutron star progenitors, depending on the evolutionary stage and final orbital separation. Therefore, binaries experiencing first stable mass transfer may more easily survive subsequent CE events and result in possibly wider final separations compared to current predictions. Despite their high mass, our accretors also experience extended “blue loops,” which may have observational consequences for low-metallicity stellar populations and asteroseismology. 

As active galactic nuclei (AGN) ‘turn on’, some stars end up embedded in accretion discs around supermassive black holes (SMBHs) on retrograde orbits. Such stars experience strong headwinds, aerodynamic drag, ablation, and orbital evolution on short time-scales. The loss of orbital angular momentum in the first ∼0.1 Myr of an AGN leads to a heavy rain of stars (‘starfall’) into the inner disc and on to the SMBH. A large AGN loss cone (θAGN, lc) can result from binary scatterings in the inner disc and yield tidal disruption events (TDEs). Signatures of starfall include optical/UV flares that rise in luminosity over time, particularly in the inner disc. If the SMBH mass is $M_{\rm SMBH} \gtrsim 10^{8}\, \mathrm{M}_{\odot }$, flares truncate abruptly and the star is swallowed. If $M_{\rm SMBH}\lt 10^{8}\, \mathrm{M}_{\odot }$, and if the infalling orbit lies within θAGN, lc, the flare is followed by a TDE that can be prograde or retrograde relative to the AGN inner disc. Retrograde AGN TDEs are overluminous and short-lived as in-plane ejecta collide with the inner disc and a lower AGN state follows. Prograde AGN TDEs add angular momentum to inner disc gas and so start off looking like regular TDEs but are followed by an AGN high state. Searches for such flare signatures test models of AGN ‘turn on’, SMBH mass, as well as disc properties and the embedded population.

 

Abstract The recent detection of GW190521 stimulated ideas on how to populate the predicted black hole (BH) pair-instability (PI) mass gap. One proposal is the dynamical merger of two stars below the PI regime forming a star with a small core and an oversized envelope. We outline the main challenges this scenario faces to form one BH in the gap. In particular, the core needs to avoid growing during the merger, and the merger product needs to retain enough mass, including in the subsequent evolution, and at core collapse (CC). We explore this scenario with detailed stellar evolution calculations, starting with ad hoc initial conditions enforcing no core growth during the merger. We find that these massive merger products are likely to be helium-rich and spend most of their remaining lifetime within regions of instabilities in the Herzsprung–Russell diagram, such as luminous blue variable eruptions. An energetic estimate of the amount of mass loss neglecting the back reaction of the star suggests that the total amount of mass that can be removed at low metallicity is ≲1 M ⊙ . This is small enough that at CC our models are retaining sufficient mass to form BHs in the PI gap similar to the recent ones detected by LIGO/Virgo. However, mass loss at the time of merger, the resulting core structure, and the mass loss at CC still need to be quantified for these models to confirm the viability of this scenario. 

We report on the search for the optical counterpart of the gravitational event GW170814, which was carried out with the VLT Survey Telescope (VST) by the GRAvitational Wave Inaf TeAm. Observations started 17.5 h after the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo alert and we covered an area of 99 deg2 that encloses $\sim 77{{\ \rm per\ cent}}$ and $\sim 59{{\ \rm per\ cent}}$ of the initial and refined localization probability regions, respectively. A total of six epochs were secured over nearly two months. The survey reached an average limiting magnitude of 22 AB mag in the r band. After assuming the model described in Perna, Lazzati & Farr, that derives as possible optical counterpart of a BBH (binary black hole) event a transient source declining in about one day, we have computed a survey efficiency of about $5{{\ \rm per\ cent}}$. This paper describes the VST observational strategy and the results obtained by our analysis pipelines developed to search for optical transients in multi-epoch images. We report the catalogue of the candidates with possible identifications based on light-curve fitting. We have identified two dozens of SNe, nine AGNs, and one QSO. Nineteen transients characterized by a single detection were not classified. We have restricted our analysis only to the candidates that fall into the refined localization map. None out of 39 left candidates could be positively associated with GW170814. This result implies that the possible emission of optical radiation from a BBH merger had to be fainter than r ∼ 22 (Loptical ∼ 1.4 × 1042 erg s−1) on a time interval ranging from a few hours up to two months after the gravitational wave event.