Mergers of neutron stars and black holes with cores of giant stars: a population synthesis study

Abstract: We perform population synthesis of massive binaries to study the mergers of neutron stars (NSs) and black holes (BHs) with the cores of their giant secondaries during common envelope evolution (CEE). We use different values of the efficiency parameter $\alpha_{\rm CE}$ in the framework of the energy formalism for traditional CEE ($\alpha_{\rm CE} \leq 1$) and including additional energy sources to unbind the envelope ($\alpha_{\rm CE} > 1$). We constrain the possible values of $\alpha_{\rm CE}$ by comparing the results of our simulations with local rate densities of binary compact object mergers as inferred from gravitational-wave observations. We find two main evolutionary pathways of binary systems that result in NS-core mergers, while only one of them can also lead to the merger of a BH with the core of the giant star. We explore the zero age main sequence (ZAMS) statistical properties of systems that result in NS/BH-core mergers and find that the two evolutionary channels correspond to a bimodal distribution of orbital separations. We estimate the percentage of the mergers' event rates relative to core collapse supernovae (CCSNe). We include the effect of mass accreted by the NS/BH during CEE in a separate set of simulations and find it does not affect the mergers' event rates.