COMMON ENVELOPE: ON THE MASS AND THE FATE OF THE REMNANT

One of the most important and uncertain stages in binary evolution is the common envelope (CE) event. Significant attention has been devoted in the literature so far to the energy balance expected to determine the outcome during the CE event. However, this question is intrinsically coupled with the problem of what is left from the donor star after the CE and its immediate evolution. In this paper we argue that an important stage has been overlooked: the post-CE remnant thermal readjustment (TR) phase. We propose a methodology for unambiguously defining the post-CE remnant mass after it has been thermally readjusted, namely, by calling the core boundary the radius in the hydrogen shell corresponding to the local maximum of the sonic velocity. We argue that the important consequences of the TR phase are (1) a change in the energy budget requirement for the CE binaries and (2) a companion spin-up and chemical enrichment, as a result of the mass transfer (MT) that occurs during the remnant TR. More CE binaries are expected to merge. If the companion is a neutron star (NS), it will be mildly recycled during the TR phase. The MT during the TR phase is much stronger than the accretion rate during the CE, and therefore satisfies the condition for hypercritical accretion better. We also argue that the TR phase is responsible for the production of mildly recycled pulsars in double NSs.