Population synthesis of substellar companions surviving common-envelope evolution

Abstract

Predicting the population of substellar companions around white dwarfs (WDs) requires accounting for both companions that avoid engulfment during stellar evolution and those that survive common-envelope (CE) evolution. While the former population was investigated in our previous population-synthesis study, a comprehensive population synthesis of the latter remains lacking. We investigated the fate of engulfed substellar companions within the same population-synthesis framework. We aim to predict the occurrence rates and properties of companions surviving CE evolution, identify the conditions permitting survival, and assess their detectability. We determined the stellar structure at the onset of CE evolution and calculated CE outcomes for different ejection efficiencies considering gravitational and thermal energy or the full internal energy of the envelope. Survival is rare and strongly depends on the evolutionary stage at engulfment. No companions entering CE evolution during the red-giant branch or early asymptotic-giant branch survive. Survivors originate exclusively from the thermally pulsing asymptotic-giant branch, when stellar envelopes can become weakly bound, allowing even relatively low-mass companions to survive. The resulting systems typically have orbital periods of a few hours. We predict occurrence rates of up to 0.16% among WDs, corresponding to an upper limit of 2.53 companions to WDs within 50 pc. Transit detection is primarily limited by orbital geometry, whereas all predicted survivors would produce detectable infrared excess with JWST out to 50pc. We predict that CE evolution produces a small but observable population of close substellar companions to WDs. Survival is governed not only by companion mass but critically by the envelope structure at engulfment, particularly during thermal pulses, and by the contribution of recombination energy to envelope ejection.