Violent stellar merger model for transient events

We derive the constraints on the mass ratio for a binary system to merge in a violent process. We find that the secondary-to-primary stellar mass ratio should be 0.003 ≲ (M₂/M₁) ≲ 0.15. A more massive secondary star will keep the primary stellar envelope in synchronized rotation with the orbital motion until merger occurs. This implies a very small relative velocity between the secondary star and the primary stellar envelope at the moment of merger, and therefore very weak shock waves, and low-flash luminosity. A too low-mass secondary will release small amount of energy, and will expel small amount of mass, which is unable to form an inflated envelope. It can, however, produce a quite luminous but short flash when colliding with a low-mass main-sequence star. Violent and luminous mergers, which we term mergebursts, can be observed as V838 Monocerotis-type events, where a star undergoes a fast brightening lasting days to months, with a peak luminosity of up to ∼10 ⁶L_⊙ followed by a slow decline at very low effective temperatures.