Channels for the formation of double degenerates and a Monte-Carlo analysis

Abstract

I consider three evolutionary channels leading to the formation of double degenerate (DD) binary systems, “stable Roche lobe overflow (RLOF) plus common envelope (CE)”, “CE plus CE”, and “exposed core plus CE”. I carry out Monte-Carlo simulations and examine the effects of model parameters (tidal enhancement of wind, mass transfer efficiency in the stable RLOF, CE ejection efficiency, the initial mass ratio distribution) on the results. My model successfully explains the formation of DDs and the observed distributions of their masses, mass ratios, and orbital periods, as well as the birth rates of DDs and Type Ia supernovae. Specifically, (1) Stable RLOF plus CE and CE plus CE are the main channels, (2) the Galactic birth rate of DDs is 0.03 yr −1, and that of DDs with helium white dwarfs (He WDs) as primaries is 0.017 yr −1, (3) the number of detectable DDs (cooling time less than 10 8 yr) in our Galaxy is 3 million, and DDs with He WD primaries make up 56%, (4) the distribution of orbital periods of detectable DDs peaks around 6 hr, (5) WD 0957-666, 1101+364 and Feige 36 are formed by “stable RLOF plus CE”, and WD 0135-052 is probably a carbon-oxygen WD pair rather than a helium WD pair, (6) mass transfer during stable RLOF is not conservative and 7) a tidally enhanced stellar wind exists.