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

I consider three evolutionary channels leading to the formation of double d egenerate (DD) binary systems, "stable Roche lobe overflow (RLOF) plus comm on envelope (CE)", "CE plus CE", and "exposed core plus CE". I carry out Mo nte-Carlo simulations and examine the effects of model parameters (tidal en hancement 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 distri butions of their masses, mass ratios, and orbital periods, as well as the b irth rates of DDs and Type Ia supernovae. Specifically, (1) Stable RLOF plu s CE and CE plus CE are the main channels, (2) the Galactic birth rate of D Ds is 0.03 yr(-1) and that of DDs with helium white dwarfs (Re WDs) as prim aries 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 mak e up 56%, (4) the distribution of orbital periods of detectable DDs peaks a round 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 t han a helium WD pair, (6) mass transfer during stable RLOF is not conservat ive and 7) a tidally enhanced stellar wind exists.