The formation of double degenerates and related objects

I systematically investigate the formation of double degenerates (DDs) via binary interactions. I consider three evolutionary channels for their formation stable Roche lobe overflow (RLOF) plus common envelope (CE), CE plus CE, exposed core plus CE, and carry out Monte Carlo simulations. I explore the effects of model parameters, such as the tidal-enhancement parameter for stellar wind, the mass transfer efficiency for stable RLOF and the CE ejection efficiency, on my results. I also explore the effects of various assumptions about age, metallicity, mass ratio distribution and wind velocity. My results show that the model is successful in the explanation of the formation of DDs. I explain satisfactorily the distributions of masses, mass ratios, orbital periods and birth rate of the observed DDs. The main conclusions are the following. (i) Stable RLOF plus CE and CE plus CE are the main evolutionary scenarios leading to the formation of DDs. (ii) The Galactic birth rate of DDs is 0.03 yr(-1), and the birth rate of DDs with helium (He) white dwarfs (WDs) as brighter components is 0.017 yr(-1). (iii) The number of detectable DDs in our Galaxy is 3 x 10(6), and DDs with brighter He WDs make up 56 per cent. (iv) The distribution of orbital periods for detectable DDs peaks around 6 h. (v) WD 0957-666 and WD 1101+364 are formed through the stable RLOF plus CE scenario, and WD 0135-052 is possibly a carbon-oxygen (CO) WD pair rather than a helium (He) WD pair. (vi) The Galactic birth rates of close i, respectively. (vii) The mergers of two WD binaries and DD mergers are 0.074 and 0.029 yr(-1), respectively. (vii) The merges of two He WDs and the mergers of He and CO WDs have masses of 0.61 +/- 0.09 and 0.96 +/- 0.13 M., respectively. (viii) Mass transfer during stable RLOF is not conservative. (ix) A tidally enhanced stellar wind exists. I also investigate the formation of Type Ia supernovae (SNe Ia), cataclysmic variables (CVs), subdwarf O-type (sdO) stars and R Coronae Borealis (R CrB) stars. The birth rates of SNe Ia and CVs are successfully explained in the above model. The model also shows that CVs with long orbital periods tend to have CO WDs. The birth rates of the mergers of two He WDs (sdO stars) and the mergers of He and CO WDs (R CrB stars) are 0.006 and 0.018 yr(-1) in the Galaxy, respectively. The birth rates of CVs, DDs, DD mergers and SNe Ia are more sensitive to the recent stellar formation history than to the past one.