Population synthesis of Be/white dwarf binaries in the Galaxy

Using the Scenario Machine (a numerical code that models the evolution of large ensembles of binary systems) we study the number and physical properties of binary Be stars with white dwarfs taking account of the compact object cooling and we discuss the ways of their formation. Ill our calculations we take into account the influence of tidal synchronization on the evolution of stars in a close binary. The synchronization time scale may be less than the life-time of a Be star on the main sequence after tire first mass transfer. It has strong effects on the resulting number distribution of binary Be stars over orbital I,periods. In particular, it carl explain the lack of short period Be binaries. According to our calculations the number of binary systems containing a Be star paired with a white dwarf in the Galaxy is very large -70% of all Be stars formed as a result of binary evolution must have a white dwarf as a companion. Based on our calculations we conclude that the compact companion in these systems must have a high surface temperature. The number distribution over the surface temperature peaks at 2 10(4) K for all white dwarfs and at 4 10(4) K for white dwarfs paired with early-type Be stars (between B0 and B2). The registration of white dwarfs in such systems is hampered by the fact that the entire orbit of a white dwarf is embedded in the dense circumstellar envelope of the primary star (our calculations show that the majority of Be/WD systems have orbital periods less than one year) and all extreme-UV and soft X-ray photons of a compact companion are absorbed by the Be star envelope. The detection of a white dwarf is possible during the period when the Be star disc-like envelope is lacking by the detection of a white dwarf extreme-UV and soft X-ray emission. This method of registration appears to be particularly promising for single early-type Be stars because in these systems the white dwarfs must have a very high surface temperature. However, the loss of the Be disc-like envelope does not often occur and it is a rather rare event for many Be stars. The best possibility of white dwarf detection is given by the study of helium spectral lines found in emission from several late-type Be stars. The ultraviolet continuum energy of these Be stars is found to be not enough to produce the observed helium emission. Besides, we also discuss the orbital properties of binary Be star systems with other evolved companions such as helium stars and neutron stars and give a possible explanation for the lack of Be/black hole binaries.