The Formation of Cataclysmic Variables with Brown Dwarf Secondaries

The present-day formation of cataclysmic variables (CVs) with brown dwarf (BD) secondaries (0.013 M_sun < M_sec < 0.075 M_sun) is investigated using a population synthesis technique. Results from the latest, detailed models for BDs have been incorporated into the population synthesis code. For our models, we find that ZACVs with BD secondaries have orbital periods in the range 46 min to 2.5 hrs. We also find that ZACVs with BD secondaries comprise 18% of the total, present-day ZACV population. In addition, we find that 80% of ZACVs with BD secondaries have orbital periods < 78 minutes. This implies that 15% of the present-day ZACV population should have orbital periods shorter than the observed orbital period minimum for CVs. We also investigate the dependence of the present-day formation rate of CVs with BD secondaries on the assumed value of the common envelope efficiency parameter, alpha_CE, for three different assumed mass ratio distributions in ZAMS binaries. Surprisingly, we find that the common envelope process must be extremely inefficient (alpha_CE < 0.1) in order for CVs with BD secondaries not to be formed. Finally, we find that the progenitor binaries of ZACVs with BD secondaries have ZAMS orbital separations < 3 AU and ZAMS primary masses between ~1-10 M_sun, with ~75% of the primary masses less than ~1.6 M_sun. Interestingly, these ranges in orbital separation and primary mass place the majority of the progenitor binaries within the so-called ``brown dwarf desert.‘’