The Formation of Cataclysmic Variables with Brown Dwarf Secondaries

Abstract

The present-day formation of cataclysmic variables (CVs) with brown dwarf (BD) secondaries (0.013 M? ? Ms ? 0.075 M?) is investigated using a population synthesis technique. Results from the latest, detailed models for BDs have been incorporated into the population synthesis code. The present-day orbital period distribution of zero-age CVs (ZACVs) that form with BD secondaries is calculated. For our models, we find that ZACVs with BD secondaries have orbital periods in the range 46 minutes to 2.5 hr. 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 shorter than 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, ?CE, for three different assumed mass ratio distributions in zero-age main-sequence (ZAMS) binaries. Surprisingly, we find that the common envelope process must be extremely inefficient (?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 of smaller than 3 AU and ZAMS primary masses between ~1 and 10 M?, with ~75% of the primary masses of smaller than ~1.6 M?. Interestingly, these ranges in orbital separation and primary mass place the majority of the progenitor binaries within the so-called brown dwarf desert. The implications of these results are discussed in the context of V485 Cen and 1RXS J232953.9+062814, the only known CVs with orbital periods shorter than the period minimum, CVs above the period minimum that are strongly suspected of containing BD secondaries (such as WZ Sge), common envelope evolution involving very low mass secondaries, and the BD mass function in ZAMS binaries.