On the existence of low-luminosity cataclysmic variables beyond the orbital period minimum

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Abstract
Models of the present-day intrinsic population of cataclysmic variables predict that 99 per cent of these systems should be of short orbital period (Porb ≾ 2.5 h). The Galaxy is old enough that ~ 70 per cent of these stars will have already reached their orbital period minimum (~80 min), and should be evolving back toward longer periods. Mass-transfer rates in these highly evolved binaries are predicted to be ≾10−11 M⊙ yr−1, leading to Mv of ∼10 or fainter, and the secondaries would be degenerate, brown dwarf-like stars. Recent observations of a group of low-luminosity dwarf novae (TOADs) provide observational evidence for systems with very low intrinsic Mv and possibly low-mass secondaries. We carry out population synthesis and evolution calculations for a range of assumed ages of the Galaxy in order to study Porb and Ṁ distributions for comparison with the TOAD observations. We speculate that at least some of the TOADs are the predicted very low-luminosity, post-period-minimum cataclysmic variables containing degenerate (brown dwarf-like) secondaries having masses between 0.02 and 0.06 M⊙ and radii near 0.1 R⊙. We show that these low-luminosity systems are additionally interesting in that they can be used to set a lower limit on the age of the Galaxy. The TOAD with the longest orbital period currently known (123 min), corresponds to a Galaxy age of at least 8.6 × 109 yr.

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