Distances and Absolute Magnitudes of Dwarf Novae: Murmurs of Period Bounce

Abstract

We redetermine the relationship between absolute magnitude and orbital period for dwarf novae, based on 46 stars with good distance estimates. This improves upon Warner's previous relation, building upon today's improved estimates of distance and binary inclination, and greater wavelength coverage. Together with other distance and dynamical constraints, this calibration is then applied to a set of ~300 known or likely dwarf novae of short orbital period, to study the dependence of quiescent M_v, time-averaged M_v, mass ratio q, and white-dwarf temperature T_WD, on P_orb. These distributions show that stars become much fainter as they approach minimum P_orb, and appear to show /evolutionary tracks/ as the secondary is whittled down by mass loss. Stars on the lower branch have the expected properties of "period bouncers" -- with a feeble secondary, faint accretion light, cool white dwarf, and long recurrence time between eruptions. Period bounce seems to occur at a mass of 0.058+-0.008 M_sol. Stars on the lower branch may also have higher velocities and heights above the Galactic plane, consistent with a greater age. Some are very nearby, despite strong selection effects discriminating against the discovery of these faint binaries accreting at very low rates. Period bouncers appear to be very common, and probably would dominate a complete census of cataclysmic variables.