On the Properties of Galactic Novae and Their Orbital Period Distribution

Abstract

Using population synthesis techniques, we analyze the orbital period distribution and other ensemble properties of Galactic novae. We find that the frequency of nova outbursts in the Galactic disk should be about 30 events yr-1 (to within a factor of ~5). This frequency is in agreement with previous theoretical estimates and the observationally inferred rates (but we caution that there are many uncertainties inherent in the calculations). We also find that the frequency-averaged mass of degenerate dwarfs in nova systems is in the range of 0.95 ± 0.15 M☉, which is in good agreement with the observational estimate of 0.90 M☉. If period-dependent observational selection effects are not substantial, we show that any theoretical model (and/or region of parameter space) pertaining to the formation and evolution of Galactic novae can be constrained by comparing the predicted ratio of novae above the period gap to those below the gap (i.e., νabove/νbelow) with the observed one. Using the observationally inferred lower limit and given that the temperature of the accreting degenerate dwarfs has a significant effect on the estimated nova frequencies for cataclysmic variables in the orbital period range of 1-2 hr (i.e., below the period gap), we conclude that (on average) the degenerate dwarfs in systems below the gap are relatively cold (<3 × 107 K), and that they may be significantly cooler than the dwarfs found in the high-period systems. We note that this result is in accord with recent observations and underscores the need for theoretical calculations of the thermal evolution of degenerate dwarfs in nova systems.