Late Evolution of Cataclysmic Variables

Abstract

We study the evolution of hydrogen-rich cataclysmic variables (CVs) near minimum orbital period at ∼78 minutes. As has been known for many years, these are among the most intrinsically common CVs, but they hide fairly well because of their faintness and low incidence of eruptions. We discuss their number and observational signatures, paying special attention to those that may have passed minimum orbital period-the "period bouncers." The status of binaries near minimum period is best determined by the mass ratio, and this is best constrained by measuring the accretion disk precession frequency, because that frequency is readily accessible to observation and proportional to the secondary star's mass. This method reveals four stars that are good candidates to have survived period bounce; two appear to have secondaries as puny as 0.02. But each star M , can have bounced only recently if at all. There is still no strong evidence of any long era of evolution in a state of increasing period. This conflicts sharply with discussions of observational data that have identified dozens of known CVs with this state. The total space density of cataclysmic variables is ∼10 Ϫ5 pc Ϫ3 , with short-period systems constituting ∼75% of the total. Both estimates are far less than predicted by simple theories of evolution. It is probably necessary to have some means of destroying CVs before they reach the predicted very high space densities. This can be done by invoking an angular momentum loss mechanism that does not quickly subside as the mass ratio becomes very low.

The census of cataclysmic variables in the ROSAT Bright Survey

“…Our limit of the space density is in good agreement with the recent estimate by Patterson (1998), who derives a density of ∼10 −5 pc −3 . The reader is refered to his paper, where a thorough discussion of the possible selection effects in various surveys is given.…”

Finding the most variable stars in the Orion Belt with the All Sky Automated Survey

“…Their super-cycles are stable both in length and outburst pattern (Osaki 1996;Patterson 1998;Downes et al 2001). V1159 Ori was discovered by Wolf & Wolf (1906), who gave it the name "Var.,Orionis 36.1906", and first investigated by Kippenhahn (1953) and Jablonski & Cieslinski (1992).…”

Dwarf novae in the Hamburg quasar survey: rarer than expected

“…(b) Double-humped orbital light curves are observed in a large number of short-period dwarf novae, e.g. WX Cet (Rogoziecki & Schwarzenberg-Czerny 2001), WZ Sge (Patterson 1998), RZ Leo, BC UMa, MM Hya, AO Oct, HV Vir (Patterson et al 2003), HS 2331+3905 (Araujo-Betancor et al 2005a), and HS 2219+1824 (Rodríguez-Gil et al 2005a); the origin of those double-humps is not really understood, but most likely associated with the accretion disc/bright spot. In longperiod dwarf novae, double-humped light curves are observed in the red part of the spectrum caused by ellipsoidal modulation of the secondary star, e.g.…”

“…CV population models result in space densities in the range 2 × 10 −5 pc −3 to 2 × 10 −4 pc −3 (de Kool 1992;Politano 1996), whereas the space density determined from observations is (0.5−1) × 10 −5 pc −3 (Patterson 1984;Ringwald 1996;Patterson 1998). It appears therefore that we currently know about an order of magnitude less CVs than predicted by the models.…”