BK Lyncis: the oldest old nova?... and a Bellwether for cataclysmic variable evolution

Abstract

We summarize the results of a 20-year campaign to study the light curves of BK Lyncis, a nova-like star strangely located below the 2-3 hour orbital period gap in the family of cataclysmic variables. Two apparent “superhumps” dominate the nightly light curves – with periods 4.6% longer, and 3.0% shorter, than Porb. The first appears to be associated with the star's brighter states (V~14), while the second appears to be present throughout and becomes very dominant in the low state (V~15.7). It's plausible that these arise, respectively, from a prograde apsidal precession and a retrograde nodal precession of the star's accretion disk. Starting in the year 2005, the star's light curve became indistinguishable from that of a dwarf nova – in particular, that of the ER UMa subclass. No such clear transition has ever been observed in a cataclysmic variable. Reviewing all the star's oddities, we speculate: (a) BK Lyn is the remnant of the probable nova on 30 December 101, and (b) it has been fading ever since, but has taken ~2000 years for the accretion rate to drop sufficiently to permit dwarf-nova eruptions. If such behavior is common, it can explain other puzzles of CV evolution. One: why the ER UMa class even exists (because all members can be remnants of recent novae). Two: why ER UMa stars and short-period novalikes are rare (because their lifetimes, which are essentially cooling times, are short). Three: why short-period novae all decline to luminosity states far above their true quiescence (because they're just getting started in their postnova cooling). Four: why the orbital periods, accretion rates, and whitedwarf temperatures of short-period CVs are somewhat too large to arise purely from the effects of gravitational radiation (because the unexpectedly long interval of enhanced postnova brightness boosts the mean mass-transfer rate). And maybe even five: why very old, post-period-bounce CVs are hard to find (because the higher mass-loss rates have “burned them out”). These are substantial rewards in return for one investment of hypothesis: that the second parameter in CV evolution, besides Porb, is time since the last classical-nova eruption.