Abstract
We summarize the results of a 20-yr campaign to study the light curves of BK Lyn, a nova-like star strangely located below the 2 to 3 h orbital-period gap in the family of cataclysmic variables (CVs). Two apparent superhumps dominate the nightly light curves, with periods 4.6 per cent longer, and 3.0 per cent shorter, than the orbital period. 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 is plausible that these arise, respectively, from a prograde apsidal precession and a retrograde nodal precession of the star's accretion disc. 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 CV before. Reviewing all the star's oddities, we speculate: (a) BK Lyn is the remnant of the probable nova on 101 December 30, and (b) it has been fading ever since, but it has taken ∼2000 yr for the accretion rate to drop sufficiently to permit dwarf-nova eruptions. If such behaviour 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 nova-likes 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 are just getting started in their post-nova cooling). Four: why the orbital periods, accretion rates and white dwarf 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 post-nova 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 orbital period, is time since the last classical-nova eruption.
Highlights
BK Lyncis was discovered in the Palomar-Green survey for objects with ultraviolet excess (Green et al 1986), and was listed as PG0917+342 in the preliminary catalog of cataclysmic-variable stars in that survey (Green et al 1982)
Each identified signal is fairly stable in frequency, but the years of long-baseline coverage show that differences are measurable: the apsidal superhump apparently increased by ~0.2% in frequency between 1992-4 and all subsequent years, and the nodal superhump apparently decreased in frequency by ~0.1% between 2002 and 2012
When the star declines in light, it probably means that Mand the outer disk radius are declining too
Summary
BK Lyncis was discovered in the Palomar-Green survey for objects with ultraviolet excess (Green et al 1986), and was listed as PG0917+342 in the preliminary catalog of cataclysmic-variable stars in that survey (Green et al 1982). Two years of time-series photometry revealed “superhumps” in the star's light curve – large-amplitude waves interpreted as resulting from apsidal precession of the accretion disk (Skillman & Patterson 1993, hereafter SP) These studies showed only small variability in the range V=14.5-14.7. Several studies of ancient Chinese records have suggested that a nova appeared very close to its position on 30 December 101 (Hsi 1958, Pskovskii 1972, Clark & Stephenson 1977), and Hertzog (1986) concluded that BK Lyn is the remnant of Nova Lyncis 101 This would certainly qualify as the oldest old nova – far exceeding the closest challenger, WY Sge = Nova Sagittae 1783 (Shara et al 1985). That transformation may provide a powerful clue to the long-term evolution of cataclysmic variables
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