(cba:news) those baffling luminous stars at short period... (Joe Patterson) [2012-02-17T17:00:31Z]

                                                               2/17/12
Dear CBAers,

  We've gone about 60 days now on BK Lyn and ER UMa, and accumulated a huge
amount of coverage.  To my amazement, those light curves look practically
identical!  ER UMa is just its usual self, flashing superoutbursts and
their usual accompanying superhumps, normal outbursts, and brief stays
at quiescence... with negative superhumps of constant amplitude (in
intensity units) throughout.  That's what it usually does (as we learned
last year).  But you have to be an expert on these stars to tell which is
which from the light curve.  In other words, it looks like BK Lyn is turning
into a dwarf nova this year - in particular, a super-charged ER UMa-type
dwarf nova.

  I rubbed my eyes on this one.  A long-standing mystery is why no
classical nova has ever turned into a dwarf nova.  Practically all of them
should, and certainly all of them below the period gap.  Yet none has.
The simplest resolution is that novae take more than ~100 years to
return to their true quiescent level.  Could it be that we're witnessing
now BK Lyn's first return to a dwarf-nova state?  A plausible case has
been made (Hertzog 1986) that BK Lyn is the remnant of Nova Lyn 101; and
if that's so, then the timescale for return would be about 2000 years.
Is this reasonable?  It would explain why no such return has occurred in
the ~100 years we've been looking... and it would also satisfy the
observed space densities (dwarf novae are more populous than known
classical novae by about 50 to 1).  But isn't it a little far-fetched
that this important transition occurred... ahem... while we were looking?

  Actually, I think not.  There are roughly 8 known ER UMas, and roughly
5 BK Lyns.  We have been observing all of these stars for durations of 5
to 20 years.  As long as the BK->ER transition is reasonably long-lasting
(more than a few years), the probability of observing this transition
in one star is not prohibitively low.

  So the bottom line is: keep up the pressure on BK Lyn!  And ER UMa too,
both for at least another month.

  And this renewed interest in ER UMa stars leads me to put another on the
list: RZ LMi.  So far we have only got 1 good season of observation on it;
there's still time to make 2012 the second.  These three stars all compete
heavily with each other, so a good strategy is to adopt one as your
favorite, and keep after that one assiduously.  You can look at the CBA
homepage to see how much attention each one is getting.  Long light curves
are definitely the way to go - but avoid large airmasses (>2.0).

  In the south we want to do this for two stars:
1. CP Pup, still well-placed, a star with variability that is large and
mysterious, and the only southern old nova below the period gap  (that's
bright enough for us).

2. T Pyx.  To see what new surprises this lovely star will throw at us.

Finally, there's the new star identified in ATel 3927: 0753+04, obviously
suitable for all hemispheres.  Likely a new VY Scl, and very, very
interesting to us.  Its complete name appears to be 1RXS J075330.1+044606.

  I've been away from email each weekend, and that is likely to continue
for a while.  Back in the saddle MOnday.

joe

P.S. a bunch of us are planning to attend the SAS/AAVSO meeting in late
May at Big Bear Lake, CA.  A great opportunity to get together.  Hotel space
might be tougher this year, because of the joint meeting... so early
reservations would be a good idea.  There are several other hotels very
near the Northwoods Resort, so that should accomodate overflow.  I'll
also be staying afterwards for at least a day in the area, more or less
for the Riverside Telescope Makers Conference.