(cba:news) CC Scl: ready, set, go; and AO Psc: faith restored?
jop at astro.columbia.edu
Sat Sep 1 06:52:33 EDT 2012
I just read a fascinating paper (on astro-ph: arxiv.org/abs/1208.5936)
by Patrick Woudt, Brian Warner, et al. (mostly the South Africans) about
CC Scl, long known as a respectable dwarf nova. They show clear
evidence of a 6.5 minute period, which certainly looks like that of a DQ
Her star (intermediate polar). Yet it is seemingly well credentialed as
a short-Porb, max-plus-supermax dwarf nova, which superhumps just when
dwarf-nova rules require it, and never when those rules forbid it.
Who invited that? Maybe we have to change the rules somewhat... and at
the very least we need to take a very close look at this star. It's
right up our alley, transiting near local midnight and just begging for
long time series. Perfect for the australites, and acceptable for some
of the borealites too. I think it's generally between 13 and 17.5,
hence suitable for most of us. To properly resolve the 6.5 minute
signal, a 90-second cycle time will be adequate (as long as it's not
*exactly* commensurate with 389.5 seconds!), although faster would be
This 6.5 minute signal is no barn-burner, though; you might not see it
directly in the light curve, though probably will in the power spectrum.
More of a barn-burner is the 14 minute signal of AO Psc, which is
about 0.05-0.07 mag in a star which generally reports for duty at
V=13-14 - and is therefore another alluring target in September-October.
Partly because of the following tale.
In 1999 we carried out a long campaign on AO Psc. We found periodic
signals at 14.3 minutes, 13.4 minutes, and 3.6 hours - standard issue
for this star. And we also found a clear signal at 4.0 hours. It
certainly looked like a bona fide superhump, and therefore exciting:
first superhump found in a DQ Her star! But the signal was very, very
close to exactly one-sixth of a sidereal day, and I suspected that it
was an artifact of differential extinction. This is a known bugaboo of
us white-light enthusiasts, and when I studied it carefully I saw that
the ephemeris for maximum light of the new signal positioned it pretty
close to culmination in Arizona, where much of the data was taken.
Super-suspicious, so I never published it. But over the past twenty
years we have observed hundreds of stars with comparable zeal - just as
blue as AO Psc, but we never saw anything like this. One, two, and even
three cycles/sidereal day - yeah, we're familiar with those, and I just
yawn then away. But never six. If you observe enough stars and enough
superhumps, eventually some numbers will pop up which look suspicious,
but are mere coincidences. Now that we are much more global than in
1999, we could check this with a similarly energetic campaign.
So let's do it. Being equatorial, nobody can get long runs on AO Psc.
But on the other hand, everyone can see it. So we should be able to
patch together a very nice global light curve. The field is practically
blank, so there's a comparison-star issue. It would be good to come up
with two comparison stars of good quality, and observers could choose
the one convenient for them (field of view, brightness, etc.). Some of
you guys are awfully good at this (Jim Jones is an ace) - and sitting
here in NYC with no telescope, I'm certainly not. White light is OK,
and V (or Sloan g. or something close) would be somewhat better if you
have lots of photons to spare.
Finally, if you have the option to supply the airmass with each point,
that would be helpful; I plan to correct the white-light observations
for differential extinction (which I normally don't, in the spirit of
keeping human hands off the data as much as possible).
Oh, and if you can, avoid multiple-comparison-star photometry (i.e.,
merging several to form a composite V-C). That taxes my diminishing
supply of brain cells. Buena suerte!
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