(cba:news) CC Scl: ready, set, go; and AO Psc: faith restored?

Joe Patterson jop at astro.columbia.edu
Sat Sep 1 06:52:33 EDT 2012

Dear CBAers,

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 
slightly better.

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!

joe p

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