(cba:news) QR And, WZ Sge, mostly

Joe Patterson jop at astro.columbia.edu
Sun Oct 1 07:39:22 EDT 2017

Dear CBAers,

Some great (northern) fall weather this year, and a lotta work keeping 
up with all the CBA data.  Fortunately I have my every-7-years 
sabbatical this semester, so I can do a better job.

Our paper on WZ Sge is about ready for circulation (to the numerous 
authors - you know who you are out there).  It spans many years, but 
another couple of weeks would really help nail things down for 2017. 
It's a tough target - 15.5 with a neighbor star ~7 arcsec away - and 
calls for (somewhat) fast integrations, since eclipse ingress and egress 
are brief.  I recommend a 20-30 s cycle time, but I notice that slower 
is usually OK too.

The bottom line won't change.  The eclipse times wander very slightly on 
a timescale of decades.  This prevents a measurement of long-term 
evolution (systematic orbital-period increase or decrease), but the 
timings seem to wander on the ~25 year ERUPTION cycle.  That would be 
pretty interesting... although we can't prove it since we only have 55 
years of data.  I look forward to another 55.

Of all CVs, WZ Sge is the nearest, brightest in eruption, most studied, 
possibly oldest, and a dozen other distinctions.  It's the Andromeda 
Galaxy of CVs.  But the fast variations in light mean that only the 
larger CBA scopes (14 inch and up) can contribute substantially.

We can definitely quit on QR And.  Some really great data - long and 
top-quality runs - has been arriving on this 12th mag eclipsing binary. 
Jim Sergeant, Joe Ulowetz, Stephen Brincat, and James Boardman have been 
leading the way.  I'm just about to do the period-change analysis, but 
am sure that we have enough for 2017.

Some morning-target talk, on BRIGHT stars...

FY Per is a total mystery star.  A well-determined spectroscopic period 
of 0.2585 d, but every so often, a 90-minute photometric period pops up 
-just a few hundredths of a magnitude, but not particularly difficult to 
study since the star is 12th mag.  One of these years, we should figure 
it out!

GK Per has a 351-second signal - an intermediate polar (DQ Her star). 
But it's *really* difficult to spot in quiescence, which is 95% of the 
time.  So no one has figured out its long-term ephemeris; and this 
matters a lot, because it has by far the highest accretion rate among 
all DQs (and thus should spin up fast).  The reason for failure is that 
there's a K subgiant in the binary, overwhelming the light from the 
white dwarf.  This can be subdued with ULTRAVIOLET photometry.  If you 
have an ultraviolet filter (and CCD with decent UV sensitivity), you can 
be the hero with a long program of UV time series.  Calibration is of no 
importance - you just want to make sure a lot of UV photons, and not 
many others, reach your CCD.  It's just a timing project.

Airmass is usually a big problem with UV photometry - but less so here, 
since the point of interest is a 6-minute *periodic* signal.

Don't fall in love with your U filter, though.  Accurate UV photometry 
is famously tricky, for many reasons (extinction, low sensitivity, 
weirdness of the filter itself, etc.)  For nearly all purposes - except 
maybe this one - it's a tool to AVOID.

The AAVSO fall meeting is in Nashville this year (Nov 2-4), and it looks 
like a pretty good collection of CBAers will be there.  So I guess I 
will, too.

joe p

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