stars for August... but mainly WZ Sge

From: Joe Patterson <*jop_at_astro.columbia.edu*> Date: Fri, 17 Aug 2001 07:29:56 -0400 (EDT) · jop_at_astro.columbia.edu

Dear CBAers,                                                8/17/01

Just back from Germany, spending many pleasant hours analyzing the
treasure trove of WZ Sge data.

So far, the star has been flashing all the main features seen in the 1978
outburst.  Namely:
* very rapid rise to peak brightness at 8.0
* quick fall to 8.5, then slow decline at dwarf novae's official rate of
  ~0.13 mag/d
* large photometric humps at or nearly at Porb, dominating light curve for
  ~12 days
* large superhumps growing suddenly around day 12, with a period ~1%
  longer than Porb, decaying on a timescale of a couple of weeks
* a few dips suggestive of eclipse features.

    Of course, the observational material is ~10 times richer now,
thanks to the felicitous timing of the outburst (Sagitta in July, not
December!).  Thus some of these features are provable now, not merely
suggested, and in general we can greatly sharpen the detail of our
measurements.  As the brightest and probably nearest of all dwarf novae,
WZ Sge in 2001 will likely teach us lessons we'll still be meditating on
20 years from now.

     Of course every imaginable space-borne telescope will be watching WZ
Sge in the days ahead, and most already have.  In the hard UV/soft X-ray,
the star shows a forest of high-excitation emission lines.  Some less
imaginable telescopes may be heard from, too.

     The star is now down to 10.6, and for most of you this requires a
change in observing strategy.  That 8th mag star you've been using - I
believe GSC 1621-1830 - is now too bright, since it will saturate if you
expose WZ Sge itself adequately.  I recommend changing comparison stars;
Bob Rea has switched to 1621-1939, so consider that one and use 10-20 s
exposure times, which might give good signal-to-noise.

     Since the bright-spot eclipse seems to have returned in some guise,
producing narrow notches in the light curve, it might seem a good idea to
keep the time resolution short so as to define those notches properly.
For small telescopes, though, you really can't get enough signal in a few
seconds - so the data needs averaging and the nominal time resolution is
lost.  In the bargain, all this ugly dead time creeps in.  So in general
I'd recommend for telescopes under 0.6 m that you concentrate on getting
the WZ Sge images properly exposed, and accept whatever exposure time that
entails (under 35 s I hope - if you need to go longer, well, relax your
criterion for "properly exposed").  The bigger scopes can do the faster
stuff.

     A related question is filters.  Professional astronomers love
filters, since they don't have to pay for them, and have plenty of
aperture which allows tham to reject 90% of the light without penalty.
(Oh yeah, because they sometimes bring useful astrophysical info too.)
But three circumstances make filter use nearly always inappropriate for
us.
1. We are usually starving for light, and can't bear to reject 90% of it.
2. We often want high time resolution, which makes problem 1 even more
   severe.
3. The luminous parts of CVs are nearly always (and absolutely always in
   the case of bright erupting CVs) hot sources of continuous radiation,
   and the periodic signals are broad-band phenomena depending only
   weakly on wavelength.  Is any such dependence (there probably is
   some) of interest?  Yes, perhaps.  But in the process you will have
   paid a heavy price in the quality of your light curves.

Professional astronomers who have not used amateur equipment often fail to
realize these points, because really the *opposite* is true when you have
a big enough telescope.  With the 52-inch we nearly always use filters,
because photons are abundant, and filters enable both calibration and much
better treatment of extinction effects.

     Starting about now we'll get a WZ Sge page on the website, though
many of you have your own WZ Sge features now (Masi, Martin, Richmond, as
well as the usual excellent job by VSNET).  Feel free to advertise your
own site in this space (and I'll do so next message, when I'll collect the
addresses).

     Now... on to STARS.  There's still a lotta life left in WZ Sge, so
that's the prime object for the north.  Gianluca Masi, Brian Martin, and
Michael Richmond have been accumulating great riches of data, with
contributions also from Tonny, Rudolf, Donn Starkey, Lew Cook, Dave
Skillman, and some intrepid New Zealanders (mainly Bob Rea and Bill
Allen).  The NZ contribution has been really helpful since we still have
no northern coverage from San Francisco Bay westward to the Black Sea.

     The other objects I wanted to promote are KL Dra (big northern
scopes only), V1223 Sgr (south only), and the equatorial DQ Her stars AO
Psc and FO Aqr [we *always* love these stars, but they're to be visited
just once in while, with attention to accurate timing, not in big
campaigns].

     Enough for now.  Happy observing!

               joe