(cba:news) april-may stars
jop at astro.columbia.edu
Thu Apr 26 08:43:05 EDT 2007
As most of you know, the sky has recently furnished us with beautiful
new bookended superhumpers: BZ UMa and GW Lib. Great rewards for these
long nights! GW Lib will likely keep going for a long time - a few more
weeks at least - and will get even better placed in the sky. So that
will remain the prime choice in the southern sky. Superhumps in these
"WZ Sge stars" (SU UMas of very long recurrence time) are little known -
practically all our knowledge is based on WZ Sge itself, and it remains
unknown whether those lessons are general to the class. So keeping the
faith on GW Lib is nighty desirable - it's sure to pay off.
The BZ UMa outburst may be about to end - or maybe not. In any case, it
too is an excellent star for a lot more vigilance. It may have echoes,
and it only falls to about 15.5 anyway, so you're likely to be able to
track it pretty well all the way to quiescence. Except maybe for the
increasingly difficult position, which will get awkward in a few weeks.
So, let's keep going on both. No deadlines for BZ UMa or GW Lib. Just
for IR Aq.
YY Sex we can say goodbye to. Bob Rea has been observing up a storm,
and his observations show that the period has been stable for 5 years.
That plus the extremely red color of the signal establishes that it is
an AM Her star, not a DQ as generally catalogued. Point proved; time to
Moonlight is getting bright now, and a good northern target is AM CVn.
The goal is the tracking of the 1028 s signal. We now have enough to
establish cycle count over a 15-year (and likely 29-year) interval, and
this is our main objective. But the 2007 timings, though establishing a
secure cycle count, don't specify the absolute phase as accurately as
we'd like. The last really good year was 2004, so there's about a 50 s
uncertainty (about 0.05 in phase) now. Kind of annoying. If we could
narrow this to about 20 s, no one would have to do this period study for
another 5-10 years. Let's tune it up! However, since there are more
powerful signals at 525 and 1011 s, you really have to get long
observations (4 hrs or more) to separate the various signals. A good
target, but only if you can get long runs.
A good equatorial target is CR Boo. Hardly anyone responded to my last
plea for it - those flashy superhumpers bumped it off, I think. But
it's in good sky position, available to all observers, and its
superhumps are still not much studied - really nothing of consequence
since 1996. Pretty strange for a bright AM CVn star with a very short
period. I dunno, maybe we have to wait till GW Lib fades... but I urge
you to consider CR Boo. The usual guidelines apply: long runs very very
desirable, unfiltered data best unless you have >20" of aperture (in
which case I recommend V).
Finally there's EX Hya. Good for bright skies. We're putting the
finishing touches on a long-term ephemeris, and it's well-placed as an
evening southern target.
Happy observing! My life has gotten a little complicated lately, so I
won't be seeing any of you in Big Bear. We may very well organize
something for 2008, quite possibly at the Hawaii meeting Russ Genet is
organizing at the end of the year. Or maybe earlier and cheaper -
anyone want to make a proposal?
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