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(cba:news) april-may stars (Joe Patterson) [2007-04-26T12:43:06Z]


Dear CBAers,

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 move on.

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?

joe