(cba:news) stars for May
jop at astro.columbia.edu
Sat May 14 09:24:47 EDT 2011
Apologies for my long silence. Just as T Pyx suddenly rose, I suddenly
fell - with pneumonia. It didn't have any great complications, but kept
me out of action for 6-7 weeks. I'm mostly OK now - and will see some
of you at the AAVSO/AAS meeting in a week.
Spending so much time at home, I've had the opportunity to analyze
thoroughly the data from the 3 big springtime (northern) campaigns: ER
UMa, BK Lyn, and AM CVn. ER UMa has proved to a pretty good clone of
V503 Cyg, and suggests the hypothesis that all of these oddly-frantic
and short-period dwarf novae may have "permanent" (i.e. not freshly
reborn in superoutbursts) negative superhumps. Or, more expansively,
that any short-period nonmagnetic star that is sufficiently bright for
sufficiently long will have 'em. It's just a hypothesis, but so far
there is no obvious counterexample. Questions remain: how bright? how
long? and of course, whether this true. BK Lyn is certainly evidence
for the prosecution, displaying simultaneous positive and negative
But these guys at RA=9 hr have had their say, so it's definitely time to
end coverage of ER UMa and BK Lyn. AM CVn is a different story. So far
we have excellent coverage over a 125-day season - the best ever. But
the long and close coverage enables a study we've never been able to
carry out for any star: tracking the small and simultaneous period
changes in the negative and positive superhumps. Are they mirror images
of each other? They ought to be, based on the simple hypothesis that
they represent the natural apsidal and nodal precession of a disk of
slightly varying effective radius. This study requires a lot of
patience, because the period wiggles are quite small and slow; if we
get as many as two complete up-and-down wiggles over a 5-month observing
season, I'll be a happy camper! Can Ven is still around - so let's
definitely keep going for another 3-4 weeks.
A side benefit of AM CVn is the measurement of orbital period change.
We've been keeping very close watch on Porb (1028.7322 s) since 1991...
and since the weak orbital signal (0.007 mag) is heavily assaulted by
the stronger superhump signals at 1011.43 and 525.56 s, it takes long
time series to cleanly separate all these effects (and to build up the
*significance* of the weakest signal). Another few weeks of coverage
would add several new and valuable timings of orbital minima. So keep
the faith! Josch, Enrique, Tom, Tut/George, and Shawn Dvorak have been
the main players so far in the AM CVn story.
I guess because of the southern Milky Way swinging into view, our
southern menu is suddenly crowded! The following stars are now
available, and are all subjects of our long-term period (and Pdot) studies:
V4743 Sgr (too faint??)
* means (mainly) rotational period study
^ means (mainly) orbital period study
GW Lib and T Pyx are obvious choices (but their riches will be
intellectual, not monetary). We're looking for GW Lib's post-eruption
pulsation behavior - we found some quite fascinating behavior in 2010,
and now it's time for an encore. At V=16, should be available to
practically everyone. As for T Pyx, I was very surprised that our
southern observers didn't jump all over it. I've not seen credible
reports about periodic signals. But on the other hand, the periodic
signal at quiescence was never - even to this day - seen by anyone else
but us... so I conclude that it's basically OUR JOB to figure this out.
So it would be a very, very good target for our southern observers for
a few weeks. Sorry for the great brightness; it probably presents some
challenges, but give it a try!
Now for northern/equatorial targets. As some of you know, I've always
been very interested in pulse-timing observations of the DQ Her stars
(intermediate polars) early and late in the observing seasons. These
are the key to precise many-year measurement of the periods - a
responsibility which, as things have played out, has fallen entirely to
us. The late-season guys are now: RX0704+26, RX0636+35, Swift 0732-13,
and MU Cam; the early-season guys are V2306 Cyg, V2069 Cyg, FO Aqr, and
AO Psc. A 2-3 hour observation of each usually produces a decent pulse
timing... and a few such timings of each star, unless they disagree,
usually suffices to nail down the all-important cycle count (to bridge
between observing seasons).
Lotta stars in this message! Pick out some which are well-placed for
your aperture, latitude, observing style, etc. - and let the time-series
I hope to see a bunch of you in Boston next week. We'll certainly plan
to go out for a CBA dinner together, possibly on Monday, May 23.
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