(cba:news) stars for May

Joe Patterson jop at astro.columbia.edu
Sat May 14 09:24:47 EDT 2011

Dear CBAers,

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:
RX J1654-19*
XMM 1151-62^
V4743 Sgr (too faint??)
V617 Sgr^
WX Cen^
V1223 Sgr*
GW Lib$
Swift 0732-13*
T Pyx$
* 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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