(cba:news) New stars for May-June... and HP Lib in particular (Joe Patterson) [2017-05-25T08:08:55Z]

Hi CBAers,

I just finished the analysis of our vast reservoir of AM CVn data - 
stretching over 25 years of very dense coverage (all "CBA", although 
with significant participation by MDM scopes in a few years).  This has 
provided by far - by light years - the best-ever record of the intricate 
structure of positive and negative superhumps.  But the big goal has 
always been to measure the long-term dP/dt of the *orbital* signal, 
because that would tell us the rate (and direction) of evolution. 
Theory predicts P/(dP/dt) ~ 10^^8 years.

Counting each >3 hour CBA observation as a "night", we have ~1200 nights 
of observation.  We also ~100 nights (mostly KPNO) during 1978-1991, but 
I exclude these, since sparse coverage basically adds more noise than 
signal (and also because the data are on inaccessible media, like tapes, 
punched cards, floppy disks).  With the exception of one 1978 point (a 
week-long run).

The data analysis is lengthy, since to study the orbital signal, you 
have to subtract the more powerful and ever-shifting positive and 
negative superhumps. But, aside from baseball and golf, that's what they 
make summers for.  So I finished... and indeed found dP/dt of the 
expected magnitude.

But of the wrong sign.  The orbital period is not increasing, but 
decreasing, with P/(dP/dt) ~ minus 5x10^^7 years.  This means that AM 
CVn is not a "true" AM CVn star, because the donor star must be 
shrinking on mass loss (in contrast to a true white dwarf).  And it 
suggests that many of the ultracompacts may be of this type also.

It also suggests that we should be increasing our coverage of this 
class.  The best candidate is definitely HP Lib - bright, seasonally 
appropriate, and appropriate for both hemispheres.  I suggest that star 
for a fire-at-will approach, starting right now (Shawn Dvorak has 
already started).  If you don't include airmass in your data report, and 
is convenient for you to include, then please do so.  I might decide to 
correct for differential extinction, since it's a very blue star and 
we'll need some significant range in airmass if we want long tuns (which 
we DEFINITELY do).

Greater than 2.5 airmasses is still a definite no-fly zone, and greater 
than 2.0 is getting dicey (on a good-quality night, you can violate it 
somewhat).

Other new stars for May-June:

V617 Sgr and WX Cen.  Need eclipse timings.  Just a few would be great.
This data is also on the verge of ready-for-prime-time, nut needs a few 
2017 eclipse timings.  Easy project.

CR Boo and V803 Cen.  Close cousins of AM CVn, and definitely worth 
watching.  If they go faint (17th magnitude), then they zoom to the top 
of the priority list. They don't do it much... and if they do, start 
your run immediately, and tell the rest of us.

V418 Ser and RW UMi.  Both about 19th mag, and only included here 
because some people with access to quite large scopes read these 
messages.  Both stars which would handsomely repay a few nights of close 
coverage!  (to dovetail with what we already have)

V4743 Sgr and V1223 Sgr.  Both DQs (IPs) which need a start for a new 
season of coverage.  If you don't mind Sagittarius (most of us 
borealites hate it).

V1974 Cyg.  A little early in the season, but a great northern target if 
you're looking for an morning-sky target.

OV Boo.  Should keep some coverage throughout the summer. What a team! 
(You know who you are out there.)

That'll do for a start.  Enrique will surely add.  Enrique, do you have 
enough now on ES Dra?

I hope to see a bunch of you at the SAS-AAVSO meeting next month in 
Ontario (California)!

joe
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