(cba:news) AM CVn and HP Lib... and new stars for June

Joe Patterson jop at astro.columbia.edu
Fri Jun 14 22:17:53 EDT 2019

Hi CBAers,

Old and new campaigns.....

1. AM CVn.  Great coverage all around, and I've used it to establish a 
unique cycle count for the orbital signal, going back to 1978.  The 
period is slowly decreasing... and we can definitely quit now.

2. HP Lib.  Another winner.  No long-term ephemeris (yet), but a clear 
detection of the orbital signal, despite its meager amplitude (0.005 mag 
peak-to-trough).  Amazing how crowd-sourcing brings out the really weak 
signals.  Still a worthy target for southerners (~ another month?) , but 
getting borderline for northerners.

FOR BOTH OF THESE STARS (AM and HP): if you've been observing these 
stars, can you study your data for presence/absence of night-to-night 
variability, and send me the result?  My time-series analysis is 
maximally sensitive if I subtract the mean light (and trends).  And 
because different people observe with different filters, airmasses, and 
comparison stars, there's no straightforward way to compare them.  You 
can, though, with your own data.  I'm basically interested in just one 
thing:  how much night-to-night variability do these two stars have? 
Not within the time series, but *night-to-night*.  It's very low, but 
HOW low?

Because both stars are very blue, they will appear slightly fainter at 
higher airmass.  I estimate a likely 0.08 mag/airmass correction (if 
you're inclined to apply it).  BTW this is a correction to the 
*differential* magnitude (it would be zero if the comparison star were 
equally blue).  This correction is not worth fussing over, if you 
observe with under fairly repeatable circumstances (airmass and 
comparison star).

3. CR BOO. Time to quit on this one.  Unlike the other two helium 
variables, this one varies a lot - fast, slow, periodic, nonperiodic - 
and it's a project to figure it out.  It has never done us the courtesy 
of staying put for a few weeks on end.

And new stars (first installment).

4. MAXI J1820+070 = ASASSN-18ey, the great X-ray nova of 2018.  Back at 
18th mag now... but there's a good chance of seeing the orbital signal 
now, since the X-rays are gone.  We know the period is *near* 17 hours 
(because of the superhumps), but now the star is probably ripe for 
measurement of the actual Porb.

5. V339 DEL.  A recent nova ripe for Porb measurement.

6. V1974 CYGNI.  One of our favorites.  Dubbed "the nova of the century" 
in 1992.  Let's make it surrender some secrets *this* century, too. 
This star has three clocks: orbit, positive superhump, negative superhump.

Other good seasonal stars in a day or two.  As usual, an optimum 
strategy for these stars is: pick a favorite, and follow as long as 
airmass and sky conditions tolerate.


Also, since most of you observe (mostly) unfiltered, can you take an 
hour to observe the star through a V filter and establish a calibrated V 
brightness.  No time variability needed, just one number!

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