(cba:news) the "ultracompact" project: AM CVn, HP Lib, CR Boo

[Joe Patterson]

Hi CBAers,

I wanted to write in some detail about the AM CVn project underway.  In 
our Skillman et al. 1999, we spelled out the star's basic period 
structure in full detail: an orbital period of 1028.7322 s, a positive 
superhump at 525.6 s (actually its "subharmonic" at 1051.2), and a 
negative superhump at 1011.4 s.  That paper also listed many other sum 
and difference frequencies present at lower amplitude; but these are the 
basics.

Many theoretical and observational papers on superhumping CVs have 
ascertained the origin of positive superhumps: an eccentric instability 
at the 3:1 orbital resonance in the disk.  Because it's seated in the 
disk, it doesn't have high stability, but wanders in phase on a 
timescale of days to weeks.  The orbital signal is weaker (just 0.008 
mag full amplitude), which is why we're the only people (so far; TESS 
will see it easily) who have ever detected it.  Nevertheless, we've 
obtained timings over 27 years (at least) and found the orbital period 
slowly decreasing, on a timescale of 60 million years.  This arises from 
a combination of mass transfer (which increases the period) and 
gravitational radiation (which decreases the period).  AM CVn is 
expected to be one of the brightest steady Galactic sources in the next 
generation of space-based gravitational-wave detectors.

Thanks in no small measure to David Cejudo's relentless coverage from 
Madrid, the 2019 coverage has confirmed the Porb change - our main goal. 
  Great!  But now we're within reach of another goal, not yet achieved 
for any star.  Do the positive and negative superhumps change their 
periods in phase, in anti-phase, or uncorrelated?   If the *negative* 
superhump arises also at the 3:1 resonance (not known!), then it's 
reasonable to expect that the signals will be in anti-phase.  Any other 
result... well, we'll have to think that one through!

I doubt that any other star will enable such a test, because AM CVn's 
constancy in brightness is unparalleled (basically within 0.02-0.03 
mag).  So let's KEEP IT GOING - at least another month.

BTW the preferred term nowadays is "ultracompacts", rather than AM CVns, 
because there is also a collection of similar binaries (Porb<50 min) 
which have neutron-star or black-hole accretors.  Much less is known 
about them, but the physics is likely identical (except very, very close 
to the accretor).

Two other AM CVns (oops, ultracompacts) are candidates, and bright 
enough for a similar, but less detailed, test.  These are HP Lib and CR 
Boo.  Negative superhumps haven't even been *discovered* in these stars 
(yet), so we have a way to go.  Nor is the orbital period known for 
sure.  But we can find these things, if they're present at anything 
exceeding ~0.003 mag.  Behold the power of time series!  They're also 
seasonally appropriate, and with friendly coordinates - accessible to 
everyone.  Let's get going on these stars!

joe p

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