(cba:news) the "ultracompact" project: AM CVn, HP Lib, CR Boo
jop at astro.columbia.edu
Sun Mar 24 11:04:31 EDT 2019
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
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!
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