Among those stars:
T Pyx done for the season. We have enough to nail down the ephemeris
- and hopefully the physics!
HP Lib and CR Boo: It's important to start the HP Lib season, which
would be about now (late March). But about a week of coverage would
nail down the early-season phase of the periodic signal(s??). After
that, CR Boo wins. Because it has outbursts, unlike AM CVn and HP
Lib, it's hard to say in advance what the priority-setting standard is
(are outbursts more or less informative than quiescence?) Except for
one thing: its transitions to 17th magnitude, which used to be rare
and probably still are, are of VERY high interest, A few weeks of
coverage will make these priority decisions easier.
There are some other southern stars asking for coverage.
1. IM Nor. Got a paper almost ready to roll. A few 2019 eclipse
timings will fortify the long-term ephemeris. It's far-southern and
normally mag 18 out of eclipse, so likely only in Gordon's territory.
2. WX Cen. Another "V Sge" star, like IM Nor and T Pyx, with very
rapid change in Porb. A bright star, friendly for all scopes... but
very desirous of long runs, since Porb is long and the eclipses
somewhat shallow (~0.5 mag).
3. NR TrA. We've been tracing out the orbital light curve since its
2008 outburst. Let's get a high-quality result for 2019!
4. V407 Lup. Time to start the season on this fascinating and
possibly unique star, a 2016 nova. Paper in an advanced state on this
guy, but needing another season.
We've gotten great results from our surveys of the post-eruption
orbital light curves, in general. Koji Mukai's list of recent novae
is an excellent source of material to study:
https://asd.gsfc.nasa.gov/Koji.Mukai/novae/novae.html
The great majority of these targets are desirous of LONG
observations... but the AM CVn stars are less finicky than the others.
joe p
On 3/24/2019 11:54 PM, Gordon Myers wrote:
Joe, what's priority among CR Boo, HP Lib and T Pyx?
Gordon
-----Original Message-----
From: cba-chat <cba-chat-bounces@xxxxxxxxxxx> On Behalf Of Joe Patterson
Sent: Sunday, March 24, 2019 8:05 AM
To: cba-news@xxxxxxxxxxx; cba-chat@xxxxxxxxxxx
Subject: (cba:chat) the "ultracompact" project: AM CVn, HP Lib, CR Boo
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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