(cba:news) CR Boo, HP Lib, IM Nor, WX Cen, NR TrA, V407 Lup

[Joe Patterson]

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 at cbastro.org> On Behalf Of Joe Patterson
> Sent: Sunday, March 24, 2019 8:05 AM
> To: cba-news at cbastro.org; cba-chat at cbastro.org
> 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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