(cba:news) AM CVn - one long St. Patrick's night
Joe Patterson
jop at astro.columbia.edu
Sat Mar 20 03:42:45 EDT 2021
Hi cbaers,
More chat than news, but...
Walt Cooney just sent a very long AM CVn time series. I wanted to show
it, to illustrate how I analyze this (and nearly all) data. The
original light curve is AM9291WC.TXT. Plot it up. You'll see the 8.5
minute wiggles which made this star so famous back in the 1970s. Their
amplitude appears to vary through the night... and it probably does, but
most of that is due to interference by the other periodic signals, which
are too weak to see in isolation, but show up in a power spectrum. The
main signals are:
525 s = positive (apsidal) superhump (actually a double-humped 1051 s)
1011 s = negatuve (nodal) superhump
1028 s = true orbital period
Then calculate a power spectrum, or periodogram, or whatever
period-finding gadget you might have available. PERANSO is a good choice.
The single-night light curve does is not quite long enough to resolve
the 1028 and 1011 s signals, but combining with adjacent nights makes it
obvious that these are the basic periods. In our analysis (read the
Skillman et al. 1999 paper on AM CVn if you're deeply interested), we
typically combine data on consecutive nights to get fine frequency
resolution.
AM9291WC.TXT also shows a slow brightening towards the middle of the
night, then fading after ~1 a.m. (meridian passage). This arises from
differential extinction (AM CVn is a VERY blue star). The CV is near
2.0 airmasses at the beginning, 1.0 near meridian passage, and 1.7 as
the end. If you study this a little, you'll see that the differential
extinction is about 0.06 mag/airmass. If we were really diligent
photometrists, we would carefully measure that and correct for it. But
we're not; our typical atmospheric conditions are just not good enough
to rely on such slow wiggles as being strictly due to airmass. (This
was an exceptionally good night - that's why I chose it.)
So my common practice is to subtract the very slow variations which are
at least in part due to airmass, as well as the mean magnitude. In that
way I obtain a "zeroed" light curve (AM9291WCZ.TXT). +This* (always the
Z files) is what gets spliced with adjacent nights to get a really
accurate value of the period.
If you look at the Z file, you'll see a few wiggles that haven't been
removed. Are they real, or atmospheric, or what? Who knows? Probably
real; after all, this is a CV (though the least "C" and the least "V" of
any such star in the sky). Anyway, these wiggles are small and
irregular, so they probably won't do much damage.
If you're deeply interested in such things and would like to do the
analysis, I'll send you a spate of z files, and you can compute away.
It's very addictive, though.
JOE
p.s. Don't be intimidated by the data quality here. THis was an
excellent night from an excellent observer at an excellent site. The
technical term for such a thing in the Astrophysical Journal is "a
typical light curve".
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