(cba:news) comments re HP Lib... and AM CVn
jop at astro.columbia.edu
Tue Jun 18 11:01:40 EDT 2019
These stars - one celebrated, and the other deserving to be - are really
unique among CVs. Their orbital periods are very short of course (17
and 18 minutes), they have dominant superhumps, and the spectra sre
basically pure helium. But the property I find most remarkable, and
never commented on, is the lack of variability in the light curve -
apart from the orbital and superhump periods. The "flickering" never
exceeds 0.02 mag, and night-to-night variations never exceed ~0.05 mag.
The underlying reason for such constancy is a puzzle, but not yet one
with a convincing explanation. I thought I would append one long night
of HP Lib light curve, courtesy of Gordon Myers, to illustrate what the
raw light curve looks like. You can see the effect of atmospheric
extinction (actually differential extinction, arising from the bluer
color - and thus greater extinction - of HP Lib). To analyze the light
curve(s) over weeks, as we do, we (I) have to correct for this
extinction - although sometimes by simply *deleting* high-airmass
observations, since data quality sometimes degrades quickly there.
Anyway, you can see the 0.05 mag superhumps, and the extinction. With a
lot of data spread around the world, you can dig out the orbital signal
as well, although it's ~7X weaker (and thus invisible, except in a power
spectrum). Gordon and Berto have been carrying most of the water on
this one, with LONG time series.
Great evening target in April-May-June.
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