(cba:news) more on bk lyn... the magnificent
jop at astro.columbia.edu
Wed Apr 6 16:07:11 EDT 2011
I wanted to talk a little more about this star, which I'm now writing
up after several intensive years of study. It's probably the most
interesting of the small family of negative superhumpers.
In what was basically the first CBA paper, Skillman & Patterson
(1993) found a common superhump: a 0.05 mag signal at a period 5% longer
than Porb. In retrospect, no great surprise there - we know now that
essentially all stars of sufficiently high Mdot and sufficiently short
Porb manufacture such signals. We went back to BK Lyn with 50-day
campaigns in 1999 and 2002, and short campaigns in two other years. The
two long campaigns showed a complex and fascinating power spectrum: very
strong signals at 0.394 and 13.739 c/d, and weaker but definite and
sharp signals at 12.760, 13.344 (the radial-velocity frequency), 25.521,
and 27.475 c/d. Each with an error of 0.002 c/d. It was confusing at
first, since the two superhumps are displaced by ~1.0 c/d - so if you
don't have very good frequency resolution, they look like simple daily
aliases. In my lexicon (not in general use yet), the signals are at N,
wo+N, wo-A, wo, 2(wo-A), and 2(wo+N). And the superhumps are
*extremely* stable: basically perfect over the ~50-day baselines.
Other than a few mentions in conferences, I still - shamefully -
haven't written this stuff up. But I am now!
And so it goes this year, too. Because of competition in the evening
sky (mainly YZ Cnc and ER UMa), the coverage hasn't been great - except
for that of Josch Hambsch. Though the 2011 coverage is sparse compared
to previous years, there appears to have been an outburst this year,
around March 29. I can't tell for sure since we haven't standardized
our comparison stars (and there's not enough overlap to calibrate
'em)... but I think so. This could be of great interest, since it could
be a distance calibrator (if it's substantially like a dwarf-nova), and
also because it may offer some insight into why the star selects between
apsidal and nodal superhumps.
I say all this to encourage a few more weeks of work on BK Lyn before
saying goodbye for the year. Also, it would be nice to get calibrated
broadband colors of the star (which we never have).
The stability of the nodal superhump is unprecedented - to the point
of being suspicious. We should follow the signal deep into evening
twilight in May, and pick it up again in morning twilight in October -
to see if it is actually phase-stable over the six months. And if it
is, then we can keep stretching the baseline to get a good measure of
BK Lyn is a pretty bright star: usually 14.5-15.5 and with beautiful
periodic signals. A light curve suitable for framing!
In the south, Berto's newly found SU UMa star (OT J132900.9+365859, a
Catalina-discovered transient) is a very good target.
I'm still confused about DR UMa. Anyone want to chime in? Is this a
good target, or not?
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