(cba:news) more on bk lyn... the magnificent

Joe Patterson jop at astro.columbia.edu
Wed Apr 6 16:07:11 EDT 2011

Dear CBAers,

   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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