[vsnet-alert 16039] stage A superhump paper and project

Taichi Kato tkato at kusastro.kyoto-u.ac.jp
Tue Jul 23 11:14:21 JST 2013

   Here is our new paper.

   Now we should declare the "stage A superhump project" !

   The basic idea is to "observe as many stage A superhumps as
possible" to determine mass ratios.  This method can be applied
to almost all SU UMa-type dwarf novae (if the orbital period
is known). 

   So our strategy is:

   (1) Detect superoutbursts (or precursors) as early as possible 
(best if they are caught on the rise or during the precursor).

   (2) Make announcement (please post to vsnet-alert) as soon as possible.

   Recent ASAS-SN CV detection may be able to detect outburst in 
the very beginning.  So please let us know outbursts (even if they are 
already known objects) of SU UMa-type dwarf novae as soon as possible 
(Dr. Stanek, simply post a message to vsnet-alert as soon as you detect 
one, rather than a daily batch).

   (3) Make time-series photometry as soon as possible.  Typical
duration of stage A is a few days, so the observation on the initial
day of the outburst (detection) is crucial.  World-wide participation
is a key to this project!  Please start observing if you catch
information of the outburst, even if there is not my request yet.

   (4) Once a transition to stage B is confirmed, we can relax
a bit.  This is the different point from the past project;
people tend to start observing when superhumps become evident,
and there are a lot of observations after the peak of the superhump
amplitude.  What is needed in the present project is the growing
stage of superhumps (i.e. when superhumps are not always clearly seen).
Of course, stage B superhumps and the origin of Pdot is still
poorly understood, and keep observing until the object fades.

   (5) As written in the paper, post-superoutburst superhumps
may give clue to the mass ratio, too.  So observations in this
stage are also important.

   (6) The priority targets are the object with known orbital
periods (you can check RKCat for this).  There are many objects
whose orbital periods are known, but without stage A observations.
Please pay attention to "our old friends", too.
There are many southern hemisphere SU UMa stars, but are not
sufficiently observed in relation to the modern superhump stages.
Early detections e.g. by Rod Stubbings and prompt follow-up
time-resolved photometry e.g. by Josch Hambsch or Berto Monard
will greatly improve the situation.
The majority of WZ Sge-type dwarf novae show early superhumps,
so WZ Sge-type candidates can be regarded as objects with
known orbital periods.  (So please observe ASAS-SN CVs as quickly
as they are announced!  CRTS CVs have longer delays in detection,
and ASAS-SN CVs may be better targets.  MASTER CVs appear to be
between them).  Other SU UMa-type dwarf novae without
known orbital periods should be observed as well, since
their orbital periods can be determined by later observations
(or even detectable during the superoutburst), but observable
stage A superhumps may not be so frequent.


From: Taichi Kato <tkato at kusastro.kyoto-u.ac.jp>
Date: Mon, 22 Jul 2013 03:55:14 GMT   (137kb)

Title: New Method to Estimate Binary Mass Ratios by Using Superhumps
Authors: Taichi Kato (Kyoto U.), Yoji Osaki (U. of Tokyo)
Categories: astro-ph.SR
Comments: 13 pages, 10 figures, accepted for publication in PASJ
License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
  We propose a new dynamical method to estimate binary mass ratios by using the
period of superhumps in SU UMa-type dwarf novae during the growing stage (the
stage A superhumps). This method is based on a working hypothesis in which the
period of the superhumps at the growing stage is determined by the dynamical
precession rate at the 3:1 resonance radius, a picture suggested in our new
interpretation of the superhump period evolution during the superoutburst
(Osaki, Kato 2013, arXiv:1305.5877). By comparison with the objects with known
mass ratios, we show that our method can provide sufficiently accurate mass
ratios comparable to those obtained by quiescent eclipse observations. This
method is very advantageous in that it requires neither eclipses, nor an
experimental calibration. It is particularly suited for exploring the low
mass-ratio end of the evolution of cataclysmic variables, where the secondary
is undetectable by conventional methods. Our analysis suggests that previous
estimates of mass ratios using superhump periods during superoutburst were
systematically underestimated for low mass-ratio systems and we provided a new
calibration. It suggests that most of WZ Sge-type dwarf novae have secondaries
close to the border of the lower main-sequence and brown dwarfs, and most of
the objects have not yet reached the evolutionary stage of period bouncers. Our
result is not in contradiction with an assumption that the observed minimum
period (~77 min) of ordinary hydrogen-rich cataclysmic variables is indeed the
period minimum. We highlight the importance of early observation of stage A
superhumps and propose a future desirable strategy of observation.

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