[vsnet-alert 22212] Re: SDSS J141118.31+481257.6: stage transition

[Taichi Kato]

Re: SDSS J141118.31+481257.6: stage transition

> then the object has continued to show stage B superhumps with
> a period of 0.03216 (1) d.

   Note that this period is extremely close to that of
early superhumps (0.0321 d in vsnet-alert 22181), which
must be very close to the orbital period.

   As pointed out in Kato and Osaki (2013) PASJ 2013, 65,
superhump periods can become short than the orbital one
if the mass ratio is low enough and pressure effect
is considered (in the paper, q < 0.035 objects can have
superhump periods short than the orbital one).

   This finding illustrates the importance of the pressure
effect in interpreting fractional superhump excesses.
Traditional methods like Patterson (1998) 110, 1132
and Patterson et al. (2005) 117, 1204 did not consider
this effect and assumed the fractional superhump excess
becomes zero to the limit of q=0.  As stated in
Kato and Osaki (2013), these traditional methods give
systematically lower q values for very low-q objects.
This issue appears to be confirmed by the finding
in SDSS J141118.31+481257.6 (let's apply the traditional
method to this object and see how ridiculous the resultant
value is!).  The preliminary period of stage A superhumps
suggests q=0.047 and it is consistent with other periods
(expected fractional superhump excess for stage B superhumps
when the pressure effect is considered).

   The important lesson: the traditional method like
Patterson (1998) gives systematically biased q values
for low-q objects, and should not be used for AM CVn-type
superoutbursts.  The same would be true for period bouncers,
which also have low-q values.