[vsnet-chat 2663] Re: SU UMa stars

[Taichi Kato <tkato>]

Re: SU UMa stars

> I'm a little intrigued by the tendency to call all stars which show hump
> structure during outbursts SU UMa stars. The GCVS defines these as stars
> with short and long outbursts - normal and super outbursts. I've always
> thought this confusing as stars such as BV Cen have both short and long
> outbursts but don't appear to have the superhump phenomenon.

   Note that GCVS classification was written more than ten years ago.
This inevitably includes a number of "UGSU" stars with no observed superhumps
and a number of "UGSS" stars, which are now known as SU UMa stars.

   The cuurent classification of SU UMa stars at least requires superhumps
during long outbursts (cf. reviews like Warner 1995, Osaki 1996 etc.).
BV Cen may be another good example of "inside-out" type disk instability,
associated with stagnation periods.  The northern counterpart may be GK Per
(see Kim et al. 1992, ApJ 384, 269).  Other long-period dwarf novae with
a low outburst frequency more or less tend to show similar behavior.

> Getting back to RXJ as above, has anyone found a normal outburst? Or can
> anyone produce an orbital period to compare with the hump period? Does it
> show the change in superhump period common in stars such as VW Hydri? Apart
> from the steep rise to start each hump they resemble orbital humps with
> their strong flickering. So is it really an SU UMa star or is it, like many
> others which don't seem to have normal outbursts, a separate type of
> object?

   Almost all (well-studied) dwarf novae showing prominent hump structures
during long outburst have been confirmed to have a diffenent underlying
orbital period.  And most of them also show short outbursts.  We can
therefore safely attribute the modulation to superhumps, and expect
a slightly shorter orbital period, and missed normal outbursts.
Exceptions to this view are, of course, keenly sought.

> The large amplitude objects of this type are called WZ Sagittae
> objects (it would be nice to have a southern hemisphere prototype for a
> change) but this seems to be a low amplitude object. 

   There may be a new class of low-amplitude SU UMa-type systems, which
only show infrequent superoutbursts.  Or shall we call them low-amplitude
TOADs ;-)

Regards,
Taichi Kato

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