[vsnet-chat 357] Re: Last-Chance Saloon of CVs

[Taichi Kato <tkato>]

Re Joe Patterson's comment in [vsnet-chat 356]:

  I mostly use these terms (ER UMa stars and WZ Sge stars) just in the
sense:

> Now these many terms are fine for discussion among experts, since 
> they summarize observable differences.

  I do not object using "SU UMa stars" when referring to these dwarf
novae as a whole; actually I use abbreviations in VSNET CV circulars
"UGSU(ER)" and not "UGER", which some people have suggested.

  My standpoint on ER UMa stars and WZ Sge stars are almost just the
same as Thorstensen et al. (1997) wrote in PASP 477, 482: "we will
provisionally refer to this subgroup as ER UMa stars, while reserving
judgement as to whether they present a physically distinct class".
It is still in debate by a number of researchers whether there are
(observationally) continuous transitions betwwen ER UMa stars and
"classical" SU UMa stars, and WZ Sge stars and classical SU UMa stars.
Though I am not sure what Patterson's "plotting the observable quantities"
has resulted, a simple 2-D plot of Porb(Psh) vs. supercycle (as presented
in Daisaku Nogami's poster in the Keele CV Conference) shows a smooth
evolutionary track for well-known classical SU UMa stars and WZ Sge stars,
but not for what is called ER UMa stars.  The interpretation seems to me
still open whether this means a distinction, or a (continuous) diversity
predominantly among short period system.  I hope Nogami-san will discuss
on this matter in more details in the coming Wyoming Conference (and make
some comments in this thread).

  As you probably know, there is some theoretical motivation to discriminate
ER UMa and related systems from "classical" SU UMa systems.  Classical
SU UMa stars generally obey what is predicted for the standard picture
of nonmagnetic CVs below the period gap: outbursts governed by the combination
of thermal and tidal instabilities, mass-transfer by gravitational wave
radiation (GWR).  ER UMa stars seem to violate this picture in some points:
the required mass-transfer rates a few to ten times higher than the GWR
theory predicts.  RZ LMi (and DI UMa) further needs an assumption (cf. Osaki
1995, PASJ 47, L25).  The reasons why only these stars require such abnormal
input conditions -- the problem whether they reflect a difference in
underlying physics, stellar systems or evolutionary paths is one of the
main foci of the discussions on ER UMa stars.  To me WZ Sge stars seem to
make a more smooth transition to classical SU UMa stars, within the
framework of standard CV evolution.  We can still point out which star
is of ER UMa-type, but the distinction between WZ Sge stars and usual SU UMa
stars has apparently become more vague than ever (probably most of visual
observsers may agree in this point).  However, we still use the term
WZ Sge-type dwarf novae since this "summarizes observable differences",
also for which we are searching for explanations from the theoretical side.
The situation might become similar for ER UMa stars, if systems having
supercycle lengths between 50 days and 80 days are discovered (to my
knownledge that range is lacking dwarf novae, just as in the famous
"period gap" in CV period distribution), and are found to comprise a
continuous spectrum.  Even that time, we will be continuing to understand
what makes these objects such active, or having such high mass-transfer
rates, than the standard picture predicts.

  Regarding excessive Balkanization of subclasses, one of our VSOLJ
observers once complained that the current (UGSU, UGSS, UGZ) classification
of dwarf novae are extremely too rough.  He instead proposed UGAL (AL Com
stars), UGUZ (UZ Boo stars), UGVY (VY Aqr stars), UGHT (HT Cas stars)... ;-)
(some description exists in the past documents on the VSNET Home Page,
but unfortunately or fortunately enough they were written in Japanese).

  Looking forward to seeing you and your colleagues again at Wyoming!

Regards,
Taichi Kato

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