[vsnet-chat 3429] Re: Permanent Superhumpers...

[Alon Retter <ar@astro.keele.ac.uk>]

>    As I understand, exactly speaking, "permanent superhumps" and "permanent
> superhumpers" are a different entity.  An object can show "permanent"
> superhumps, in constrast to dwarf nova superhumps, when a certain instability
> condition -- it's not a great surprize since there had been already a number
> of calculations (dating back to Whitehurst 1988) which determine the
> instability criterion.  This possibility was sought by several authors,
> including Osaki 1989 [I personally thought of one for V795 Her, too]
> However, the concept of "permanent superhumpers" is in the framework of
> Osaki's unified theory.  At that time of our knowledge, it was a surprize
> to find systems (other than postnovae) below the period gap, which can
> permanently attain high-mass transfer to exhibit superhumps.  It is still
> a mystery, as you know.  Although there was a suggestion of superhumps,
> there had been a long standing discussion between models of intermediate
> polars and preceessing disks to explain the phenomenon (e.g. see the
> history of V795 Her).  As I understand, the final unambiguous solution
> was achieved by Osaki's unification theory and his numerical simulations
> with Hirose and Ichikawa, powered by the discovery of ER UMa stars in 1994,
> the "missing link" between SU UMa stars and supposed permanent superhumpers.
> The concept of "permanent superhumpers" was thus established by Osaki's
> theory.  When we speak of permanent superhumps in long-period novalike
> systems, we don't usually use the category "permanent superhumpers".

To my opinion permanent superhumps occur in permanent superhumpers. 
They occur when the system is tidally and thermally unstable. I agree 
that there is some confusion in terminology as the field is relatively 
new. For example is a permanent superhumper a nova like or are these two
groups separate? One can regard permanent superhumpers as a subclass
of nova likes (as SU UMa systems are a subclass of dwarf novae) or might
say that permanent superhumpers are tidally unstable (as SU UMa systems),
while nova likes are tidally stable (as U Gem systems). One might also
say that a VY Scl system that show permanent superhumps is not a permanent
superhumper as it doesn't show (as far as I know) superhumps during the
low phase. I suggest that the system is both a permanent superhumper and
a VY Scl system (as V446 Her is both a post-nova and a dwarf nova). It is
really confusing. 

As far as I understand there is no difference between a permanent
superhumper below the gap and above the gap. Osaki suggested that
the period gap limits between tidally stable and unstable systems based
on the theoretical relation between the orbital period and the secondary
mass. Thus CVs above the gap should have mass ratio larger than the limit
(then thought to be 0.25), while observations show that you can have small
mass ratios above the gap, and the limit was modified to q=0.33 by further
simulations (I bet that in the future it will be larger - see TV Col).

As to the presence of permanent superhumpers below the gap, there is no
problem with that according to the general ideas. Indeed one cannot
explain their presence (temporal conservation of angular momentum) if
their current mass transfer rates represent their mean values (meaning 
that they are permanently permanent superhump systems), however people
believe that they spend most of the time at low mass transfer rates.
Thus they must have mass transfer cycles. Indeed, the observations seem 
to support these cycles. Note also that the only `nova-like' system above
the gap is BK Lyn that probably had a nova event in the year 101. So, it
is likely that the nova outburst can change the thermal stability of 
SU UMa system, and turn them into permanent superhump systems for a while
(which is presumably short compare with the life time of the system).

Let, me finally say, that I believe that we should expect to find many
more CVs with orbital periods below the period gap that show permanent
superhumps. In addition the ideas I have described in the previous section
might be found wrong.

Regards,
Alon
-------------------------------------------------------------------------
   Dr. Alon Retter         Tl. (work)     +44-(0)1782-58-3493
   Physics Dept.           Fax  (work)     +44-(0)1782-711093
   Keele University        -----------------------------------
   Staffordshire           'As a scientist I don't believe myself, so
   ST5 5BG, U.K.            why should I believe you?' (A.R. 1965-2085)
-------------------------------------------------------------------------

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp