[vsnet-chat 459] V1974 Cyg Campaign - critical value - answer to Taichi Kato

[Alon Retter <alon@wise1.tau.ac.il>]

>    What does this "critical value" mean?  The lower limit of the surface
> density (of the disk) which can maintain the hot state?  Will the nova
> become an SU UMa-type dwarf nova afterwards?
> 
> Regards,
> Taichi Kato
> 
------------------------------------------------------------------------
Osaki's equation (PASP 1996, 108, 39, equation #4) fixes the border
line between the thermally stable and thermally unstable disc.  For
systems below the period gap, it is the border between the permanent
superhumps systems and the SU UMa stars.

For Nova Cyg 1992, with Porb=2hr it results in a critical Mdot=8^16
gr/sec.  Now, one can assume that the disc is the main light source,
and using constraints on the nova parameters (Mwd, d, ...), one gets a
critical visual mag. For Nova Cygni, at about Mv=16 now-days, we got a
range of 17.5-18.5, which is well above its 19.5 pre-outburst
brightness.

The meaning of these calculations is that when the nova decays by
1.5-2.5 more mag. (or before if the other sources contribute
significantly to the light curve), the critical line will be crossed -
the permanent superhumps nova will evolve towards an SU UMa system -
regular superhumps!  To our knowledge this fascinating scenario has
never been observed before.

Similar calculations for the 2 other permanent superhumps novae (CP Pup
1942 and V603 Aql 1918) yield critical values, which are below their
present quiescent states, and this explains the presence of permanent
superhumps in these systems. An alternative possibility for the fate of
Nova Cygni 92 is that the system will be stuck in a higher state than
its pre-nova luminosity. This might be the case in CP Pup and V603 Aql.

We are writing a letter to the MNRAS on this subject. It will probably
be prepared in a week or so. I'll mail it to the vsnet as soon as it is
ready. I would like to use this occasion to thank Prof. Osaki for
discussing this issue with me in Wyoming and afterwards.

Best wishes                       Alon Retter

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