[vsnet-chat 4027] on the presence of helium novae (V445 Pup)

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

Dear vsneters,

I've been following with great interest the eruption of V445 Pup,
the extended discussion on its peculiarities, and the suggestion
by Kato that it might be a helium nova. I apologize for my reply
that has been delayed due to various reasons. Dr. Kato might remember
that I speculated the presence of helium novae in a talk given in
the Kyoto conference in 1998. The presence of helium novae is indeed
speculative, and most theoreticians believe that it cannot exist.
I appreciate the work that has been done by Taam, Mariko Kato, Nomoto,
Livne, Woosley and others. However, it should be noted that according
to (Mariko) Kato, Saio & Hachisu  (1989) a helium nova is possible if
the mass transfer rates are very high - ~5^{-8} Msolar/yr. This limit
exceeds by 2 factors the values estimated for the AM CVn systems. We
should all remember, however, that theoreticians (as well as observers)
have been wrong many times in the past. In fact, they have `killed' the
existence of magnetic novae. These systems are widely accepted nowadays.
In addition, old models suggested that a SN detonation is inevitable in
light (~0.4 Msolar) helium (but with hydrogen) white dwarf primaries,
while modern models showed that a nova outburst is possible. Nova
outbursts were also rejected for CVs with high mass transfer rates, while
new models permit them in these systems. Therefore, I think that we
shouldn't believe the theoreticians about this point, and should still
search for helium novae. As there are only 7 or 8 recorded AM CVn systems,
it is very likely that it would be very hard though to find such a beast.

As for V445 Pup, since we don't have any simple explanation for its
observational behaviour, it is my personal opinion, that we cannot reject
this model for the object. The helium nova scenario for V445 Pup can be
easily ruled out in the future, if in late spectroscopy hydrogen lines
will be found, or if we won't be able to detect a short term periodicity
in its light curve. I think that at the moment it might be, however, too
early to observe the binary system, so the failure to detect a short term
periodicity in V445 Pup still doesn't rule out this idea.

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