Issue |
A&A
Volume 382, Number 1, JanuaryIV 2002
|
|
---|---|---|
Page(s) | 124 - 129 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20011627 | |
Published online | 15 January 2002 |
Implications of the HST/FGS parallax of SS Cygni on the disc instability model
1
Universitäts-Sternwarte, Geismarlandstr. 11, 37083 Göttingen, Germany
2
Department of Astronomy, University of Cape Town, Private Bag, Rondebosch 7700, Cape Town, South Africa
Corresponding author: M. R. Schreiber, mschrei@uni-sw.gwdg.de
Received:
2
November
2001
Accepted:
6
November
2001
We analyse the consequences of the recently measured
parallax of SS Cygni (Harrison et al. [CITE]) on the accretion disc
limit cycle model. Using the observed long term light curve of
SS Cyg and , we obtain for the mean mass
transfer rate
.
In addition, we calculate the vertical structure of the accretion disc
taking into account heating of the outer disc by the stream impact.
Comparing the mean accretion rate derived from the observations with
the calculated critical mass transfer rate, we
find that the disc instability model disagrees with the observed long
term light curve of SS Cyg as
is greater or similar to the critical mass
transfer rate.
The failure of the model indicated by this result can be confirmed by
considering that the accretion rate at the onset of the decline should
be exactly equal to the value critical for stability. In contrast to
this prediction of the model, we find that the accretion rate required
to explain the observed visual magnitude at the onset of the decline
must be significantly higher than the critical mass transfer rate.
Our results strongly suggest that either the usually assumed temperature
dependence of the viscosity parameter α is not a realistic description
of the disc viscosity,
that the mass transfer rate in SS Cyg noticeably
increases during the outbursts or, finally, that the HST distance of
pc, is too high.
Key words: accretion, accretion discs / binaries: close / stars: individual: SS Cygni / novae, cataclysmic variables
© ESO, 2002
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