[vsnet-alert 1137] Re: AB Dra entering standstill?

[Fred Ringwald <ringwald@astro.psu.edu>]

Dear Observers,

Please watch AB Dra carefully.  Some time ago I started compiling light
curves of Z Cam stars (see vsnet 517, 1996 February 6) (Z Cam stars are
defined by having flat standstills about one magnitude fainter than
outburst maximum, which last for months or even years.) Guy Hurst kindly
sent me the BAA light curves of several Z Cam stars, including that of AB
Dra, which extends as far back as 1969.  Guy pointed out that it didn't
show any standstills, and wondered whether AB Dra should be classified as
a Z Cam star.  I wonder, too, since it has an unusually short orbital
period, of 3.6475 hours (Thorstensen & Freed, 1985): most other Z Cam
stars have orbital periods between 5 and 7 hours. 

If AB Dra really is going into standstill, it would be significant, since
it would remove these observational doubts.  It would be theoretically
important, too.  The standard model explaining standstills (Meyer &
Meyer-Hofmeister 1983) hypothesizes that Z Cam stars have average
mass-transfer rates near that of the cutoff between dwarf novae and
nova-likes, which do not have outbursts; they can be thought of as dwarf
novae stuck in outburst all the time.  Z Cam stars, therefore, are thought
to get stuck in outburst only some of the time: these are standstills. 

An outburst might therefore trigger the mass transfer over this threshold,
making it a nova-like for a while, until it runs out of gas, the
atmosphere of the mass-losing star becomes unable to supply it fast
enough.  Still unexplained is why the standstills occur one magnitude
fainter than maximum, rather than at maximum; and why standstills only a
few months or years, and not 10^5 years, the diffusion timescale of a red
dwarf atmosphere.  If average mass transfer rate is a function of orbital
period (Patterson 1984), knowing that AB Dra is a Z Cam star would provide
a unique constraint on physical models for Z Cam stars. 

Thorough photometric monitoring of AB Dra (every night you can, even more
than once per night if you can) over the next few nights would be
important, to check whether this really is a standstill.  If it is, less
thorough (several times per week, but nevertheless consistent), monitoring
would be important, to see how long it lasts.  The light cuve might also
show small outbursts through the standstill (see Szkody & Mattei (1984),
for an example). 

Alternatively, the light curve might more resemble that of WW Cet---which
has in the past been called a Z Cam star, but it doesn't display an
essential property of standstills: it never stands still. It wanders,
outside of outburst, by well over one magnitude.  For light curves of WW
Cet, see Ringwald et al. (1996), for which I got vsnet data kindly
provided by Kato-san...and the BAA, VSS,RASNZ, and AAVSO, thanks very much
for all your observing effort. 

Checking the list of AAVSO Program Stars kindly provided by Janet Mattei,
as well as Szkody & Mattei (1984), shows that Z Cam varies between 11.0 <
V < 15.3.  The present hangup, however, is near m_v = 13.6.  I have no
doubt this is real, since it's represented by several points, but this is
much fainter than one magnitude fainter than outburst maximum.  I
therefore think it's likely this is *not* a Z Cam standstill, but perhaps
more like the behavior of WW Cet, or perhaps something new.  Only
continued observations can tell, of course...Best,


References

Meyer, F., & Meyer-Hofmeister, E. 1983, Astronomy & Astrophysics, v. 121,
p. 29

Patterson, J. 1984, the Astrophysical Journal Supplement Series, v. 54, p.
443

Ringwald, F. A., Thorstensen, J. R., Honeycutt, R. K., & Smith, R. C. 
1996, the Astronomical Journal, 111, 2077

Szkody, P., & Mattei, J. A. 1984, Publications of the Astronomical Society
of the Pacific, v. 96, p. 988

Thorstensen, J. R., & Freed, I. W. 1985, the Astronomical Journal, 90,
2082

=======================================================================
Fred Ringwald                    ... to further the progress of science,
Dept. of Astronomy & Astrophysics    to guide to an understanding
The Pennsylvania State University         of the majesty of the heavens,
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Email: ringwald@astro.psu.edu   http://vsnet.astro.psu.edu/users/ringwald/

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