BT Mon: eclipser with a hump
Observed: 26 Oct, 1 Nov, 24 Dec 2005, 19 Jan 2006
4 Feb 2006
Light curves of this cataclysmic star are presented. They show
the eclipse with a large hump. They are modelled with BinaryMaker3.
BT Mon is a cataclysmic variable, that is a binary with an accreting
white dwarf. The orbital period is 8h. In 1939, BT Mon exploded as a nova
(N Mon 1939) and today a faint shell can be observed around it (Gill &
The observations were carried out with a 203mm f/6.3 SC telescope, a
Clear filter and a SBIG ST7E camera (KAF401E CCD). 443 images were obtained,
each with an exposure duration of 60s.
For the photometry, the comparison star is GSC 4803-00206 with an assumed unfiltered
magnitude of 13.5. The check star is GSC 4803-00332 with a measured average
magnitude of 13.972 (standard deviation 0.034). BT Mon is very close to a
brighter star, then the photometry aperture circle encompasses both this star
and BT Mon. For this reason, the origin of the magnitudes is considered
An example of a light curve:
Red: BT Mon, Blue: the check star, shifted by +2.2 mag. The error
bars are +/- the 1 sigma statistical uncertainties.
All the light curves are HERE.
The data may be folded with the ephemeris from Smith et al (1998) to obtain the
The period being almost exactly (1 day)/3, there is a gap in the data
around phase 0.6.
There is a large hump on the ingress side of the eclipse. Such a feature
is mentioned by Warner (1995) (page 253) but as "low amplitude".
Other observers did not observe it at all (Robinson et al (1982), Smith
et al (1998)).
The bottom of the eclipse appears to be ahead of the ephemeris by about 15 mn:
According to Smith et al (1998) the inclination is 82.3° and the mass
ratio is 0.837, from spectroscopy. I use this in BinaryMaker3, adjusting
by hand the other parameters to fit the phase light curve. An animation
showing the result:
Among the parameters that fit the observations:
the fillout for the primary is -0.407, that is a disk radius of 60% of the Roche
lobe, in agreement with Robinson et al (1982);
the temperature of the disk is 9000°K (7000°K for the G8V (Smith et
al (1998)) secondary). This is not very hot. But I need a very hot (temperature
factor 1.8) spot at longitude 80° and another one, not that hot (temperature
factor 1.2), almost opposite to the first one (longitude 240°).
The spots may be not that hot and be bulges of matter?
there is a reflection from the secondary that gives a smooth bump in
the light curve around phase 0.5.
All the parameters are HERE.
Gill C.D., O'Brien T.J. (1998) MNRAS Deep optical imaging of nova remnants II.
A southern-sky sample [arXiv:astro-ph/9806076].
Robinson E.L., Nather R.E., Kepler S.O. (1982) ApJ 254 646.
Smith D.A., Dhillon V.S., Marsh T.R. (1998) MNRAS 296 465.
Warner B. (1995) Cataclysmic Variable Stars Cambridge U.P..
Telescope and camera configuration.
Computer and software configuration.
Model with BinaryMaker3.