MBCAA Observatory

BT Mon: eclipser with a hump

Observed: 26 Oct, 1 Nov, 24 Dec 2005, 19 Jan 2006

Michel Bonnardeau
4 Feb 2006

Abstract

Light curves of this cataclysmic star are presented. They show the eclipse with a large hump. They are modelled with BinaryMaker3.

Introduction

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 & O'Brien (1998)).

Observations

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 as arbitrary.

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.

Plase plot

The data may be folded with the ephemeris from Smith et al (1998) to obtain the phase plot:

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:

BinaryMaker3 model

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.