CG Draconis - a Dwarf Nova, Light Curve and Mini-Review

[Maxim Usatov]

Here is the recent light curve I have made of CG Draconis - an eclipsing dwarf nova star with a twist. The data was taken with 0.5 m CDK telescope in New Mexico during the night of May 16, following the call by J. Shears from the British Astronomical Association Variable Star Section.

 

 

Being a dwarf nova, we are looking at a binary star system with the primary component being a white dwarf that accretes material from a main sequence secondary star:

 

 

(illustration taken from NASA/CXC/M.Weiss.)

 

As the mass falls onto the dwarf, it forms an accretion disk around it which is thermally unstable, producing periodic outbursts. The CG Dra system is very active.

 

In addition to outbursts, CG Dra is an eclipsing binary. As the secondary component orbits the white dwarf and the extended accretion disk around it, it obscures them every period which results in an eclipse detectable with photometry (Shears et al. 2008).

 

In brief, the CG Dra system can be characterized as following:

 

- Outbursts approximately every 11 days.
- Two types of outbursts: short and long.
- Short outbursts endure for about 4 days, long for about 8 days.
- Shallow eclipses every 4 hours or so, of about 0.16 magnitudes depth, 18 minutes duration.

 

The system had only a few observations since 2008, so the goal was to check on the current behavior and see if there's anything new going on.

 

I have observed the system remotely for three hours in the Johnson V filter, starting 06:02 and ending 09:04 UTC. The individual FITS frames were calibrated and processed with the MetroPSF photometry program I wrote. MetroPSF calibrates the images astrometrically, matches sources with AAVSO Photometric All Sky Survey (APASS) DR9 and VSX catalogs, identifies variables, conducts photometry and generates the reports and light curves pretty much automatically once all the settings are done correctly.

 

In the light curve obtained we can see a ~ 0.2-magnitude dip beginning at about 59351.28 JD. This does not look like an actual eclipse. More likely it is just irregular modulations in the accretion disk - the 0.15 mag flickering this system is known of makes it a difficult target. Perhaps, the accretion disk is clumpy.

 

The problems of CG Dra - why this dwarf nova is interesting:

 

1) Spectroscopic observations by Bruch et al. (1997) on a 3.5 m telescope at Calar Alto in Spain show that the secondary component is of K5 ± 2 spectral type. For main sequence stars there are known mass-type relationships that allow us to estimate the mass of a main sequence star if we know its type. If we know the mass we can estimate its period in a binary system. A type-mass relation taken from Schmidt-Kaler (1982) would imply a 6-hour period for CG Dra, inconsistent with 4-hour period observed by others. This can be explained by the secondary having higher density that a main sequence star, but this has never been observed in any dwarf nova before. 

 

2) If CG Dra is a normal dwarf nova then the main sequence star of mass corresponding to the K5 ± 2 spectral type would not fit into its Roche lobe. The material begins to transfer when it overflows the Roche lobe, which appears to be impossible in this situation. Mass transfer and outbursts shouldn't be taking place!

 

3) Radial velocity measurements in the spectrum of the K5 V secondary are on the order of 30 times smaller than predicted for this system. It appears there is not enough wobble of the secondary in orbit due to the gravitational pull.

 

Bruch et al. (1997) suggest the K5 spectrum may actually originate from a star that is not a part of CG Dra system. An optical coincidence? It would be interesting to capture an eclipse and follow this system.

References:

Bruch, A., Schimpke, T., Kochsiek, A. 1997, A&A, 325, 601
Schmidt-Kaler, T. 1982, in Landolt-Bornstein, 2b, ed. K. Schaifers, & H. H. Voig (Heidelberg: Springer)
Shears, J., Boyd, D., Brady, S., Pickard, R. JBAA, 118, 6

[JeremyS]

Nice work, Maxim 👍🏻

[Maxim Usatov]

Thank you, Jeremy!