The second “Bulgarian” nova-like cataclysmic
star was discovered at NAO-Rozhen
August 27, 2011
Observations at the National Astronomical Observatory (NAO) Rozhen at the beginning of 2011 revealed an unusual light variability of the star 2MASS J01074282+4845188. Their analysis led to the conclusion that it is a cataclysmic star. Thus it turns out that the second “Bulgarian” cataclysmic star has been discovered. Accidentally this event happened immediately after 50-years anniversary of the first nova-like cataclysmic star KR Aur discovered by Bulgarian astronomer professor Malina Popova (1922-2011) on the basis of her observations at Sonneberg Observatory (Germany).
The cataclysmic stars are rare type binary stellar systems containing normal cool star and white dwarf, i.e. a star which thermonuclear fuel has been exhausted and that slowly cools down. The distance between the components of the cataclysmic systems is very small and their components revolve around the mass center for a several hours. Due to the closeness and the big gravitational attraction the shape of the normal star is quite distorted. A mass transfer from the normal star to the white dwarf leads to formation of huge gaseous structure around this compact object, called accretion disk (Fig. 1). This structure is not stationary and causes different observational effects: fast light variability during minutes as well as sporadic alternates between two brightness levels (high and low) during months and years.
The most important brand of the cataclysmic stars, as their name shows, is the burst event. In dependence of the parameters of the explosions the cataclysmic stars have been separated into several types. But stars belonging to one of them, called nova-like type, do not undergo bursts whereas reveal all rest observational effects of the cataclysmic stars.
Fig. 1. 3D art model of a nova-like cataclysmic star
The discovery of cataclysmic stars is difficult because they are weak objects with emission power about ten times smaller than that of our Sun. Mostly they have been found during their explosions when their light increases hundred times. But this method is not applicable for the nova-like stars and correspondingly their number is quite small. The list of these stars includes slightly above 200 targets. Most of them are discovered and classified during the last 2-3 years.
Among the all nova-like binaries eclipses of the accretion disk by the normal star have been registered only for several stars. This observational effect requires the observer’s line-of-sight to be hardly inclined to the orbital plane of the binary but this is very seldom fulfilled. At the same time only the eclipsing nova-like cataclysmic stars allow to get the masses and radii of their stellar components and accretion disks, important parameters for the astrophysics.
Whereas the first “Bulgarian” nova-like cataclysmic star KR Aur does not show eclipses, the new-discovered 2MASS J01074282+4845188 is eclipsing one. The photometric observations obtained with the 60-ñm telescope of NAO-Rozhen at the end of January 2011 revealed one asymmetric, deep minimum with V-type shape (Fig. 2) and a hump before the eclipse that are important features of the nova-like cataclysmic stars. The determined orbital period of about 5 hours supports this classification.
Fig. 2. The light curve of 2MASS J01074282+4845188obtained by the 60 ñm telescope of NAO-Rozhen (à)
and the light curve of the prototype UX UMa (b)
It turned out that 2MASS J01074282+4845188 is not only a new member of the small family of the eclipsing nova-like stars but also the depth of its eclipse of 2.7 mag is one of the biggest for this type objects.
In order to confirm the nova-like classification of 2MASS J01074282+4845188 a week after its discovery spectral observations of the star were carried out with 2 m telescope of NAO-Rozhen equipped with the focal reductor FoReRo-2. The obtained several spectra show that the red hydrogen line Íα is in emission and its width corresponds to a velocity above 700 km/s of the accretion disk around the white dwarf. These characteristics of the Ía line are another important criterion about the belonging of the object to the cataclysmic stars.
The study of the new-discovered cataclysmic star will contribute to get more knowledge about the final evolution of the life of binaries with masses of the order of the Sun’s mass.
The classification and investigation of the star 2MASS J01074282+4845188 is an achievement of Dinko Dimitrov from the Institute of astronomy with NAO (BAS) and Diana Kjurkchieva from Shumen University as a result of the joint work in the framework of the project DO 02-362 (SMARTNET) financed by the Scientific Foundation of the Bulgarian Ministry of education and Science.
The results of this study are accepted for publication in the scientific journal New Astronomy. The paper is available at http://arxiv.org/abs/1106.2115 or at http://www.sciencedirect.com/science/journal/13841076.