DETERMINATION OF NATURE FOR ELEVEN DOUBLE STARS Z. Cvetković, S. Ninković, R. Pavlović Astronomical Observatory Belgrade, Serbia Astronomical Observatory Belgrade, Serbiaand S. Boeva, G. Latev BAS Astronomical Institute, Sofia, Bulgaria “9 th BULGARO-SERBIAN ASTRONOMICAL CONFERENCE” - Sosfia, July 03, 2014

There has existed an intense and fruitful cooperation between the two institutions: Astronomical Institute of Bulgarian Academy of Sciences and Astronomical Observatory in Belgrade, including also research field of double stars. In the framework of this cooperation we have had observations at both NAO Rozhen and AS Vidojevica., as well as common publications including famous journals like the Astronomical Journal. This time the topic is nature of eleven double stars, observed at both Rozhen and Vidojevica. “9 th BULGARO-SERBIAN ASTRONOMICALCONFERENCE” - Sofiia, July 03, 2014

A double star is more general than a binary. To form a binary a star pair must be gravitationally bound. Consequently, to establish the nature means to examine if a given pair is gravitationally bound or, at least, how probable this is. Why probable, because of data lack, very often the data body is very scarce. There exist criteria aimed at establishing the probability of being bound for two stars observed very close to each other on the celestial sphere. Many of these criteria correspond to the case when the data concerning the velocity (kinematical characteristics) are known for both stars. Then we can use dynamics (e. g. energy conservation law, Kepler’s third law) to estimate the probability of bounding since the distances are poorly known. “9 th BULGARO-SERBIAN ASTRONOMICAL CONFERENCE - Sofia, July 03, 2014

INPUT DATA “9th BULGARO-SERBIAN ASTRONOMICAL CONFERENCE” - Sofia, July 03, 2014

PROCEDURE For all eleven double stars there exist measurements (sufficient number) on the basis of which one can obtain the apparent orbit. Nevertheless, we prefer a combined approach using both apparent orbits and estimation. The first step in the estimation is in fact testing the hypothesis that both stars are at the same distance from the Sun. In the cases of very large separations, even the minimum one is of order of 10 as, this hypothesis cannot be realistic, even if the heliocentric distance is sufficiently small, say pc. Also the astrophysical consequences of ascribing a too short heliocentric distance, the intrinsic brightness would be extremely low. Finally, in a few cases when the proper motion is available for both stars, we see that it is very different, i. e, its modulus for the star suspected to be the companion suggests that this star cannot be at the proposed distance.

CONCLUSION In all eleven cases, neither apparent orbits nor the probability found under the hypothesis that two stars are at the same heliocentric distance is in favour that we have binaries, i. e. really gravitationally bound pairs. The pairs listed in the table here, according to our conclusion,are optical. “9 th BULGARO-SERBIAN ASTRONOMICAL CONFERENCE” - Sofia, July 03, 2014

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