A dynamical study of the multiple system 17 Cygni ABFG

Abstract

Adynamical study of the relative motions of the components of the inner pairs AB (ADS 12913) and FG (ADS 12889) of the quadruple heirarchical system 17 Cygni (WDS 19464+3344) is presented, as well as analysis of themotions of the outer pair AB–FG. The study is based on CCD observations obtained on the 26-inch refractor of the Pulkovo Observatory (2003–2013), position observations from the WDS catalog, Hipparcos parallaxes, and radial velocities of the components from literature data. A family of orbits for 17 Cyg AB is obtained for the first time, and has a most probable period of 6200 yrs. The apparent motion parameters (AMP) method is used, since the entire visible arc of the orbit over 1832–2013 is only ~4◦. The AMP method is also used to calculate the orbit of the 17 Cyg FG pair, which has a period of 238 yrs, yielding results in good agreement with the orbits derived in other studies. The ephemerides of the obtained AMP orbits, the position data for the AF pair from the WDS catalog (11 positions during 1893–2002), and Pulkovo CCD observations for 2007–2013 are used to calculate the apparent motion parameters of AB–FG outer pair, as well as a family of close-to-parabolic orbits with periods of 3.7 million years ormore. All the orbits (for both the inner and the outer pairs) are steeply inclined to theGalactic plane. Monte Carlo simulations are used to compute the probability that the outer pair is gravitationally bound, which is 47%. The similarity of the proper motions and radial velocities of all the components provides evidence that they all belong to a single stellar stream. Data from the CNS3 catalog are used to compose a list of candidate members of this stream.