Kinematic study of the association Cyg OB3 with Gaia DR2

Abstract

We study the stellar kinematic properties and spatial distribution of the association Cyg OB3 with the most precise and accurate astrometric solution available so far with $Gaia$ DR2. All known O- and B-type stars in the Cyg OB3 region with 5-parameter astrometric solutions are considered in the sample, which consists of a total of 45 stars. The majority of the stars are found to be concentrated at a heliocentric distance of about 2 kpc. The corresponding standard deviation is 0.57 kpc, significantly higher than the expected physical dimension of the association along the line of sight. The mean peculiar velocity of the sample after removing Galactic rotation and solar motion is ~20 km/s, dominated by the velocity component towards the Galactic center. The relative position and velocity of the black hole X-ray binary Cyg X-1 with respect to the association suggest that Cyg OB3 is most likely its parent association. The peculiar kinematic properties of some of the stars are revealed and are suggestive of past stellar encounters. HD 225577 is one of the stars exhibiting the lower value of proper motion than the rest of the sample and moving with a peculiar velocity smaller than its velocity relative to the association. The high peculiar velocity of ~50 km/s of a previously known runaway star HD 227018 is confirmed with Gaia. The slowly expanding nature of the association is supported by the small relative speeds <20 km/s with respect to the association for a majority of the sample stars.

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ORCID for Poshak Gandhi: orcid.org/0000-0003-3105-2615

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