2019A&A...623A..22S

The open cluster IC 4996 in Cygnus and its vicinity are investigated by applying a two-dimensional photometric classification of stars measured in the Vilnius seven-color photometric system. Cluster members are identified by applying distances based on the Gaia DR2 parallaxes and the point vector diagram of the Gaia DR2 proper motions. For some B-type stars, spectroscopic MK types are also obtained from the Asiago spectra and collected from the literature. New parameters of the cluster are derived. The interstellar extinction A_V covers a wide range of values, from 1.3 to 2.4mag; the mean value in the central part of the cluster is 1.8mag. The cluster distance is 1915±110pc, and its age is within 8-10 Myr. The cluster exhibits a long sequence from early-B to G stars, where stars cooler than B8 are in the pre-main-sequence stage. The plot of extinction versus distance shows a steep rise of A_V up to 1.6mag at 700-800pc, which is probably related to dust clouds at the edge of the Great Cygnus Rift. The next increase in extinction by an additional 0.8mag at d≥1.7kpc is probably related to the associations Cyg OB1 and Cyg OB3. The cluster IC 4996 does not belong to the Cyg OB1 association, which is located closer to the Sun, at 1682±116pc. It seems likely that the cluster and the surrounding O-B stars have a common origin with the nearby association Cyg OB3 since Gaia data show that these stellar groups are located at a similar distance.