CCD UBV photometric and Gaia astrometric study of eight open clusters-
  ASCC 115, Collinder 421, NGC 6793, NGC 7031, NGC 7039, NGC 7086, Roslund 1
  and Stock 21
  In this study, we carried out CCD UBV photometry of eight open clusters, ASCC
115, Collinder 421, NGC 6793, NGC 7031, NGC 7039, NGC 7086, Roslund 1, Stock
21, and determined their reddening, metallicity, distance, age, and mass
functions. We used new Gaia Data Release 2 (DR2) astrometric data to separate
cluster member stars from the field stars and obtain precise structural and
astrophysical parameters. To identify cluster member stars we utilized an
unsupervised membership assignment code (UPMASK), which is based on the
photometric and astrometric data. The density distributions for the open
clusters show good fits with the empirical King model except for Roslund 1 and
Stock 21 not having central concentration. The colour excesses and
metallicities were derived separately using U-B vs B-V two-colour diagrams.
Keeping these parameters as constants, we simultaneously calculated distance
moduli and ages of the clusters from V vs B-V and V vs U-B colour-magnitude
diagrams using PARSEC theoretical isochrones. Taking into account Gaia DR2
proper motion components and parallaxes of the member stars, we also calculated
mean proper motions and distances for the clusters. Distances derived both from
isochrone fitting to colour-magnitude diagrams of the clusters and Gaia DR2
trigonometric parallaxes are compatible with each other. Slopes of the mass
functions of the eight open clusters are in good agreement with Salpeter (1955)
value of 1.35.
