uvby – Hβ CCD photometry and membership segregation of the open cluster NGC 2682 (M 67)

Abstract

Following deep astrometric and photometric study of the cluster NGC 2682 (M 67), we are able to accurately determine its fundamental parameters. Being an old and rich cluster, M 67 is relevant for the analysis of the Galactic disk evolution. M 67 is well studied but lacks a wide and deep Strömgren photometric study. The brightest stars of the open cluster M 67 were used as uvby − Hβ standard stars in our studies of NGC 1817 and NGC 2548, and the extension of the field covered, as well as the number of observations, allowed us to obtain the best set of Strömgren data ever published for this cluster. We discuss the results of our CCD uvby − Hβ intermediate-band photometry, covering an area of about 50' × 50' down to V ∼ 19. Moreover, a complete membership segregation based on astrometric and photometric criteria is obtained. The photometric analysis of a selected sample of stars yields a reddening value of E(b − y) = 0.03 ± 0.03, a distance modulus of V0 − MV = 9.7 ± 0.2 and [Fe/H] = 0.01 ± 0.14. Through isochrone fitting we found an age of log t = 9.6 ± 0.1 (4.2 ± 0.2 Gyr). A clump of approximately 60 stars around V = 16, (b − y) = 0.4 could be interpreted as a population of pre-cataclysmic variable stars (if members), or as a stream of field G-type stars placed at twice the distance of the cluster (if non-members).