Revisiting the area of the open cluster Stock 16

Abstract

Deep CCD UBV(RI)C photometry has been carried out in the area of the open cluster Stock 16. The different photometric diagrams indicate the presence of three superimposed stellar groups, all composed of early type stars. The first of them, the cluster Stock 16, suffers from slightly variable reddening with a mean color excess 〈EB-V〉 = 0.51. The second group, reddened by 〈EB-V〉 = 0.84, includes the Wolf-Rayet star WR50 (a WC7+OB), and the third one, with a mean 〈E B-V〉 = 1.18, also includes another Wolf-Rayet star, WR 51 (a WN4+OB?). Our results confirm the distance and reddening already stated for Stock 16 but include new members. Concerning the other two highly reddened groups, we show that the first one - located at a distance of 3600 pc - is probably related to an anonymous OB association behind Cen OB1 also at 3600 pc already mentioned in the literature; the other seems to be a distant OB association - at more than 9000 pc - which may belong to the Scutum-Crux spiral arm. In these two groups we find evidence that the absorption law, A V/EB-V = R amounts to 4.0 approximately. The ages of the different star populations in the zone range from 5 × 106 yr to 6.4 × 106 yr for Stock 16, 8 × 106 for the second most reddened group and 5 × 106 for the probable association in Scutum-Crux. A determination of the IMF slope of Stock 16 was also carried out for stars in the mass range 1.8 < M < 14 M ⊙, giving a normal value x = 1.3.