Revisiting the area of the open cluster Stock 16

Abstract

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 E B−V = 0.51 . The second group, reddened by E B−V = 0.84 , includes the Wolf-Rayet star WR 50 (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 /E B−V = R amounts to 4.0 approximately. The ages of the different star populations in the zone range from 5 × 10 6 yr to 6.4 × 10 6 yr for Stock 16, 8 × 10 6 for the second most reddened group and 5 × 10 6 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. IntroductionThe very young open cluster Stock 16 lies close to the east border of the Coal Sack in Crux (l = 306.11, b = +0.14) where it appears projected against the H region RCW 75 (Rodgers et al. 1960), probably excited by the UV radiation field from the O7.5 V ((f)) star HD 115455 (Walborn 1973). A few photometric studies of the brightest members of Stock 16 have been carried out in the past, e.g. Lyngå (1970), Crampton (1971 and Lundström & Stenholm (1984). Later on, Turner (1985; hereafter T85) performed a detailed analysis in Stock 16 and the surrounding area by means of UBV photometry for 33 stars finding a color excess E B−V = 0.49 and a distance of 1.90 ± 0.08 kpc which coincides with the distance estimation, also quoted by T85, of Cen OBI and Cen Rl (1.88 ± 0.24 kpc). Since the age computed for Stock 16, 5 × 10 6 yr, is comparable to the ages of Cen OBI and the R association (Herbst 1975) Cen R1 (from 10 6 to 8 × 10 6 yr) and the distances of all Based on observations collected at the University of Toronto Southern Observatory (UTSO), Las Campanas, Chile. Feinstein et al. (2003), who have studied several moderately bright stars, has allowed determination of the properties of the ISM toward the cluster finding a mean polarization percentage P ≈ 2.5%.The present photometric investigation is aimed at performing a deeper study of the region including the open cluster Stock 16 and the surrounding area. We will try to confirm the earlier assumption of T85 who proposed that the star formation process in the area has taken place along the axis joining the elephant trunk stru...

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