A&A 388, 179–188 (2002) Astronomy DOI: 10.1051/0004-6361:20020540 & c ESO 2002 Astrophysics Two highly reddened young open clusters located beyond the Sagittarius arm?,?? A. E. Piatti1 and J. J. Clari´a2 1 Instituto de Astronom´ıa y F´ısica del Espacio, CC 67, Suc. 28, 1428, Buenos Aires, Argentina 2 Observatorio Astron´omico, Universidad Nacional de C´ordoba, Laprida 854, 5000 C´ordoba, Argentina Received 10 October 2001 / Accepted 3 April 2002 Abstract. We present the results of CCD BV I Johnson-Cousins photometry down to V ∼ 19 mag in the regions of the unstudied stellar groups Pismis 23 and BH 222, both projected close to the direction towards the Galactic centre. We measured V magnitude and B − V and V − I colours for a total of 928 stars in fields of about 40×40. Pismis 23 is conclusively a physical system, since a clear main sequence and other meaningful features can be seen in the colour-magnitude diagrams. The reality of this cluster is also supported by star counts carried out within and outside the cluster field. For Pismis 23 we derive colour excesses E(B−V )=2.0 0.1andE(V −I)=2.6 0.1, a distance from the Sun of 2.6 0.6kpc(Z = −19 pc) and an age of 300 100 Myr (assuming solar metal content). BH 222 appears to be a young open cluster formed by a vertical main sequence and by a conspicuous group of luminous, typically red supergiant stars. We derived for this cluster a colour excess of E(V −I)=2.4 0.2, a distance from the Sun of 6.0 2.7kpc(Z = −46 pc) and an age of 60 30 Myr. The resulting reddening and distance estimates place these two young objects among the most reddened and distant open clusters known in the direction towards the Galactic centre. They are located beyond the Sagittarius arm, close to the direction where this arm probably bifurcates into two arms. Key words. open clusters and associations: individual: Pismis 23, BH 222 – open clusters and associations: general – techniques: photometric 1. Introduction sizes. The present paper belongs to a series devoted to the study of numerous, mostly unstudied, poorly popu- Open clusters constitute one of the most important fields lated open clusters (or candidates), preferably located to- of research in observational astronomy. Aside from the in- wards the central parts of the Galaxy (see, e.g., Piatti & trinsic interest of these objects, the study of the open clus- Clari´a 2001a, and references therein). The primary pur- ter system lends itself readily to a set of applications in pose of this systematic survey is to determine whether several areas of modern astrophysics, such as the assess- the observed objects are genuine physical systems or if, ment of chemical abundance gradients in the disk (see, on the contrary, the apparent enhancement of stars in e.g., Piatti et al. 1995; Twarog et al. 1997), galactic struc- a sky region is a consequence of random fluctuations of ture and evolution (e.g., Janes & Adler 1982; Janes & the stellar density in that zone. Information on the dis- Phelps 1994), interactions between thin and thick disks tance, interstellar reddening, age and membership of those (e.g., Sandage 1988), as well as theories of stellar forma- objects confirmed as open clusters, is obtained through tion and evolution (e.g., Meynet et al. 1993; Phelps & the analysis of their colour-magnitude diagrams (CMDs). Janes 1993). In particular, open clusters located towards the Galactic The open cluster catalogue by Lyng˚a (1987) includes centre play an important role in that they provide the 1151 entries. However, very little is known for many of advantage of tracing the structure and evolution of the them, except their positions and estimates of their angular inner disk more precisely. Many of these objects, how- ever, have not been studied in detail yet because they Send offprint requests to: A. E. Piatti, e-mail: [email protected] are either relatively faint objects or located in obscured ? Based on observations made at the University of Toronto and/or crowded fields. This is the case of the southern h m s (David Dunlap Observatory) 24-inch telescope, Las Campanas, open cluster candidates Pismis 23 (α2000 =1623 58 , ◦ 0 00 ◦ ◦ Chile. δ2000 = −48 53 33 ; l = 334.66, b = −0.42) and BH 222 ?? h m s ◦ 0 00 ◦ Tables 2 and 3 are only available in electronic form at the (α2000 =1718 47 , δ2000 = −38 17 22 ; l = 349.12, CDS via anonymous ftp to b = −0◦.44), both projected close to the Galactic centre. cdsarc.u-strasbg.fr (130.79.128.5) or via Pismis 23, also known as Lyng˚a10(Lyng˚a 1965), OCL-967 http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/388/179 Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20020540 180 A. E. Piatti and J. J. Clari´a: Highly reddened open clusters Table 1. Journal of observations. Cluster Date Filter Exposures seeing (UT) (00) Pismis 23 1995 June 30 B 1 × 60 s, 2 × 900 s 1.5 V 1 × 60 s, 2 × 600 s 1.5 I 1 × 60 s, 2 × 300 s 1.5 BH 222 1995 June 28 B 1 × 60 s, 2 × 900 s 1.7 V 1 × 60 s, 2 × 600 s 1.7 I 1 × 30 s, 2 × 60 s 1.7 (Alter et al. 1970), BH 190 (van den Bergh & Hagen 1975) or ESO 226-SC5 (Lauberts 1982), was first identi- fied as an open cluster in Norma by Pismis (1959), while BH 222, located in Scorpius, was detected as an open clus- ter in a uniform survey of clusters in the southern Milky Way by van den Bergh & Hagen (1975). According to Lyng˚a (1987), their angular diameters are about 1.0 and 2.0 arcmin, respectively. As far as we are aware, none of these two stellar groups have been previously studied. The next section is dedicated to a brief description of the data acquisition and reduction. We confirm in Sect. 3 the existence of Pismis 23 and BH 222 as genuine open clusters and derive their fundamental parameters. A com- parison with other clusters located in similar directions is given in Sect. 4, and the final conclusions are presented in Fig. 1. Magnitude and colour photometric errors provided by Sect. 5. DAOPHOT as a function of V for the richest cluster of the sample (Pismis 23). They are typical in our sample. 2. Observations and reductions observations in the standard way at the Observatorio We obtained CCD BV I Johnson-Cousins images in the Astron´omico de la Universidad Nacional de C´ordoba fields of Pismis 23 and BH 222 during two photomet- (Argentina). The b, v, i instrumental magnitudes were ric nights in June 1995, with the University of Toronto obtained following the reduction procedure described in Southern Observatory 0.6 m telescope, situated at Las Piatti et al. (1999) and transformed into the standard sys- Campanas Observatory (Chile). The observations were tem using the following relations: carried out using a PM 512×512 METACHROME UV 00 coated chip, with a scale of 0.45 /pixel, covering a to- bj,n = b1 + V +(B − V )+b2(B − V )+b3Xj,n, (1) tal field of about 40×40. The mean seeing for the entire vk,n = v1 + V + v2(B − V )+v3Xk,n, (2) run was about 1.600 and the quality of the nights was i = i + V − (V − I)+i (V − I)+i X , (3) photometric. Details of the number of observations, ex- l,n 1 2 3 l,n posure times, and typical seeing for each object and fil- where V ,(B −V )and(V −I) are the standard magnitude ter, are shown in Table 1. The airmasses of the observed and colours and X the corresponding airmass for the j, fields were always less than 1.35, lying just within the k, lth measured standard star. We solved Eqs. (1) to (3) airmass range of the standard stars. Besides the clus- for all coefficients simultaneously, for each night n, with ter fields, we observed nightly an average of 12 standard the PHOTCAL package in IRAF. The mean rms errors stars from the Selected Area Standard fields (Landolt range between 0.005 mag and 0.020 mag in all passbands. 1992). These stars were carefully selected to cover a wide Figure 1 shows the trend of the photometric magnitude range of spectral types and airmasses. Instrumental signa- and colour errors with V provided by DAOPHOT, for tures were removed using bias and a combination of dome the richest cluster observed (Pismis 23). For each selected and twilight sky flats, while instrumental magnitudes field, we generated a master table containing a running were produced via point spread function (Stetson 1987). star number, the X and Y coordinates, the V magni- 1 The IRAF /DAOPHOT package was used to reduce the tudes, the (B − V )and(V − I) colours, the observational 1 IRAF is distributed by the National Optical Astronomy errors provided by the IRAF INVERTFIT task σ(V ), Observatories, which is operated by the Association of σ(B − V )andσ(V − I), and the number of observations Universities for Research in Astronomy, Inc., under contract n. These tables were built by combining all the indepen- with the National Science Foundation. dent measurements using the stand-alone DAOMATCH A. E. Piatti and J. J. Clari´a: Highly reddened open clusters 181 Table 2.