CCD Photometric Search for Peculiar Stars in Open Clusters
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A&A 373, 153–158 (2001) Astronomy DOI: 10.1051/0004-6361:20010577 & c ESO 2001 Astrophysics CCD photometric search for peculiar stars in open clusters II. NGC 2489, NGC 2567, NGC 2658, NGC 5281 and NGC 6208?,??,??? E. Paunzen1,2 and H. M. Maitzen1 1 Institut f¨ur Astronomie der Universit¨at Wien, T¨urkenschanzstr. 17, 1180 Wien, Austria 2 Zentraler Informatikdienst der Universit¨at Wien, Universit¨atsstr. 7, 1010 Wien, Austria Received 20 February 2001 / Accepted 20 April 2001 Abstract. High accurate photometric data in the three filter, narrow-band ∆a-system for five open clusters (NGC 2489, NGC 2567, NGC 2658, NGC 5281 and NGC 6208) are presented permitting one to detect apparent chemically peculiar stars. The ages and distances are wide spread to improve the statistics of peculiar stars in our Milky Way. A total of 235 stars within these clusters have been observed. We have detected ten apparent peculiar objects (six with significant positive and four with negative ∆a-values) in four open clusters. Although three of them within the boundaries of NGC 2567 are probably not members of this cluster. Since no Str¨omgren uvbyβ data are available from the literature, Johnson UBV photometry was used to test and calibrate our photometric data. The results from a comparison of both systems are excellent. Key words. stars: chemically peculiar – stars: early-type – techniques: photometric – open clusters and associations: general 1. Introduction In total, we detected six objects with significant posi- tive ∆a-values and four with negative ones. Three of the We continue our search for chemically peculiar objects of latter found in NGC 2567 are most probably not members the upper Main Sequence using the three filter ∆a-system of the cluster but background stars. (Maitzen 1976) and applying the CCD technique. The first paper of this series (Bayer et al. 2000, Paper I hereafter) 2. Observations and reduction presented the characteristics of the telescopes, instrumen- tation, filters and the reduction processes which were used. Observations of the five open clusters were performed with the Bochum 61 cm (ESO-La Silla) as well as the Helen- Another five open clusters (NGC 2489, NGC 2567, Sawyer-Hogg 61 cm telescope (UTSO-Las Campanas NGC 2658, NGC 5281 and NGC 6208) with distances Observatory) in 1995. The instrumentation and filters from 1000 to 3800 pc and ages from 0.01 to 1 Gyr have used are described in Paper I. The observing log is listed been investigated. A broad variety of distances and ages in Table 1. brings us closer to find a possible correlation of the ap- The basic reductions (bias-subtraction, dark- parent number of chemically peculiar depending on these correction, flat-fielding) were carried out within standard parameters. IRAF routines. For all frames we have applied a point- In addition to (g1 −y) measurements, Johnson (B−V ) spread-function-fitting within the IRAF task DAOPHOT values from the literature were used to derive ∆a-values. (Stetson 1987). Photometry of each frame was performed Color-magnitude diagrams are shown for all five open clus- separately and the measurements were then averaged and ters in Fig. 2. weighted by their individual photometric error. Send offprint requests to: E. Paunzen, 3. Results e-mail: [email protected] In Paper I, we compared our photometric indices with re- ? Based on observations at ESO-La Silla and UTSO-Las Campanas. sults from published Str¨omgren uvbyβ photometry. The ?? Tables 4 to 8 are only available at the CDS via anonymous situation for the five open clusters presented here is dif- ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via ferent. None of them has yet been investigated, to our http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/373/153 knowledge, via Str¨omgren uvbyβ photometry. However, ??? Figure 1 is only available in electronic form at Johnson UBV photometry is available for all of them and http://www.edpsciences.org has been used to test and calibrate our photometric data. Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20010577 154 E. Paunzen and H. M. Maitzen: CCD photometric search for peculiar stars in open clusters. II. 11 11 12 12 13 13 V V 14 14 15 15 NGC 2489 NGC 2567 16 16 0.80 0.70 0.78 a = 0.904 + 0.381 (g - y) 0.68 0.76 0 1 0.74 0.66 0.72 0.64 0.70 a a 0.62 0.68 0.66 0.60 a = 0.754 + 0.217 (g - y) 0 1 0.64 0.58 0.62 0.56 0.60 -0.57 -0.54 -0.51 -0.48 -0.45 -0.42 -0.39 -0.36 -0.75 -0.70 -0.65 -0.60 -0.55 -0.50 -0.45 -0.40 -0.35 (g - y) (g - y) 1 1 6 13.0 7 NGC 2658 NGC 5281 13.5 8 14.0 9 14.5 10 V V 15.0 11 15.5 12 16.0 13 14 16.5 0.69 0.66 0.68 0.64 0.67 0.62 0.66 0.60 0.65 a a 0.58 0.64 0.56 0.63 0.54 a = 0.740 + 0.181 (g - y) a = 0.735 + 0.285 (g - y) 0.62 0 1 0 1 0.52 0.61 -0.58 -0.56 -0.54 -0.52 -0.50 -0.70 -0.65 -0.60 -0.55 -0.50 -0.45 -0.40 (g - y) (g - y) 1 1 11.5 NGC 6208 12.0 12.5 13.0 V 13.5 14.0 14.5 15.0 0.84 0.82 0.80 0.78 0.76 a 0.74 0.72 0.70 0.68 a = 0.822 + 0.228 (g - y) 0.66 0 1 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 (g - y) 1 Fig. 2. V versus (g1 − y)anda versus (g1 − y) for NGC 2489 (left upper panel), NGC 2567 (right upper panel), NGC 2658 (left middle panel), NGC 5281 (right middle panel), NGC 6208 (left lower panel). Filled circles indicate apparent peculiar stars whereas open circles are non−peculiar objects. The solid line is the normality line whereas the dotted lines are the confidence intervals corresponding to 99.9 %. The error bars for each individual objects are the mean errors. The measurement errors of V are much smaller than the symbols and have been omitted. E. Paunzen and H. M. Maitzen: CCD photometric search for peculiar stars in open clusters. II. 155 Table 1. Observing log. apparent (B − V )0 values as well as the 3σ detection limit are given. Cluster Site Nights #g1 #g2 #y The finding charts of our open clusters are shown in NGC 2489 ESO 3 3 4 3 Fig. 1. The size of the symbols are inversely proportional UTSO 1 1 1 1 to the apparent visual magnitudes of the objects in the NGC 2567 ESO 4 5 7 5 sense that larger symbols denote brighter objects. NGC 2658 ESO 3 4 5 3 UTSO 1 1 1 1 The complete tables with all data for the individual NGC 5281 ESO 4 10 13 9 cluster stars are available both from SIMBAD (via anony- NGC 6208 ESO 2 3 4 5 mous ftp) or upon request from the first author. These tables include the cross identification of objects from the literature, the observed (g1 − y)anda values with their corresponding errors, V magnitudes, the (B − V )values from the literature and ∆a-values derived from the nor- The normality lines were, therefore, independently de- mality lines of (g − y)and(B − V ), respectively. rived for (g − y)and(B − V ), respectively. We have only 1 1 Table 3 lists the apparent peculiar stars found within taken published (B − V ) values for this analysis and not our investigation. The individual objects are discussed in tried to transform (g − y) values in the corresponding 1 more detail in the following subsections. system. There are several reasons for this. Firstly, most The V versus (g −y)anda versus (g −y) diagrams for of the data are based on photographic plates resulting in 1 1 all five open clusters are shown in Fig. 2. Furthermore, the larger uncertainties. An unknown error is introduced by normality lines and the confidence intervals corresponding unresolved visual binaries not taken into account by the to 99.9% are plotted. Filled circles indicate the apparent photometry. From a statistical point of view, the “mix- peculiar objects. ture” of different data sets will give misleading results. A comparison of the ∆a-values for both systems results in an excellent agreement. There are only a few exceptions 3.1. NGC 2489 which are discussed in the relevant sections. Lindoff & Johansson (1968) used photographic plates to However, for the peculiar objects reported in this paper investigate this rather rich open cluster at a distance of − without (B V ) values from the literature, a heuristic about 1450 pc from the Sun. The earliest members found − transformation of measured (g1 y) values was applied. have spectral types of about B8. The Johnson UBV mea- Starting with a list of all stars observed in both systems, surements of Ramsay & Pollacco (1992) were also used for objects which deviate more than 5 σ from a preliminary our analysis. Figure 2 shows a well defined Main Sequence linear least square fit were rejected.