A&A 373, 153-158 (2001)
DOI: 10.1051/0004-6361:20010577

CCD photometric search for peculiar stars in open clusters

II. NGC 2489, NGC 2567, NGC 2658, NGC 5281 and NGC 6208[*][*] [*]

E. Paunzen1,2 - H. M. Maitzen1


1 - Institut für Astronomie der Universität Wien, Türkenschanzstr. 17, 1180 Wien, Austria
2 - Zentraler Informatikdienst der Universität Wien, Universitätsstr. 7, 1010 Wien, Austria

Received 20 February 2001 / Accepted 20 April 2001

Abstract
High accurate photometric data in the three filter, narrow-band $\Delta a$-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 $\Delta a$-values) in four open clusters. Although three of them within the boundaries of NGC 2567 are probably not members of this cluster. Since no Strömgren $uvby\beta$ 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

We continue our search for chemically peculiar objects of the upper Main Sequence using the three filter $\Delta a$-system (Maitzen 1976) and applying the CCD technique. The first paper of this series (Bayer et al. 2000, Paper I hereafter) presented the characteristics of the telescopes, instrumentation, filters and the reduction processes which were used.

Another five open clusters (NGC 2489, NGC 2567, NGC 2658, NGC 5281 and NGC 6208) with distances from 1000 to 3800pc and ages from 0.01 to 1Gyr have been investigated. A broad variety of distances and ages brings us closer to find a possible correlation of the apparent number of chemically peculiar depending on these parameters.

In addition to (g1-y) measurements, Johnson (B-V) values from the literature were used to derive $\Delta a$-values. Color-magnitude diagrams are shown for all five open clusters in Fig. 1.

In total, we detected six objects with significant positive $\Delta a$-values and four with negative ones. Three of the latter found in NGC 2567 are most probably not members of the cluster but background stars.

  \begin{figure} \psfig{figure=ms1177f2a.eps,height=73mm,width=16.cm}\par\psfig{fi... ...idth=16.cm}\par\psfig{figure=ms1177f2c.eps,height=73mm,width=16.cm} \end{figure} Figure 1: V versus (g1-y) and a 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.
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2 Observations and reduction

Observations of the five open clusters were performed with the Bochum 61cm (ESO-La Silla) as well as the Helen-Sawyer-Hogg 61cm telescope (UTSO-Las Campanas Observatory) in 1995. The instrumentation and filters used are described in Paper I. The observing log is listed in Table 1.

The basic reductions (bias-subtraction, dark-correction, flat-fielding) were carried out within standard IRAF routines. For all frames we have applied a point-spread-function-fitting within the IRAF task DAOPHOT (Stetson 1987). Photometry of each frame was performed separately and the measurements were then averaged and weighted by their individual photometric error.

3 Results

In Paper I, we compared our photometric indices with results from published Strömgren $uvby\beta$ photometry. The situation for the five open clusters presented here is different. None of them has yet been investigated, to our knowledge, via Strömgren $uvby\beta$ photometry. However, Johnson UBV photometry is available for all of them and has been used to test and calibrate our photometric data.

The normality lines were, therefore, independently derived for (g1-y) and (B-V), respectively. We have only taken published (B-V) values for this analysis and not tried to transform (g1-y) values in the corresponding system. There are several reasons for this. Firstly, most of the data are based on photographic plates resulting in larger uncertainties. An unknown error is introduced by unresolved visual binaries not taken into account by the photometry. From a statistical point of view, the "mixture'' of different data sets will give misleading results. A comparison of the $\Delta a$-values for both systems results in an excellent agreement. There are only a few exceptions which are discussed in the relevant sections.

However, for the peculiar objects reported in this paper without (B-V) values from the literature, a heuristic transformation of measured (g1-y) values was applied. Starting with a list of all stars observed in both systems, objects which deviate more than 5$\sigma$ from a preliminary linear least square fit were rejected. A final least square fit gives a correlation of (g1-y) with respect to (B-V). These "calibrated'' values were only used to have an estimate of the temperature range for the apparent peculiar objects (Table 2) found. No other attempt to use these values was done.

For the calibration of our y measurements we have used published V observations. A linear fit always resulted in an excellent correlation with a negligible regression coefficient. The only exception are the data for NGC 6208. This is probable caused by our larger errors as well as those in the measurements of Lindoff (1972).

Candidate peculiar stars in all five clusters were selected if their mean $\Delta a$-values are 3$\sigma$ (taking the standard deviation of the photometry) outside the corresponding confidence intervals of the normality line. If available, we have also checked the apparent peculiarity of $\Delta a$ using (B-V) from the literature. In overall, the $\Delta a$-values for both systems are in excellent agreement shows confidence for the detected apparent peculiar stars.


 

 
Table 1: Observing log.
Cluster Site Nights # $_{g_{\rm 1}}$ # $_{g_{\rm 2}}$ #y
NGC 2489 ESO 3 3 4 3
  UTSO 1 1 1 1
NGC 2567 ESO 4 5 7 5
NGC 2658 ESO 3 4 5 3
  UTSO 1 1 1 1
NGC 5281 ESO 4 10 13 9
NGC 6208 ESO 2 3 4 5


Table 2 lists our observed open clusters and their characteristics from the literature (Lyngå 1987 and the references listed in the corresponding sections). Further, the number of observed stars, the number of individual frames (Table 1) the regression coefficients for all relevant transformations and normality lines are listed. In addition, the described peculiar objects with their $\Delta a$-values and apparent (B-V)0 values as well as the 3$\sigma$ detection limit are given.

The finding charts of our open clusters are shown in Fig. 1. The size of the symbols are inversely proportional to the apparent visual magnitudes of the objects in the sense that larger symbols denote brighter objects.


 

 
Table 2: Summary of results.
Name NGC 2489 NGC 2567 NGC 2658 NGC 5281 NGC 6208
Name C0754-299 C0816-304 C0841-324 C1343-626 C1645-537
l/b 247/-1 250/+3 255/+6 309/-1 334/-6
E(B-V) 0.40 0.13 0.40 0.26 0.18
d[pc] 1450 1600 3800 1300 1000
logt 8.45 8.43 8.50 7.04 9.00
Tr-type I 2 m II 2 m I 2 r I 3 m III 2 r
n(obj) 59 50 55 30 41
$\Delta a$/(B-V)0 +0.032/-0.18 -0.065/+0.41 +0.043/+0.01 +0.049/+0.00  
  +0.052/-0.11 -0.062/+0.55 +0.026/+0.03    
  -0.061/+0.04 -0.097/+0.70      
  +0.031/+0.46        
$V=a+b\cdot(y)$ -8.16/0.997 -8.34/1.005 -8.17/0.995 -8.56/1.013 -6.67/0.916
$a_{0}=a+b\cdot(g_{1}-y)$ 0.754/0.217 0.904/0.381 0.740/0.181 0.735/0.285 0.822/0.228
$a_{0}=a+b\cdot(B-V)$ 0.618/0.056 0.627/0.130 0.640/0.093 0.532/0.096 0.674/0.054
3$\sigma$[mag] 0.013 0.021 0.010 0.006 0.018
n(frames) 13 17 15 32 12


The complete tables with all data for the individual cluster stars are available both from SIMBAD (via anonymous ftp) or upon request from the first author. These tables include the cross identification of objects from the literature, the observed (g1-y) and a values with their corresponding errors, V magnitudes, the (B-V) values from the literature and $\Delta a$-values derived from the normality lines of (g1-y) and (B-V), respectively.

Table 3 lists the apparent peculiar stars found within our investigation. The individual objects are discussed in more detail in the following subsections.

The V versus (g1-y) and a versus (g1-y) diagrams for all five open clusters are shown in Fig. 1. Furthermore, the normality lines and the confidence intervals corresponding to 99.9% are plotted. Filled circles indicate the apparent peculiar objects.

3.1 NGC 2489

Lindoff & Johansson (1968) used photographic plates to investigate this rather rich open cluster at a distance of about 1450pc from the Sun. The earliest members found have spectral types of about B8. The Johnson UBV measurements of Ramsay & Pollacco (1992) were also used for our analysis. Figure 1 shows a well defined Main Sequence with objects down to 15.5mag.

The normality lines based on both (B-V) and (g1-y), respectively, are given as:

\begin{displaymath}a_{0} = 0.618 + 0.056\cdot(B-V) \end{displaymath}

and

\begin{displaymath}a_{0} = 0.754 + 0.217\cdot(g_{1}-y). \end{displaymath}

We find four objects (Nos. 24, 39, 54 and 57) which deviate from the normality line by more than 3$\sigma$. These stars are all members of NGC 2489 according to Lindoff & Johansson (1968). Ramsay & Pollacco (1992) have only one star among their sample (No. 24). They also list this star as being a member of this open cluster. The stars with positive $\Delta a$-values have approximate spectral types of B8, A0 and F3, respectively, whereas the negative one is around A2. These types are well in the range of classical chemically peculiar stars.

3.2 NGC 2567

NGC 2567 was the subject of a detailed investigation by Clariá & Lapasset (1986). They presented photoelectric Johnson UBV photometry for 164 objects brighter than 14.6mag. Further photometry was published by Lindoff (1968) and Ramsay & Pollacco (1992). This open cluster is the most extensively investigated of our five targets.

Several red giants have been found to be members of NGC 2567. Two of them were also observed by us (Fig. 1). Furthermore, the turn off point for this cluster is nicely visible (earliest spectral type of about B8). The Main Sequence consists of late-type B, A and F stars. Clariá & Lapasset (1986) found no evidence for differential reddening across the cluster. They derived a distance of 1620pc from the Sun as well as 420pc above the Galactic plane. The latter is probably incorrect because a Galactic latitude of +3$^\circ$ and the given distance from the Sun, a distance of only 85pc above the Galactic plane has been calculated. The metallicity was found to be nearly solar.

The normality lines based on both (B-V) and (g1-y), respectively, are given as:

\begin{displaymath}a_{0} = 0.627 + 0.130\cdot(B-V) \end{displaymath}

and

\begin{displaymath}a_{0} = 0.904 + 0.381\cdot(g_{1}-y). \end{displaymath}

From Fig. 1 it is evident that three stars lie significantly below the normality line (Nos. 2, 4 and 16). Unfortunately, none of these objects was observed by Clariá & Lapasset (1986). However, No. 16 was addressed as non-member according to Ramsay & Pollacco (1992; No. 3 therein). Using the classical Q-method, they derived an interstellar reddening of about 2.1mag for this star whereas the mean value for this cluster is about 0.8mag. We also tend to believe that the other two stars are probable background stars.


   
Table 3: Stars of the investigated open clusters with peculiar $\Delta a$-values.
Cluster No1 No2 X Y V a $\sigma(a)$ (g1-y) $\sigma(g_{1}-y)$ $\Delta a$ (B-V) $\Delta a$
NGC 2489                        
  24 2 225.5 241.8 13.00 0.575 0.003 -0.511 0.007 -0.068 0.44 -0.066
  39 58 267.5 521.3 12.58 0.690 0.002 -0.532 0.003 +0.052 0.29 +0.056
  54 40 380.9 349.9 12.64 0.667 0.004 -0.546 0.003 +0.032 0.22 +0.037
  57 43 422.3 49.1 14.33 0.694 0.003 -0.418 0.003 +0.031 0.86 +0.028
NGC 2567                        
  2   20.5 441.8 15.34 0.641 0.003 -0.437 0.013 -0.097    
  4   38.8 283.0 15.08 0.657 0.003 -0.487 0.007 -0.062    
  16 3 129.0 215.4 15.67 0.643 0.003 -0.515 0.007 -0.065 0.56 -0.057
NGC 2658                        
  26 33 165.5 236.0 15.31 0.664 0.002 -0.562 0.004 +0.026 0.43 +0.020
  53 80 342.3 310.8 15.98 0.674 0.005 -0.551 0.004 +0.034 0.41 +0.032
NGC 5281                        
  24   322.2 179.0 13.09 0.604 0.006 -0.631 0.008 +0.049    


Col. 1: Cluster name.
Col. 2: Notation sorted after X and Y, respectively (Fig. 1).
Col. 3: Notation according to Lindoff & Johansson (1968; NGC 2489), Ramsay & Pollacco (1992; NGC 2567, NGC 2658).
Cols. 4, 5: X and Y coordinates in the finding charts (Fig. 1).
Col. 6: Visual magnitude.
Cols. 7, 8: mean a-index and its standard deviation.
Cols. 9, 10: mean (g1-y) value and its standard deviation.
Col. 11: Deviation from cluster line a0=a+b $\cdot\ (g_{1}-y)$ using the corresponding constants as listed in Table 2.
Col. 12: (B-V) from the literature.
Col. 13: Deviation from cluster line a0=a+b $\cdot\ (B-V)$ using the corresponding constants as listed in Table 2.


3.3 NGC 2658

With a distance of about 3800pc, NGC 2658 is the most distant open cluster investigated in the $\Delta a$-system so far. Johnson UBV colors were taken from Ramsay & Pollacco (1992). Note that they mixed up Nos. 70 and 119 in their finding chart. No other appropriate publication was found in the literature.

However, the apparent reddening and, hence, the resulting distance and age of this cluster from Ramsay & Pollacco (1992) are controversial. Lyngå (1987) lists E(B-V)=+0.04 mag, r=2000 pc, logt=9.15 and an earliest spectral type of about F8. Since Ramsay & Pollacco (1992) derived E(B-V)=+0.40 mag, the distance and age as given in Table 2 are implied. We are confident that the latter values are correct because of the results from the Q-method (Golay 1974). Ramsay & Pollacco (1992) list 38 members of this cluster together with their (B-V) and (U-B)colors based on CCD observations. These objects have spectral types from late B to mid A type (according to the Q-method) making the values given in Lyngå (1987) highly improbable. Furthermore, the results from the $\Delta a$-photometry support these findings. Taking a distance of 3800pc, we derive a distance of about 400pc above the Galactic plane.

The color-magnitude diagram (Fig. 1) shows a well defined Main Sequence with objects going down to V$\approx$16.5mag. No significant differential reddening was found for this cluster. The normality lines based on both (B-V) and (g1-y), respectively, are given as:

\begin{displaymath}a_{0} = 0.640 + 0.093\cdot(B-V) \end{displaymath}

and

\begin{displaymath}a_{0} = 0.740 + 0.181\cdot(g_{1}-y). \end{displaymath}

Two peculiar objects were detected with a high significance: No. 26 ($\Delta a$=+0.026mag; No. 33 according to Ramsay & Pollacco 1992) and 53 (+0.034mag; No. 80). Both stars are members according to Ramsay & Pollacco (1992). They listed absolute magnitudes of 1.13 ((B-V)0=+0.02mag) and 1.80mag ((B-V)0=+0.00mag), respectively. This matches very well with a spectral type of A0 suggesting that both objects are A0Si stars.

The detection of two chemically peculiar objects in such a distant open cluster lends confidence to investigate the incidence of such objects depending on different metallicities in our Milky Way in the near future.

3.4 NGC 5281

A sequence of 11 B-type stars of this open cluster was observed by Moffat & Vogt (1973). The earliest bright star found has a spectral type of about B5. We have used their photoelectric measurements to derive the normality lines based on (B-V). Together with the one of (g1-y), they are given as:

\begin{displaymath}a_{0} = 0.532 + 0.096\cdot(B-V) \end{displaymath}

and

\begin{displaymath}a_{0} = 0.735 + 0.285\cdot(g_{1}-y). \end{displaymath}

Only one star shows clearly a significant, positive $\Delta a$-value: No. 24. Its position in the color-magnitude diagram suggests that it is a member of this open cluster with a spectral type of about A0. Unfortunately, it was not measured by Moffat & Vogt (1973). Very recently, Sanner et al. (2001) presented proper motions for NGC 5281. Their result for our photometric peculiar object does by no means contradict the membership to this cluster. NGC 5281 contains also a well known supergiant (No. 21 in our sample) which exhibits a slightly enhanced $\Delta a$-value (+0.026mag). However, within the error limits, this value is not significant according to our peculiarity criterion. Using the $\Delta a$-value derived from the Johnson photometric system, it is even less significant: +0.017mag. The well known emission line star No. 10, shows only a slight negative value (-0.006mag), whereas the corresponding value from the Johnson photometric system is -0.019mag. However, it proves that the $\Delta a$ photometric system is able to detect emission line stars.

3.5 NGC 6208

Lindoff (1972) already pointed out that this old open cluster lies in a dense region, 100pc above the Galactic plane. This makes it rather difficult to investigate its membership and astrophysical properties. He finds that only half of the objects in the innermost 12 $\hbox{$^\prime$ }$are true members of NGC 6208.

The color-magnitude diagram shown in Fig. 1 coincides very well with the one given by Lindoff (1972). It already shows the rather broad and poorly defined Main Sequence of this cluster.

The normality lines based on both (B-V) and (g1-y), respectively, are given as:

\begin{displaymath}a_{0} = 0.674 + 0.054\cdot(B-V) \end{displaymath}

and

\begin{displaymath}a_{0} = 0.822 + 0.228\cdot(g_{1}-y). \end{displaymath}

Within the detection limit (0.018mag), no apparent peculiar object was found. Taking the rather old age of NGC 6208 (logt=9.0) this is well in line with prior investigations (Maitzen 1993).

4 Conclusions

In the second paper of our series on CCD $\Delta a$ photometry of open clusters, we presented high accurate data of five open clusters (NGC 2489, NGC 2567, NGC 2658, NGC 5281 and NGC 6208) for more than 230 objects. For none of these clusters differential reddening was found to play an important role. Overall we have detected ten apparent peculiar objects. But three are probably not members of the corresponding cluster. The results of the individual clusters follow:

The newly discovered apparent peculiar stars should now be observed spectroscopically to shed more light on their nature and to unambiguously establish their peculiarity type. The determination of chemically peculiar stars at larger distances than 1000pc from the Sun will allow us to analyze a possible correlation of different metallicities in our Milky Way with respect to the apparent number of chemically peculiar stars.

Acknowledgements
We are indebted to an anonymous referee for helpful comments. This work benefitted from the financial contributions of the City of Vienna (Hochschuljubiläumsstiftung project: Wiener Zweikanalphotometer). Use was made of the SIMBAD database, operated at CDS, Strasbourg, France and the WEBDA database, operated at the Institute of Astronomy of the University of Lausanne.

References

 



Online Material

   \begin{figure} \psfig{figure=ms1177f1.eps,width=155mm}\end{figure} Figure 1: Finding charts 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). North is to the right and west is upwards; 1 pixel=0.5''. The sizes of the open circles are inverse proportional to the V-magnitudes taken from Tables 4 to 8 in the sense that larger open circles denote brighter objects.
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