3. UBV analysis

3.1. Membership criteria

Figures 3 (click here) and 4 (click here) show the two colour-magnitude (CM) diagrams for all the measured stars, wherein the solid line represents the zero age main sequence (ZAMS) taken from Schmidt-Kaler (1982). Both CM diagrams reveal the presence of a reasonably broad and slightly evolved main sequence (MS), typical of an early intermediate-age open cluster. The MS extends over a range of nearly 6 magnitudes, with the faintest stars at mag. The CM diagrams also show field star contamination beyond the MS and at the red giants colour range.

  
Figure 3: The observed V,(B-V) diagram for NGC2323. Schmidt-Kaler's (1982) ZAMS has been adjusted to E(B-V)=0.25 and V-M_v=10.62

  
Figure 4: The observed V,(U-B) diagram for NGC2323. Schmidt-Kaler's (1982) ZAMS has been adjusted to E(U-B)=0.17 and V-M_v=10.62

Figures 5 (click here) and 6 (click here) are the colour-colour (CC) diagrams for stars with (B-V) smaller and greater than 1.0 mag, respectively. In particular, the star distribution in Fig. 5 (click here) suggests the existence of variable absorption across the cluster field.

  
Figure 5: Colour-colour diagram for stars with mag in NGC2323. Schmidt-Kaler's (1982) ZAMS has been plotted for E(B-V)=0.0 (solid line) and E(B-V)=0.25 (dashed line)

  
Figure 6: Colour-colour diagram for stars with (B-V) > 1.0 mag. The solid line is the intrinsic relation of Schmidt-Kaler (1982) for luminosity class III stars

Membership in NGC2323 has been determined by examining the positions of the individual stars in the CM and CC diagrams. Except for the red giants, cluster members were selected following the criteria described by Clariá & Lapasset (1986), namely by requiring the location of the star in both CM diagrams to correspond to the same evolutionary stage and that the location of the star in the CC diagram be close to the MS, the maximun departure accepted being 0.10 mag. Stars classified as probable members (m), possible members (pm) and non members (nm), are represented by filled circles, filled triangles and open circles in Figs. 3 (click here) to 6 (click here), respectively. Table 1 also includes the membership status assigned to each star.

   

Star E(B-V) E(U-B) Star E(B-V) E(U-B)

9 0.290 0.205 93 0.224 0.159

18 0.307 0.216 97 0.293 0.207

20 0.260 0.184 98 0.242 0.172

22 0.319 0.225 102 0.257 0.181

24 0.292 0.206 103 0.256 0.181

25 0.306 0.215 104 0.271 0.191

28 0.174 0.124 105 0.235 0.166

32 0.199 0.141 107 0.230 0.163

36 0.289 0.204 109 0.230 0.163

37 0.208 0.148 111 0.284 0.201

40 0.246 0.174 114 0.290 0.204

46 0.336 0.236 115 0.270 0.191

49 0.289 0.204 116 0.230 0.163

54 0.189 0.135 119 0.287 0.202

55 0.249 0.176 119' 0.274 0.193

69 0.280 0.197 123 0.252 0.178

73 0.303 0.214 137 0.223 0.158

74 0.292 0.206 138 0.265 0.188

81 0.279 0.197 151 0.260 0.184

82 0.256 0.181 X 0.263 0.186

88 0.279 0.197 Z 0.233 0.165

92 0.194 0.138

Table 4: Individual colour excesses for blue members

3.2. Foreground reddening

The reddening of NGC2323 was determined from the brightest cluster members. According to Fig. 5 (click here), all member stars with (U-B) < -0.49(B-V) + 0.19 are very likely earlier than the spectral type A2. Individual E(B-V) and E(U-B) colour excesses were derived for these stars from Eqs. (2) and (8) of Garcıa et al. (1988). The values of E(B-V) and E(U-B) for individual stars are listed in Table 4 (click here). The measured full width of the observed CC diagram from these stars is  mag, which is larger than 0.11, the lower limit estimated by Burki (1975) for clusters with differential reddening. We conclude that the reddening across NGC2323 is non-uniform. The mean values and standard deviations from 43 stars of Table 4 (click here) are: , , which have been used in subsequent discussions. The resulting total visual absorption in front of the cluster is then 0.76 mag, assuming that A_v/E(B-V)=3.0 (Clariá 1974).

The spatial correlation of reddening with position is shown in Fig. 7 (click here). Contrary to the north-south variation of reddening suggested by Schneider (1987) from Stromgrëm photometry of 10 stars, the interstellar material in front of NGC2323 appears to be distributed following an irregular pattern. However, the mean E(B-V) colour excess derived by Schneider agrees very well with our estimation. Other reddening determinations by different authors are also within the range 0.17-0.33 mag. (e.g., Becker & Fenkart 1971; Mostafa et al. 1983), which are the lower and upper limits of the reddening distribution derived in this study.

  
Figure 7: E(B-V) colour excesses of blue members plotted spatially. E(B-V) ranges: 0.17-0.23 (); 0.23-0.28 (); 0.28-0.33 (full ). X,Y coordinates were taken from Hoag et al. (1961)

3.3. Cluster distance

The ZAMS of Schmidt-Kaler (1982), appropiately reddened, can be fitted to the observed cluster sequence in the two CM diagrams by assuming an apparent distance modulus V-M_v=10.62. The resulting true distance modulus is then V₀-M_v=9.86, which corresponds to a distance of 940 pc from the sun and 21 pc below the galactic plane.

The uncertainty in E(B-V) is about 0.05 mag and the error in the fitting to the ZAMS is estimated to be 0.15 mag. Taking these uncertainties into account, we find that the cluster distance may be increased or decreased by about 10%.

There can be little doubt that some earlier distances of this cluster (e.g., Collinder 1931, 675 pc; Rieke 1935, 520 pc; Barkhatova 1950, 740 pc; Hoag et al. 1961, 1170 pc) were either over- or underestimated. However, we found a reasonable agreement with the estimations by Cuffey (1941) and Mostafa et al. (1983), who derived 910 pc and 995 pc, respectively.

3.4. Cluster diameter

To estimate the extent of NGC2323, star counts in wide concentric circles around the cluster center adopted by Hoag et al. (1961) have been made. Numbers of stars per square arcmin as a function of the distance from the cluster center were counted to limiting magnitudes of V=10, 11, 12, 13, 15 and (plate limit), the errors being estimated on the basis of Poisson statistics. The results for the counts down to V=10 and for the V ranges: 10-11, 11-12, 12-13 and 13-15 are shown in Fig. 8 (click here). The following conclusions may be drawn from the star counts: (1) The star distribution for the different V intervals seems to be very similar, suggesting that the cluster has not suffered appreciable mass segregation effects. This is not the case, however, for the range V=13-15 due to incompleteness of the data. (2) Cluster members are clearly concentrated in the central region within a radius of () arcmin, which has been adopted for NGC2323. (3) The adopted angular radius leads to a linear diameter of 5.46 pc, so that the minimum stellar density amounts to 1.28 stars pc^-3.

  
Figure 8: Star counts for five limiting magnitudes in NGC2323