4. DDO analysis

The two CM diagrams of NGC2323 reveal a relatively extensive and well-defined MS with the turn-off point in the late-B-star range and five yellow or red stars (see Table 3 (click here)) which could be giant cluster members. The best method for separating red field stars from the physical members of the cluster is probably on the basis of proper motions and/or radial velocities, but these data are not available for NGC2323. The selection process for red cluster members, however, can be performed by applying two photometric criteria (denoted A and B) described by Clariá & Lapasset (1983), which are based on combined BV and DDO data.

Before any interpretations may be made, DDO observed colours need correction for interstellar reddening. The E(B-V) colour excesses were estimated for each star using the observed (B-V), C(45-48) and C(42-45) colours and the iterative method described by Janes (1977). They are denoted and listed in Col. (2) of Table 5 (click here), while Col. (3) gives the standard deviation computed using Eq. (2) of Clariá & Lapasset (1983). Star 53 was discarded in the following analysis because it falls outside Janes's (1977) calibration. The unreddened DDO colours were then obtained from and the reddening ratios given by McClure (1976).

   

Star MK(DDO) Membership

A B Adopted

12 0.244 0.027 K3/4 III m m nm?

38 0.055 0.030 K2 III nm pm nm

42 0.052 0.038 K2 III nm m nm

121 0.237 0.037 K5 II-III m m m

It is well known that the intrinsic C₀(41-42) index of the DDO photometry measures the strength of the 4216 band of cyanogen of G and K stars, so that the larger the index the greater the absorption by this band. We have computed the DDO cyanogen strength, , for the stars of Table 5 (click here) by applying the procedure described by Piatti et al. (1993). By using Eq. (8) of Piatti et al. (1993) individual iron-to-hydrogen ratios were derived, which were in turn utilized to correct the intrinsic DDO colours for blanketing effects. The derived normal colours allowed us to assign MK spectral types to the stars using the calibration of Clariá et al. (1994). The derived MK spectral types are listed in Col. (4) of Table 5 (click here).

To discern whether or not a red star is a giant member, we applied the photometric criteria (A) and (B) proposed by Clariá & Lapasset (1983). To carry out this, we used the cluster reddening and distance modulus estimated in the present study. The results obtained from criterion (A) should be taken with caution because of the non-uniform reddening in the cluster field. The membership status derived from criteria (A) and (B) are presented in Cols. (5)-(6) of Table 5 (click here). As shown, according to the above mentioned criteria, stars 38 and 42 are likely foreground field giants, while stars 12 and 121 appear to be probable cluster members. In order to clarify the membership status of these two stars, we were greatly aided by the radial velocity observations of star 121 kindly provided by J.C. Mermilliod. Over a total of 11 CORAVEL measures of star 121, he obtained a mean radial velocity of  km/s, in good agreement with the cluster radial velocity ( km/s) published by Harris (1976). On the other hand, star 12 lies mag below the isochrone of log t=8.0 in the M_v vs. (B-V)₀ diagram (see Sect. 5), while the position of star 121 is clearly consistent with the cluster age. We then conclude that star 121 is the only giant member of NGC2323.