Since the purpose of the research at this stage is to establish the physical characteristics of those stars that belong to NGC 2422, the first step is, consequently, to determine which of the stars belong to the cluster. Strömgren photometry provides a safe method for determining the absolute magnitude of each star. The method used has been previously employed and is basically an extension of Crawford's calibrations (Crawford 1975, 1979) for A and F stars using a technique modified by Nissen (1988). A method developed by Shobbrook (1984) has been employed for the early type stars (see Peña & Peniche 1994 for details).
Table 6: Combined photometry of NGC 2422
Table 7: Reddening and unreddened photometry of NGC 2422
The final results, after the reddening and the absolute magnitude for each star were determined, are presented in Table 7 (click here). In this analysis star 34 (ADS 6216) was not considered. The data are presented in the following fashion: Col. 1 gives the identification number for each star, reddening E(b-y) and the unreddened indices; , Distance Modulus (DM), distance and metal content are also presented.
With the distances for each star established, a histogram of the distances (in parsecs) for the early type stars, the late A and F stars was carried out and is shown in Fig. 1 (click here). From this figure it is evident that the majority of the stars lie around 450 pc. A statistical probability was assigned by adjusting a Gaussian distribution to the histogram of the distances to the stars in the range between the interval 250 to 600 pcs in which the majority of the stars are found.
Mean values and a standard deviation for the member stars were determined for E(b-y) (in magnitudes), distance (in parsecs) and metal content. The numerical values were of ; and , respectively.
As in the previous papers, several conclusions can be drawn from the data in Table 6 (click here) with respect to the number of Ap stars, the number of binaries, the mean metallic content and the age of the cluster. Diagram provides information on the binaries, but none were found and no Ap stars were determined in the diagram since these stars lie in well-determined regions in such diagram. To establish the age of the cluster we first decided which stars were the hottest by plotting them on the theoretical grids of Relyea & Kurucz (1978) and of Neri et al. (1993) compiled from Kurucz (1989) and Mihalas (1972). Once the earliest stars, 36 and 41, were determined along with their effective temperatures, gravities and luminosities, their position in the evolutionary tracks of Meynet et al. (1993) was unambiguously established and the age of the cluster fixed. The numerical values of the temperature for the hottest stars is 14 000 K; the value of gravity for basically all the stars is 4.0; the mean metal content [Fe/H] of the cluster is 0.016 with a standard deviation of 0.171. With all these values, the position of these stars in the evolutionary tracks gives an age of yr. These values are in excellent agreement with those of Lang (1992) who lists an E(B-V) of 0.08, which corresponds to an E(b-y) of 0.056, a distance of 480 pc and an age of yrs. Problems still remain with respect to the evolutionary effect pointed out by Shobbrook (1984) which shows an exceedingly large trend in the diagram. This is most surely due to defects in the reddening calibration of the early type stars. In fact, the calibration employed here and that of Crawford (1970) show a systematic difference as a function of . This would definitively be an important problem to tackle in the future.