In order to carry out an exhaustive analysis of the NGC 2422 cluster, all photometric data available must be included in the study. There are two other sources of photometric data which have been already mentioned: Nissen (1988) and Shobbrook (1984). A comparison between the different measurements was necessary and desirable. To begin with, a homogenization of the identification had to be done since each previous observer lists the observations carried out with different identification numbers. The observations by Nissen were given in x and y coordinates from the map of Hoag et al. (1961); Shobbrook (1984), on the other hand, gives his data in van Shewick's and Smyth & Nandy's numbers; Smyth & Nandy (1962) report their own identification chart. Hence, in view of this chaos, we have constructed a cross identification table (Table 4 (click here)) in which all the observations with photometry and their different nomenclature have been summarized.
Nissen (1988) was looking for the metal content determination for different clusters; hence he observed only late type stars, much fainter than those measured here or by Shobbrook (1984). Therefore there was no overlap between these sets.
Table 4: Nomenclature of the stars observed in
A detailed discussion of the comparison to Shobbrook's (1984) data is necessary. To begin with, the intersection of both sets is seventeen stars, a significant number given the size of the sets (about 30 stars in each one). A comparison of the magnitudes and the colour indexes of both sets was carried out for the stars in the intersection, with the idea of taking mean values for these stars. The following findings were obtained: for V, , and the relationship was linear with correlation coefficients on the order of 0.9 and slopes close to 45 degrees. However, the relationship for the index b-y, showed an abrupt jump; a difference between the sets was found since the numerical values obtained by Shobbrook (1984) were systematically lower than those obtained here. As a first approach to confirming which set was correct, we compared the goodness of our transformation of the photometric values for our standard stars with those of Crawford (1970) in all the colour indexes and, in each case, the correlation coefficients were better than 0.997.
We carried out further comparisons of the photometric data from Shobbrook (1984) and the present paper to the previous photometric data of NGC 2422, both photographic (van Schewick 1966; Smyth & Nandy 1962) and photoelectric (Hoag et al. 1961, and the compilation of Mermilliod 1976). The following was found:
A comparison of Shobbrook's V magnitudes with those of Mermilliod's (1976) and Smyth & Nandy's (1962) gave excellent linear relations. Correlation coefficients were of equal to 0.999 and 0.853, respectively. Problems arose, however, when the b-y colour index of Shobbrook was compared to the B-V values of Mermilliod (1976) and Smyth & Nandy (1962). In each case the correlation coefficients were rather poor, equal to 0.647 and to 0.837, respectively. More puzzling is the fact that the curve that best adjusts to the b-y colour index of Shobbrook (1984) and the B-V index of S & N is not a straight line, but a quadratic fit. In fact, the quadratic adjustment gives an of 0.866, numerically better than in the linear case.
Table 5: Compilation of the photometry of the stars with large scatter in V
In view of this, a B-V versus b-y plot was constructed from the standard stars that appear in the American Ephemeris and Nautical Almanac with data in both colour indexes. In this sample a set of 45 stars within a range in b-y between -0.25 and 1.5 is found. A linear regression with a correlation coefficient of 0.9953 was obtained. A direct comparison of the line obtained from Shobbrook's data to S & N's was done and it became immediately evident that the slopes were totally different and that the b-y interval of Shobbrook's data is very narrow. On the other hand, the comparison of our data to that of S & N gave a straight line parallel and basically equal to that of the standard stars; our b-y coverage is much broader, from -0.2 to 1.3. Hence, since the standard stars of Shobbrook (1984) were chosen in a similar manner as the observed stars, i.e. with a very narrow range in b-y, this might be the cause of the differences of the b-y values in his photometry from all the other photometric sets on NGC 2422. In view of the systematic difference of his data with all other data sets, a linear regression with our data was established as and all the stars measured by Shobbrook (1984) were transformed in the b-y index according to the previous relation into our system, which, as has been previously proved, is equivalent to the standard system of Crawford (1970). Finally, mean values for the stars observed in common were obtained for magnitude and all the colour indexes. The correctness of the criteria of considering mean values is established by the fact that the mean values of the differences between Shobbrook's data and the present paper's are -0.001,-0.003,-0.004,-0.005, and 0.005 in V, b-y, , and , respectively. However, there are a few stars, particularly in V, which show large differences. A brief review of the values in different photometric compilations, presented in Table 5 (click here), supports the evidence that the scatter is large and might be due to intrinsic variability of these stars. Another example of possible variability is that of star 28 whose large difference of 0.1 mag in indicates a variable, such as Be star. The results presented in Table 6 (click here) are the values obtained from Nissen (1988), Shobbrook (1984), the mean values of Shobbrook (1984) and our photometry for those stars observed in common once the b-y values of the former were translated to our (Crawford's 1970) system and those obtained in the present research. In Table 6 (click here) the sequence number of the observed stars in decreasing is listed in Col. 1; the remaining columns list the photometric values obtained; the last column lists the source of the photometry reported.
Figure 1: Histogram of the distance of the stars in the direction of NGC 2422