THE OPEN CLUSTER NGC 2451

Abstract

This paper, the first of a series concerning the photoelectric photometry of some bright southern open clusters, is devoted to NGC 2451, an open cluster centered around the red star c Puppis ( a = 7h43TM6, 8 = _37°51' (1950); Zn = 252?4, bn = 3?5). The cluster was observed photoelectrically during February 1964 with the NSF photometer attached to the 32-inch reflector of the La Plata Observatory. Twenty-two stars were observed in the UBV system, and the results are listed in Table I. The seventh column gives the number of observations. The spectral types in the MK system were determined by M. Jaschek and C. Jaschek from spectra taken by L. Milone at the Bosque Alegre reflector of the Cordoba Observatory. The last columns give the proper motions (Cape Annals, Vol. 18, 1955) and the radial velocities. The color-magnitude array is presented in Figure 1, where the full line is the zero-age main sequence (ZAMS) according to Blaauw (1963). The color excess EB.V was found to be (M)5 ± 0.05, a value which is supported by the (17B),(B-V) plot shown in Figure 2, where the ZAMS was taken from Johnson (1963) and Allen ( 1963 ) . From the fitting of the color-magnitude diagram the distance of the cluster is 260 parsecs ± 10 ( estimated error ) . Stars Nos. 15, 16, and 21, which are foreground G stars according to their colors, and star No. 6, which is a B5 III object with an interstellar calcium line, are certainly not members of the cluster. The spectrum of star No. 10 is very poor, but according to the Jascheks it has a strong interstellar Ca n line, so its membership seems doubtful. The same conclusion can be reached from its discordant proper motion.