Deep CCD photometry in globular clusters. I. The main sequence of M 4.

Color-magnitude and color-color diagrams have been constructed for a 4 x 3-arcmin field of the globular cluster M4, on the basis of deep UBV CCD images. The color-magnitude diagram indicates that the main sequence, down to almost 3 mag below the turnoff, has an intrinsic width no greater than + or 0.02 mag in (B-V), implying either that the variation in He abundance in these stars must be less than + or - 0.07, or that the fractional variation in metallicity is not larger than + or - 0.22. Field stars in the direction of the cluster yield E(B-V) of 0.37 + or - 0.06, while the best present metallicity estimate for M4 is -0.93 + or - 0.31. The corresponding isochrones from VandenBerg (1983) are found to be in excellent general agreement with the present data.