The main sequence of the globular cluster M4=NGC 6121 from CCD NTT photometry

With the ESO NTT, we have obtained accurate CCD UBVI photometry of 1649 stars in an extended held in the globular cluster M4=NGC 6121. The position of the main-sequence turnoff is found to be at V-TO=17(m).0+/-0(m).1, (B-V)=0.863+/-0.007, (V-I)=1.224+/-0.010, (B-I)=2.084+/-0.010. Our turnoff point is redder and fainter than those found for M4 in the majority of recent studies, probably because of our field being located in the western, most reddened part of the cluster. The spread of stars around the main-sequence locus has an unusual shape (broadening near the turnoff point and close to the photometric limit), earlier noticed by Richer et al. 1995 (ApJ, 451, L17) and attributed by them to photometric effects. We have analyzed the possible reasons for this effect and concluded that, at least in our case, it is most probably due to differential absorption. We confirm the deviation of theoretical isochrones from the fainter parts of the M4 main-sequence locus found by Alcaino et al. 1988 (ApJ, 330, 569). Several curious structural details (a depression and an excess of stars) have been revealed in the brighter parts of the main-sequence luminosity function of M4. (C) 1997 American Astronomical Society.