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 164 9 stars in an extended held in the globular cluster M4=NGC 6121. The p osition 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 th e majority of recent studies, probably because of our field being loca ted in the western, most reddened part of the cluster. The spread of s tars around the main-sequence locus has an unusual shape (broadening n ear the turnoff point and close to the photometric limit), earlier not iced by Richer et al. 1995 (ApJ, 451, L17) and attributed by them to p hotometric effects. We have analyzed the possible reasons for this eff ect and concluded that, at least in our case, it is most probably due to differential absorption. We confirm the deviation of theoretical is ochrones from the fainter parts of the M4 main-sequence locus found by Alcaino et al. 1988 (ApJ, 330, 569). Several curious structural detai ls (a depression and an excess of stars) have been revealed in the bri ghter parts of the main-sequence luminosity function of M4. (C) 1997 A merican Astronomical Society.