Observing multiple stellar populations with VLT/FORS2 Main sequence photometry in outer regions of NGC 6752, NGC 6397, and NGC 6121 (M 4)

Aims. We present the photometric analysis of the external regions of three Galactic globular clusters: NGC 6121, NGC 6397, and NGC 6752. The main goal is the characterization of multiple stellar populations along the main sequence (MS) and the study of the radial trend of different populations hosted by the target clusters. Methods. The data have been collected using the FOcal Reducer and low dispersion Spectrograph 2 (FORS2) mounted at the Very Large Telescope (VLT) UT1 of the European Southern Observatory (ESO) in UBVI filters. From these data sets we extracted high-accuracy photometry and constructed color-magnitude diagrams (CMDs). We exploit appropriate combinations of colors and magnitudes, which are powerful tools for identifying multiple stellar populations, such as B versus U - B and V versus c(U,B,I) = (U - B) - (B - I) CMDs. Results. We confirm previous findings of a split MS in NGC 6752 and NGC 6121. Apart from the extreme case of omega Centauri, this is the first detection of multiple MS from ground-based photometry. For NGC 6752 and NGC 6121, we compare the number ratio of the blue MS to the red MS in the cluster outskirts with the fraction of first and second generation stars measured in the central regions. There is no evidence for significant radial trend. The MS of NGC 6397 is consistent with a simple stellar population. We propose that the lack of multiple sequences is due both to observational errors and to the limited sensitivity of U, B, V, I photometry to multiple stellar populations in metal-poor GCs. Finally, we compute the helium abundance for the stellar populations hosted by NGC 6121 and NGC 6752, finding a mild (Delta Y similar to 0.02) difference between stars in the two sequences.