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
cU,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 ω 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 (ΔY ~ 0.02) difference
between stars in the two sequences.

Based on observations at the European Southern Observatory using the
Very Large Telescope on Cerro Paranal through ESO programme 089.D-0978
(P. I. G. Piotto).Catalogs of photometry for all globular clusters
presented are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/573/A70