An aluminium tool for multiple stellar generations in the globular
  clusters 47 Tuc and M 4
  We present aluminium abundances for a sample of about 100 red giant stars in
each of the Galactic globular clusters 47 Tuc (NGC 104) and M 4 (NGC 6121). We
have derived homogeneous abundances from intermediate-resolution FLAMES/GIRAFFE
spectra. Aluminium abundances are from the strong doublet Al I at 8772-8773 A
as in previous works done for giants in NGC 6752 and NGC 1851, and nitrogen
abundances are extracted from a large number of features of the CN molecules,
by assuming a suitable carbon abundance. We added previous homogeneous
abundances of O and Na and newly derived abundances of Mg and Si for our
samples of 83 stars in M 4 and 116 stars in 47 Tuc to obtain the full set of
elements from proton-capture reactions produced by different stellar
generations in these clusters. By simultaneously studying the Ne-Na and Mg-Al
cycles of H-burning at high temperature our main aims are to understand the
nature of the polluters at work in the first generation and to ascertain
whether the second generation of cluster stars was formed in one or, rather,
several episodes of star formation. Our data confirm that in M 4 only two
stellar populations are visible. On the other hand, for 47 Tuc a cluster
analysis performed on our full dataset suggests that at least three distinct
groups of stars are present on the giant branch. The abundances of O, Na, Mg
and Al in the intermediate group can be produced within a pollution scenario;
results for N are ambiguous, depending on the C abundance we adopt for the
three groups.
