  We have obtained radial gradients in the spectral features D4000 and Mg2 for
a sample of 11 central cluster galaxies (CCGs). The new data strongly confirm
the correlations between line-strength indices and the cooling flow phenomenon
found in our earlier study. We find that such correlations depend on the
presence and characteristics of emission lines in the inner regions of the
CCGs. CCGs in cooling flow clusters exhibit a clear sequence in the D4000-Mg2
plane, with a neat segregation depending on emission-line types and blue
morphology. This sequence can be modelled, using stellar population models with
a normal IMF, by a recent burst of star formation. In CCGs with emission lines,
the gradients in the spectral indices are flat or positive inside the
emission-line regions, suggesting the presence of young stars. Outside the
emission-line regions, and in cooling flow galaxies without emission lines,
gradients are negative and consistent with those measured in CCGs in clusters
without cooling flows and giant elliptical galaxies. Index gradients measured
exclusively in the emission-line region correlate with mass deposition rate. We
have also estimated the radial profiles of the mass transformed into new stars
which are remarkably parallel to the radial behaviour of the mass deposition
rate. A large fraction (probably most) of the cooling flow gas accreted into
the emission-line region is converted into stars. We discuss the evolutionary
sequence suggested by McNamara (1997), in which radio triggered star formation
bursts take place several times during the lifetime of the cooling flow. This
scenario is consistent with the available observations.
