Lithium and oxygen in globular cluster dwarfs and the early disc
  accretion scenario
  A new scenario --early disc accretion-- has been recently proposed to explain
the discovery of multiple stellar populations in Galactic globular clusters.
According to this model, the existence of well defined (anti)-correlations
amongst light element abundances (i.e. C, N, O, Na) in the photospheres of
stars belonging to the same cluster (and the associated helium enrichment), is
caused by accretion of the ejecta of short lived interacting massive binary
systems (and single fast rotating massive stars) on fully convective pre-main
sequence low- and very low-mass stars, during the early stages of the cluster
evolution. We investigated the constraints provided by considering
simultaneously the observed spread of lithium and oxygen (and when possible
also sodium) abundances for samples of turn-off stars in NGC6752, NGC6121 (M4),
and NGC104 (47Tuc), and the helium abundance of their multiple main sequences.
These observations provide a very powerful test for the accretion scenario,
because the observed O, Li and He abundance distributions at the turn off can
be used to constrain the composition (and mass) of the accreted matter, and the
timescales of the polluting stars. In case of NGC6752 we could not find a
physically consistent solution. In case of M4, spectroscopic errors are too
large compared to the intrinsic spread, to constrain the properties of the
accreted matter. As for 47Tuc, we could find a physically consistent solution
for the abundances of He and O (and Na) in the accreted gas, and predict the
abundances of these elements in the accreted matter only if pollution happens
with timescales of ~1 Myr, hence polluters are objects with masses of the order
of several tens of solar masses (abridged).
