Radial variation of the stellar mass functions in the globular clusters
  M15 and M30: clues of a non-standard IMF?
  We exploit a combination of high-resolution Hubble Space Telescope and
wide-field ESO-VLT observations to study the slope of the global mass function
(alphaG) and its radial variation (alpha(r)) in the two dense, massive and post
core-collapse globular clusters M15 and M30. The available data-set samples the
clusters' Main Sequence down to 0.2 Msun and the photometric completeness
allows the study of the mass function between 0.40 Msun and 0.75 Msun from the
central regions out to their tidal radii. We find that both clusters show a
very similar variation in alpha(r) as a function of clustercentric distance.
They both exhibit a very steep variation in alpha(r) in the central regions,
which then attains almost constant values in the outskirts. Such a behavior can
be interpreted as the result of long-term dynamical evolution of the systems
driven by mass-segregation and mass-loss processes. We compare these results
with a set of direct N-body simulations and find that they are only able to
reproduce the observed values of alpha(r) and alphaG at dynamical ages (t/trh)
significantly larger than those derived from the observed properties of both
clusters. We investigate possible physical mechanisms responsible for such a
discrepancy and argue that both clusters might be born with a non-standard
(flatter/bottom-lighter) initial mass function.
