  [abridged] Previous analyses of lithium abundances in main sequence and red
giant stars have revealed the action of mixing mechanisms other than convection
in stellar interiors. Beryllium abundances in stars with lithium abundance
determinations can offer valuable complementary information on the nature of
these mechanisms. Our aim is to derive beryllium abundances along the whole
evolutionary sequence of an open cluster, IC 4651. These Be abundances are used
together with previously determined Li abundances, in the same sample stars, to
investigate the mixing mechanisms in a range of stellar masses and evolutionary
stages. New beryllium abundances are determined from high-resolution, high
signal-to-noise UVES spectra using spectrum synthesis and model atmospheres.
The careful synthetic modelling of the Be lines region is used to calculate
reliable abundances in rapidly rotating stars. The observed behavior of Be and
Li is compared to theoretical predictions from stellar models including
rotation-induced mixing, internal gravity waves, atomic diffusion, and
thermohaline mixing. Beryllium is detected in all the main sequence and
turn-off sample stars, both slow- and fast-rotating stars, including the Li-dip
stars, but was not detected in the red giants. Confirming previous results, we
find that the Li dip is also a Be dip, although the depletion of Be is more
modest than that of Li in the corresponding effective temperature range. For
post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung
gap than expected from classical predictions as does the Li dilution. A clear
dispersion in the Be abundances is also observed. Theoretical stellar models
including the hydrodynamical transport processes mentioned above are able to
reproduce well all the observed features.
