  Massive AGN-driven outflows are invoked by AGN-galaxy co-evolutionary models
to suppress both star formation and black hole accretion. Massive molecular
outflows have recently been revealed in some AGN hosts. However, the physical
properties and structure of these AGN-driven molecular outflows are still
poorly constrained. Here we present new IRAM PdBI observations of Mrk231, the
closest quasar known, targeting both the CO(1-0) and CO(2-1) transitions. We
detect broad wings in both transitions, tracing a massive molecular outflow
with velocities up to 800 km/s. The wings are spatially resolved at high
significance level (5-11 sigma), indicating that the molecular outflow extends
on the kpc scale. The CO(2-1)/CO(1-0) ratio of the red broad wings is
consistent with the ratio observed in the narrow core, while the blue broad
wing is less excited than the core. The latter result suggests that quasar
driven outflow models invoking shocks (which would predict higher gas
excitation) are not appropriate to describe the bulk of the outflow in Mrk231.
However, we note that within the central 700 pc the CO(2-1)/CO(1-0) ratio of
the red wing is slightly, but significantly, higher than in the line core,
suggesting that shocks may play a role in the central region. We also find that
the average size of the outflow anticorrelates with the critical density of the
transition used as a wind tracer. This indicates that, although diffuse and
dense clumps coexist in the outflowing gas, dense outflowing clouds have
shorter lifetime and that they evaporate into the diffuse component along the
outflow or, more simply, that diffuse clouds are more efficiently accelerated
to larger distances by radiation pressure.
