  Fast outflows and their interaction with slow shells (generally known as the
fossil circumstellar envelope of asymptotic giant branch stars) play an
important role in the structure and kinematics of protoplanetary and planetary
nebulae (pPNe, PNe). To properly study their effects within these objects, we
also need to observe the intermediate-temperature gas, which is only detectable
in the far-infrared (FIR) and submillimetre (submm) transitions. We study the
physical conditions of the outflows presented in a number of pPNe and PNe, with
a focus on their temperature and excitation states. We carried out
Herschel/HIFI observations in the submm lines of 12CO in nine pPNe and nine PNe
and complemented them with low-J CO spectra obtained with the IRAM 30m
telescope and taken from the literature. The spectral resolution of HIFI allows
us to identify and measure the different nebular components in the line
profiles. The comparison with large velocity gradient (LVG) model predictions
was used to estimate the physical conditions of the warm gas in the nebulae,
such as excitation conditions, temperature, and density. We found high kinetic
temperatures for the fast winds of pPNe, typically reaching between 75 K and
200 K. In contrast, the high-velocity gas in the sampled PNe is colder, with
characteristic temperatures between 25 K and 75 K, and it is found in a lower
excitation state. We interpret this correlation of the kinetic temperature and
excitation state of fast outflows with the amount of time elapsed since their
acceleration (probably driven by shocks) as a consequence of the cooling that
occurred during the pPN phase.
