Protoplanetary nebulae and young PN CO spectra

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 75K and 200K. In contrast, the high-velocity gas in the sampled PNe is colder, with characteristic temperatures between 25K and 75K, 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.