  Photodissociated gas bears the signature of the dynamical evolution of the
ambient interstellar medium impacted by the mechanical and radiative feedback
from an expanding HII region. Here we present an analysis of the kinematics of
the young Trifid nebula, based on velocity-resolved observations of the
far-infrared fine-structure lines of [C II] at 158 micron and [O I] at 63
micron. The distribution of the photodissociated regions (PDRs) surrounding the
nebula is consistent with a shell-like structure created by the HII region
expanding with a velocity of 5 km/s. Comparison of ratios of [C II] and [O I]
63 micron intensities for identical velocity components with PDR models
indicate a density of 1e4 /cm^3. The red- and blue-shifted PDR shells with a
combined mass of 516 Msun have a kinetic energy of ~1e47 erg. This is
consistent with the thermal energy of the HII region as well as with the energy
deposited by the stellar wind luminosity from HD 169442A, an O7 V star, over
the 0.5 Myr lifetime of the star. The observed momentum of the PDR shell is
lower than what theoretical calculations predict for the radial momentum due to
the shell being swept up by an expanding HII region, which suggests that
significant mass loss has occurred in M20 due to the dispersal of the
surrounding gas by the advancing ionization front.
