Tracing the Layers of Photodissociated Gas in the Trifid Nebula

Abstract

Photodissociated gas bears the signature of the dynamical evolution of the ambient interstellar medium impacted by the mechanical and radiative feedback from an expanding H ii 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 μm and [O i] at 63 μm. The distribution of the photodissociated regions (PDRs) surrounding the nebula is consistent with a shell-like structure created by the H ii region expanding at a velocity of 5 km s−1. Comparison of ratios of [C ii] and [O i]63 μm intensities for identical velocity components with PDR models indicate a density of 104 cm−3. The redshifted and blueshifted PDR shells with a combined mass of 516 M ⊙ have a kinetic energy of ∼1047 erg. This is consistent with the thermal energy of the H ii 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 H ii region, which suggests that significant mass loss has occurred in M20 due to the dispersal of the surrounding gas by the advancing ionization front.