1994A&A...291..239G

We have mapped the Orion bar and the M17 South-West interface in the 3.3 µm dust feature and the adjacent continuum with a 1" angular resolution and an unprecedented sensitivity. The 3.3µm images reveal the clumpy structure of the gas in the photodissociation region down to scales of a few 10^–3pc . The 3.3µm emitting particles are strictly confined in the neutral/molecular gas just behind the ionization/shock fronts, with no detectable emission from the ionized gas. The transition at the ionization front is not resolved, showing that the 3.3µm emitting particles are destroyed in the HII gas at a rate larger than 2.7x10^–11s^–1. More than the intensity of the radiation field, the ionized gas density is the physical parameter which shows the largest variation at the ionization front. This is why, in the frame of the PAH model, we propose that H⁺ chemisputtering in the HII gas is responsible for the destruction of the molecules. However, other destruction mechanisms such as the photo-thermodissociation of PAH can contribute, and, in conjunction with an efficient formation process operating mostly in the dense neutral/molecular gas, could explain the observations. Concerning the continuum measurements, we show it cannot be explained by the free-free emission of the ionized gas. Two components are observed. The first one is spatially correlated with the 3.3µm feature, and may be due to the same kind of particles. The second one is correlated with the HII gas, exhibits a 3.9 to 2.2µm color ratio F_λ(3.9)/F_λ(2.2) ≃0.81, and is attributed to warm dust grains. The absence of color variations correlated with the radiation field intensity argues for transiently heated very small grains in the HII region. Finally, seven extremely red sources with K-L colors larger than 3.5 are detected in the M17 south-west molecular cloud.