The molecular hydrogen emission associated with the Orion bright bar

We have measured nine infrared spectra along a strip which crosses the Orion bright bar ionization front. Emission from vibrationally excited molecular hydrogen is widespread. The molecular gas is excited both by shocks and by absorption of ultraviolet radiation. The shocked gas lies in a layer outside the ionization front; the shock is probably driven by the expansion of the H II region. The radiatively excited gas is present at every position measured, although no significant enhancement is found at the optical ionization front. This suggests that the molecular gas between the ionization front and the shock is dissociated. We note that line ratio measurements of the molecular hydrogen emission are crucial to the determination of the excitation mechanism.