PHYSICAL CONDITIONS IN PHOTODISSOCIATION REGIONS - M17 NORTHERN BAR

We present results of long-slit spectroscopy, from 2.0 to 2.5 mum, of the photodissociation region in the northern bar of the H II region M1 7. The emission from ionized material peaks to the south of that from molecular gas, in agreement with previous measurements. The diagnostic 1-0 S(1) to 2-1 S(1) line ratio is approximately 3, larger than the r atio for pure fluorescence, implying that the H-2 is slightly thermali zed. The ratio does not change along the extent of the molecular emiss ion which fills the slit (approximately 60 arcsec), suggesting that th e collisional de-excitation rate is constant. We take this to mean tha t the physical conditions are constant across the region, implying tha t in the northern bar of M17 we are viewing the face of the molecular cloud. A model is proposed, which describes the geometry. 16 H-2 lines are detected, and the column densities of the upper levels of the tra nsitions show clearly that UV excitation is responsible for the emissi on. We model the physical conditions in the region by fitting six line ratios with a two-component PDR model. This modelling suggests that t he gas is clumpy and that the observations are consistent with 76 per cent of the beam area being filled by gas at a density of 10(5) cm-3 a nd 16 per cent by gas of density 10(6) cm-3. It is shown that a lower limit to the critical density for de-excitation of the v = 1, J = 3 le vel is 10(5) cm-3. We also find that the ratio of the ortho- and para- states of H-2 cannot be equal to the LTE value of 3. We introduce two physical models, which may explain the low value of the ortho-to-para ratio that is observed.