Physical conditions in photodissociation regions: M17 northern bar

Abstract

We present results of long-slit spectroscopy, from 2.0 to 2.5 μm, of the photodissociation region in the northern bar of the H II region M17. 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 ratio for pure fluorescence, implying that the H 2 is slightly thermalized. The ratio does not change along the extent of the molecular emission which fills the slit (approximately 60 arcsec), suggesting that the collisional de-excitation rate is constant. We take this to mean that the physical conditions are constant across the region, implying that in the northern bar of M17 we are viewing the face of the molecular cloud