Sharpless 170 and the surrounding interstellar medium

Sharpless 170 is a diffuse HII region ionized by a single main sequence O-star located near the periphery of a small dense molecular cloud at a distance of similar to2 kpc. We describe wide-field observations of the region in the radio continuum, in HI and CO-lines, and in the far-infrared which delineate the major ionized, atomic, molecular and dust components of the gas affected by the exciting star. From the thermal continuum emission we estimate the mass of ionized gas at similar to350 M-. within a radius of similar to7 pc. The HI (lambda21 cm) and far-infrared observations show an extended low-density atomic component, of similar to 1000 M-. within an irregular boundary surrounding the ionized gas of mean radius similar to10 pc. Mean densities in the HI and HII are similar, in the range 9-16 nucleons cm(-3). CO emission shows a molecular cloud of similar to 1150 M-. within an area 6 pc x 4 pc with densities similar to2000 nucleons cm(-3). A compact infrared component coincides with the cloud. The exciting star is located on the near side of the cloud, just inside the southern periphery. Sh170 is an example of a young HII region with the ionized gas, seen in Halpha emission, streaming outward in the manner of a "champagne flow". Although the observed velocities of the HI are close to the mean velocity of the CO cloud, the morphology of the associated atomic hydrogen closely resembles that seen in the surrounds of other young HII regions which show clear evidence of expansion of their HI. We propose that much of the HI is a diffuse dissociation zone beyond the ionization front, in directions from the star within a wide annulus, approximately transverse to the line-of-sight, between the dense photon-bounded region on the far side of the star and the density-bounded ionized flow region on the near side. In this view, much of the associated atomic gas, like the ionized gas, has been eroded from the molecular cloud in a small fraction (less than or equal to10%) of the star's main sequence lifetime.