H-I PHOTODISSOCIATION ZONES AROUND 2 H-II REGIONS IN AURIGA, SH-217 AND SH-219

The H II regions Sharpless 217 and 219 have been mapped in H I line an d continuum emission at 21 cm wavelength with the wide-field DRAO synt hesis telescope. Both regions have prominent H I zones on the peripher ies of the ionization indicating direct photodissociation of the surro unding molecular gas by the exciting stars. At the estimated distance to the two regions of 4.7 kpc, Sh 217 comprises a mass of 340 M. in io nized form and 640 M. in atomic gas, while Sh 219 comprises 34 M. in i onized and 86 M. in surrounding atomic gas. Although the ionization of each region is principally due to a single exciting star, we identify a minor peak of emission in Sh 217 with a pair of closely spaced B st ars. The H I emission from each region extends over approximately 8 km s-1 in radial velocity but shows no indication of a shocked component or of other systematic motions. Far-infrared images from IRAS show th at the heated dust component in each case is approximately coextensive with the photodissociation region, although there is no detailed corr elation between the H I and infrared emissions. A distance-independent parameter, the observed ratio of the radio flux density to the infrar ed flux, is compared with the predicted ratio for the particular excit ing stars to provide estimates in the two cases of the fractions of st ellar emission which escape to the general interstellar medium. Parame ters, measured and derived, for the two regions are compared to those from models of the development of H I dissociation zones with time. Th e comparison indicates that Sh 217 and Sh 219 are relatively young reg ions with ages not more than a few times 10(4) yr.