HUBBLE-SPACE-TELESCOPE WFPC2 IMAGING OF M16 - PHOTOEVAPORATION AND EMERGING YOUNG STELLAR OBJECTS

We present Hubble Space Telescope WFPC2 images of elephant trunks in t he H II region M16. There are three principle results of this study. F irst, the morphology and stratified ionization structure of the interf ace between the dense molecular material and the interior of the H II region is well understood in terms of photoionization of a photoevapor ative flow. Photoionization models of an empirical density profile cap ture the essential features of the observations, including the extreme ly localized region of [S II] emission at the interface and the observ ed offset between emission peaks in lower and higher ionization lines. The details of this structure are found to be a sensitive function bo th of the density profile of the interface and of the shape of the ion izing continuum. Interpretation of the interaction of the photoevapora tive flow with gas in the interior of the nebula supports the view tha t much of the emission from H II regions may arise in such flows. Phot oionization of photoevaporative flows may provide a useful paradigm fo r interpreting a wide range of observations of H II regions. Second, w e report the discovery of a population of small cometary globules that are being uncovered as the main bodies of the elephant trunks are dis persed. Several lines of evidence connect these globules to ongoing st ar formation, including the association of a number of globules with s tellar objects seen in IR images of M16 or in the continuum HST images themselves. We refer to these structures as evaporating gaseous globu les, or ''EGGs.'' These appear to be the same type of object as the ne bular condensations seen previously in M42. The primary difference bet ween the two cases is that in M16 we are seeing the objects from the s ide, while in M42 the objects are seen more nearly face-on against the backdrop of the ionized face of the molecular cloud. We find that the ''evaporating globule'' interpretation naturally accounts for the pro perties of objects in both nebulae, while avoiding serious difficultie s with the competing ''evaporating disk'' model previously applied to the objects in M42. More generally, we find that disk-like structures are relatively rare in either nebula. Third, the data indicate that ph otoevaporation may have uncovered many EGGs while the stellar objects in them were still accreting mass, thereby freezing the mass distribut ion of the protostars at an early stage in their evolution. We conclud e that the masses of stars in the cluster environment in M16 are gener ally determined not by the onset of stellar winds, as in more isolated regions of star formation, but rather by disruption of the star formi ng environment by the nearby O stars. (C) 1996 American Astronomical S ociety.