Radio Continuum Interferometry of Dark Clouds - Part Two - a Study of the Physical Properties of Local Newly Formed HII Regions

Abstract

A radio continuum survey of dark clouds (Gilmore 1980a) revealed only a few new regions of star formation. The observations of these individual regions are presented and discussed in this paper. The regions include S140 IR, S156, and Mon R2, as well as the dark cloud complexes in Perseus, Taurus, Orion, and Ophiuchus. The H II regions detected in the radio continuum have associated visible nebulosity and are excited by early B stars. However, five infrared sources thought to have the luminosity of early B stars were not detected in the radio continuum. These five sources showed high correlation with the presence of CO self-absorption, CO emission over a wide range of velocities, and type I OH masers, but an absence of coincident visible nebulosity and detectable radio continuum emission. Therefore, it is suggested that they represent an earlier evolutionary stage than those H II regions detected in the radio continuum. This first evolutionary state marks the presence of "pre-emergent" (with respect to the molecular cloud) cocoon stars. H II regions in the second evolutionary state are marked by the presence of detectable radio continuum emission, i.e., they are stronger than 10 mJy at 2695 MHz. They have associated visible nebulosity, are relatively large, and appear to be located at the edges of molecular clouds. We designate these as "emergent edge" H II regions. The fact that many young H II regions are edge H II regions implies that massive stars may be born near the edges of clouds, a phenomenon previously suggested by several other investigators. Comparison of the results of this survey with other investigators of H II regions and newly formed massive stars indicates that the radio continuum detectability of H II regions surrounding young massive cocoon stars is a function of the age and mass of the star. 09.5 ZAMS stars and earlier types may exhibit detectable radio continuum emission before the cocoon reaches the edge of a cloud and breaks up into a core/halo-type of object. BO ZAMS stars may exhibit characteristics of both evolutionary states.