The morphologies and physical properties of ultracompact H II regions

Seventy-five ultracompact (UC) H II regions were observed with 0. 4" resolution at 2 and 6 cm using the Very Large Array (VLA), and their radio continuum brightness distributions are presented in the form of contour plots. H76α recombination line profiles were obtained for six sources using a 4. 4" beam. The flux density distributions of the sources from radio to near-infrared wavelengths are assembled using the IRAS Point Source Catalog and data from the literature. We show that these UC H II regions have electron densities >= 10⁴^ cm^-3^, emission measures >~ 10⁷^ pc cm^-6^, and diameters <~ 0. 1 pc, consistent with their being small photoionized nebulae produced by O and B stars embedded in clouds of molecular gas and dust. At high angular resolution five different morphologies of the ionized gas were seen: spherical or unresolved (43%), cometary (20%), core-halo (16%), shell (4%), and irregular or multiply peaked (17%). Cometary UC H II regions have parabolic-shaped ionization fronts, sharp leading edges, and long, narrow tails. They may be produced by the bow shock of a star moving supersonically through the molecular gas. The central cavities of shell and cometary UC H II regions are unstable, short-lived configurations, and unless some mechanism (stellar wind or radiation pressure) maintains them it is unlikely that we would observe so many. Often several UC H II regions with different morphological types are observed in the same field of view. The large far-infrared luminosities and multiple UC H II components suggest that embedded OB stars which produce the UC H II regions are associated with clusters of lower mass stars. We present evidence that the UC H II region phase of a massive star must last for a significant fraction of its main-sequence lifetime. The total number of UC H II regions found in this incomplete sample of the Galactic plane is inconsistent with a UC H II region lifetime of < 3 x 10⁴^ yr, the length of time it would take a typical object to expand and become undetectable with the VLA. The expansion of the H II is constrained by some mechanism, perhaps by the infall of the surrounding medium or the motion of the star with respect to the neutral gas, so the small sizes of UC H II regions do not necessarily indicate that they are extremely young. This work provides the basis for studying the distinctive far-infrared colors of the cool dust cocoons associated with UC H II regions. In a companion paper we search the IRAS data base for other embedded OB stars to obtain a direct measure of their population and distribution throughout the Galaxy and in the Magellanic Clouds.