A VLA Study of Ultracompact and Hypercompact H II Regions from 0.7 to
  3.6 cm
  We report multi-frequency Very Large Array observations of three massive star
formation regions (MSFRs) containing radio continuum components that were
identified as broad radio recombination line (RRL) sources and hypercompact
(HC) H II region candidates in our previous H92alpha and H76alpha study:
G10.96+0.01 (component W), G28.20-0.04 (N), and G34.26+0.15 (B). An additional
HC H II region candidate, G45.07+0.13, known to have broad H66alpha and
H76alpha lines, small size, high electron density and emission measure, was
also included. We observed with high spatial resolution (0.9" to 2.3") the
H53alpha, H66alpha, H76alpha, and H92alpha RRLs and the radio continuum at the
corresponding wavelengths (0.7 to 3.6 cm). The motivation for these
observations was to obtain RRLs over a range of principal quantum states to
look for signatures of pressure broadening and macroscopic velocity structure.
We find that pressure broadening contributes significantly to the line widths,
but it is not the sole cause of the broad lines. We compare radio continuum and
dust emission distributions and find a good correspondence. We also discuss
maser emission and multi-wavelength observations reported in the literature for
these MSFRs.
