A millimeter survey of ultra-compact HII-regions and associated molecular clouds

We report observations, using the IRAM 30 m telescope, of 30 ultracompact and hypercompact HII regions in the lines of HCO+(3-2) and/or HCO+(1-0) and H and/or H. Images are presented in both HCO+(3-2) and H toward a subset of regions (16 in HCO+(3-2), 14 in H) with a resolution of 12”. In addition, H13CO+(3-2) observations are reported toward 13 HII regions where HCO+(3-2) displays complex profiles. It is shown that the absorption dips in the HCO+ profiles are due to HCO+ self-absorption, not absorption of the HII free-free emission or warm dust emission surrounding the HII region or two velocity components along the line of sight. It was found that among the sources with self-absorbed profiles, 8 are contracting and 5 are expanding. Mass fluxes are found to be typically a few times 10-3 yr-1, implying time scales for massive star formation <105 yrs. HCO+ and H2 column densities are estimated for a subset of the sources from which masses of the dense central cloud cores were estimated. Implications of the derived column densities, masses, flow velocities, and mass fluxes are discussed.