2010A&A...513A...9C

We report observations, using the IRAM 30m telescope, of 30 ultracompact and hypercompact HII regions in the lines of HCO⁺(3-2) and/or HCO⁺(1-0) and H30α and/or H39α. Images are presented in both HCO⁺(3-2) and H30α toward a subset of regions (16 in HCO⁺(3-2), 14 in H30α) 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^–3M_☉/yr, implying time scales for massive star formation <10⁵yrs. 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.