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

¹ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter street, Madison, WI 53706, USA e-mail: churchwell@astro.wisc.edu
² Instituto de Radio Astronomía Milimétrica, Avenida Divina Pastora, 7 Núcleo Central, 18012 Granada, Spain e-mail: sievers@iram.es
³ Institut de Radio Astronomie Millimétrique, Domaine Universitaire de Grenoble, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France

Received: 1 April 2009
Accepted: 18 November 2009

Abstract

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.