2011A&A...532A.127D

Accurate distances are necessary for determining the physical properties of massive star-formation regions and the structure of our Galaxy. One way to approach these studies is by determining the kinematic distances of HII regions. This study determines the kinematic distances of HII regions located in the inner Galaxy and studies their spatial distribution. We observed 251 HII regions with the 25-m radio telescope of Nanshan Station, looking for H₂CO absorption lines and H110α radio recombination lines (RRLs). The kinematic distances were calculated using the velocity of H110α lines and a model of the Galactic rotation. The kinematic distance ambiguity was resolved using the H₂CO velocities. We detected the H110α RRLs in 28 sources and H₂CO absorption lines in 59 sources. In the latter case, 43 features had not previously been observed. H₂CO and H110α lines were simultaneously detected toward 23 HII regions. We resolved the kinematic distance ambiguities for 14 HII regions and 20 intervening molecular clouds. Our statistical analysis shows that UCHII regions are tightly confined to the Galactic plane, and their vertical height distribution is about 25.4pc. The corresponding value of the HII regions is 28.6pc. There is a good statistical relationship between the fluxes of H₂CO and infrared 100µm, for those HII regions with both H₂CO absorption lines and H110α RRLs detected. This suggests that there is a weak correlation between the continuum fluxes at 6cm and infrared 100µm.