Resolution of Distance Ambiguities of Inner Galaxy Massive Star Formation Regions. I.
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Abstract
Fifty-four ultracompact (UC) H II regions in the GLIMPSE survey region (|b| < 1° and 30° < l < 70°) were observed in H2CO and H110α using the 305 m Arecibo telescope. By analyzing H2CO absorption against the UC H II region continuum emission, we resolve the distance ambiguity toward 44 sources. This determination is critical to measure global physical properties of UC H II regions (e.g., luminosity, size, mass) and properties of the Galaxy (e.g., spiral structure, abundance gradients). We find that the distribution of UC H II regions in this survey is consistent with a "local spur," the Perseus, Sagittarius, and Scutum arms as delineated by Taylor & Cordes. However, departures from model velocities produce distance uncertainties only slightly smaller than the proposed arm separations.