SIMBAD references

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.