The Expansion Rate of and Distance to G5.89−0.39

The expansion rate of the ionized shell in the ultracompact H II region G5.89-0.39 has been measured using three VLA observations spanning 5.3 yr. The nebular shell is expanding at a rate of 2.5±0.5 mas yr -1 . From the angular radius and expansion rate, we find a dynamical age of 600 + 250 −125 yr. Combined with a model for the nebular emission and H radio recombination line spectra, we estimate an expansion velocity of about 35 km s −1 and a source distance of 2.0 + 0.7 −0.4 kpc. This is slightly closer than most previous estimates of 2.5-3 kpc. We see no evidence for northeast-southwest expansion that might be associated with the molecular outflow seen in SiO; however, our data are consistent with previous observations of a north-south flow, indicating that the SiO flow may originate from another young star along the line of sight.