HIGH-VELOCITY GAS ASSOCIATED WITH THE MASSIVE EVOLVED STAR G25.5+0.2

We have obtained single-dish observations of the extremely broad radio recombination line (RRL) H42 alpha at 85.688 GHz with the IRAM 30 m t elescope and the 2.6 mm line of (CO)-C-12 (J = 1-0) at 115.271 GHz wit h the NRAO 12 m telescope toward the Galactic object G25.5+0.2. We app ly a non-LTE model to the available RRLs and determine the average phy sical conditions in the nebula surrounding the central source: electro n number density, N-e=8x10(4) cm(-3); electron temperature, T-e=6150 K ; and volume filling factor, f=2 x 10(-5). These results are consisten t with a photoionized, optically thin shell that has been enriched by processed stellar material. From the (CO)-C-12 line profile, we find n o evidence for an interaction between G25.5+02 and any molecular mater ial. Our analysis supports the suggestion of Subrahmanyan et al. (1993 ) and Becklin et al. (1994) that G25.5+0.2 is an evolved, massive star with a thin shell of photoionized gas.