HIGH-RESOLUTION RADIO IMAGES OF THE SYMBIOTIC STAR R-AQUARII

We present MERLIN radio observations of the nearby symbiotic star R Aq uarii at 5 and 1.7 GHz with respective nominal resolutions of approxim ately 0.04 and 0.13 arcsec, comparable to the resolution of the optica l and ultraviolet images obtained with the HST. Comparison with the HS T images show that the radio emission is associated with features obse rved at optical and UV wavelengths. In particular, this comparison dem onstrates that we have resolved the radio emission from the binary sys tem at the core of R Aquarii. Using the Seaquist, Taylor & Button mode l for radio emission from symbiotic stars, we derive the mass loss rat e of the Mira to be approximately 10(-6) M. yr-1, which is typical for Miras. Observations at two frequencies provide the spectral index dis tribution. We find that the features C1, C2 and A' have spectral indic es commensurate with thermal emission. The nature of the emission from feature A is not clear, though we argue that it is also thermal. It i s apparent that the fluxes in features C2 and A' have changed between our observations and those of Kafatos et al. The 5-GHz flux of feature A' has increased by a factor of 17 over a decade. We discuss the evol ution of the emitting region, in terms of shock heating by an interact ion with an outflow from the central binary, and find that an outflow velocity of at least 300 km s-1 is required, consistent with the model of Solf.