High-resolution radio images of the symbiotic star R Aquarii

We present MERLIN radio observations of the nearby symbiotic star R Aquarii at 5 and 1.7 GHz with respective nominal resolutions of ∼0.04 and 0.13 arcsec, comparable to the resolution of the optical and ultraviolet images obtained with the HST. Comparison with the HST images shows that the radio emission is associated with features observed at optical and UV wavelengths. In particular, this comparison demonstrates that we have resolved the radio emission from the binary system at the core of R. Aquarii. Using the Seaquist, Taylor & Button model for radio emission from symbiotic stars, we derive the mass loss rate of the Mira to be 10−6M⊙yr−1 10−6M⊙yr−1 , which is typical for Miras. Observations at two frequencies provide the spectral index distribution. We find that the features C1, C2 and A′ have spectral indices commensurate with thermal emission. The nature of the emission from feature A is not clear, though we argue that it is also thermal. It is 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 evolution of the emitting region, in terms of shock heating by an interaction 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.