On the radio spectrum of CI Cygni: a test of popular models for symbiotic stars

Abstract

We have determined the spectral energy distribution at wavelengths between 6 cm and 0.85 mm for the proto-typical S(stellar)-type symbiotic star, CI Cygni, during quiescence. Data were obtained simultaneously with the Very Large Array and the SCUBA sub-millimetre (sub-mm) camera on the James Clerk Maxwell Telescope. The data have allowed us to determine the free-free turnover frequency of the ionised component, facilitating a model-dependent estimate of the binary separation to compare with the known orbital parameters of CI Cug and to critically test the known models for radio emission from symbiotic stars. In particular, our data rule out the two most popular models: ionization of the giant wind by Lyman continuum photons from its hot companion and emission resulting from the interaction of winds from the two binary components.