THE COLLECTIVE RADIO PROPERTIES OF SYMBIOTIC STARS

We present radio measurements of symbiotic stars which complement and extend our earlier survey (Paper I). These new observations extend the search for radio emission and provide multifrequency and multiepoch measurements of stars previously detected. We conclude that while a number of symbiotics are known to be radio variable, there is no evidence from the stars we have detected that there are time variations in excess of ∼30% over a period of several years. The radio flux densities are found to be correlated with brightness in the IR, particularly at the longer IR wavelengths, where dust emission dominates. Also we confirm our previous finding that the symbiotics with the latest red giant spectral types are the most luminous radio emitters. In particular, the D-types are the most radio-luminous. These properties are consistent with the view that the most radio-luminous symbiotics are those with the greatest mass-loss rates. Virtually all detected stars with measurements at more than one frequency exhibit a positive spectral index, consistent with optically thick thermal bremsstrahlung. From measurements at three frequencies, we conclude that the optically thin spectral turnover for many stars must lie at short centimeter wavelengths. A comparison between radio and Hβ fluxes confirms the probable existence of an optically thin turnover in this region for D-types, but it also indicates that the turnover for S-types may occur at much shorter wavelengths. Using a binary model for the radio emission we estimate the binary separation for a number of radio-emitting symbiotics, and find that the distribution of inferred binary separations is dramatically different for IR D-type than for S-types. In particular, the inferred separations for the D-types is much larger which is the sense expected from other evidence bearing on the different nature of these two types. Mass-loss rates derived from measurements at radio wavelengths show, in agreement with measurements in the IR, that mass-loss rates in symbiotics are somewhat larger than that in ordinary red giants of the same spectral type.