A HIGHLY SENSITIVE RADIO SURVEY OF SYMBIOTIC STARS AT 3.6 CENTIMETERS

We present the results of a highly sensitive VLA survey of symbiotic stars at 3.6 cm. We searched for radio emission in 99 stars, and detected emission in 50, showing that symbiotic stars are one of the more prolific stellar radio emitters. The median sensitivity (3 sigma) of the survey is 110 muJy. We have combined our measurements with other data, primarily in the infrared (IR) and ultraviolet (UV) to extend the study of the collective radio properties in Seaquist & Taylor. As in earlier work, the radio emission is consistent with thermal bremsstrahlung from the ionized portion of the envelopes of symbiotic stars, presumed to be associated with their mass outflow. We find that the radio emission correlates well with the mid-IR (12 and 25 mum), and less well with emission in the near and the far-IR. The correlation in the mid-IR indicates that both the ionized gas and warm dust are involved in the mass outflow, whereas the poor correlation at shorter IR wavelengths is consistent with the interpretation that the near-IR is emitted primarily by the red giant atmosphere. We briefly discuss the interpretation of the poor correlation in the far-infrared and suggest that the warmer dust is heated by the hot star while the cool dust is heated primarily by the red giant. We find a formally significant correlation between radio and UV continuum, although the data in the ultraviolet are sparse. This may signify a causal relation between mass-loss rate and UV luminosity, not unexpected for symbiotic stars. From a study of this relationship, we show that the radio-emitting regions are optically thick and/or density bounded. In addition, the S-type stars are more optically thick in the radio than D-type stars, in conformity with the ''STB model'' of Seaquist, Taylor, & Button and Taylor & Seaquist. This supports similar conclusions from independent considerations in Seaquist & Taylor. We show that the mass-loss rate measured by the radio emission is greater for symbiotic stars whose red giant spectral type is later. This behavior is somewhat similar to that seen in nonsymbiotic red giants except that the mass-loss rate is generally higher in symbiotic giants than in local nonsymbiotic giants of similar type. We discuss results for AX Per, R Aqr, and PU Vul individually.