A multi-frequency study of symbiotic stars - II. Submillimetre/millimetre observations.

It has been suggested that free-free emission from symbiotic stars should become optically thin at submillimeter and millimeter wavelengths. This paper presents 0.45-2.0 mm continuum measurements for a selected sample of symbolic stars, H2-38, HM Sagittae, V1016 Cygni, AG Pegasi, and R Aquarii. These observations clearly confirm that the D-type symbiotic stars HM Sge and V1016 Cyg exhibit a spectral turnover at a wavelength of about 3 cm. The 1.1-2.0 mm spectral indices for these sources agree well with that expected for optically thin free-free emission. Contemporaneous optical H-beta intensities are combined with submillimeter free-free flux densities to determine extinction due to circumstellar and interstellar dust. In addition, the 0.45-0.8 mm data for HM Sge and V1016 Cyg tentatively suggest that an extra emission component may be contributing to the observed flux densities at shorter wavelengths. This may be interpreted as either cold dust emitting at temperature of about 10-20 K or as emission from an optically thick, ionized stellar wind within the cavity evacuated by the last outburst.