A Search for Radio Continuum Emission Towards Long-period Variable Stars

Abstract

We hereby report on a sensitive search for radio continuum observations from a sample of 34 Mira and semi-regular variable stars. The main aim of this survey was to search for thermal free-free emission from post-shock ionised gas. Thirty-four stars were observed at 3- and 6-cm using the Australia Telescope Compact Array. Radio continuum emission was detected from one source only, the symbiotic Mira R Aqr. No continuum emission was detected from the other sources, with three-sigma upper limits of typically 0.3 mJy. From the upper limits to the radio flux densities, we have found upper limits to the gas brightness temperatures near two stellar radii at a characteristic size of $5\times10^{13}$ cm. Upper limits to shock velocities have been estimated using a shock model. For the 11 nearest sources in our sample we obtain brightness temperatures below 6 000 K and shock velocities below 13 km s$^{-1}$. For 11 out of 14 sources with previously published detections, the radio brightness temperatures are below 4 000 K. For an upper limit of 4 000 K, we estimate that the shock velocities at two stellar radii are below 10 km s$^{-1}$.