Large grains in discs around young stars: ATCA observations of WW Chamaeleontis, RU Lupi, and CS Chamaeleontis

Abstract

Context. Grains in discs around young stars grow from interstellar submicron sizes to planetesimals, up to thousands of kilometres in size, over the course of several Myr. Thermal emission of large grains or pebbles can be best observed at centimetre wavelengths. However, other emission mechanisms can contribute, most notably free-free emission from stellar winds and chromospheric activity.Aims. We aim to determine the mechanisms of centimetre emission for three T Tauri stars. WW Cha and RU Lup have recently been found to have grain growth at least up to millimetre sizes in their circumstellar discs, based on millimetre data up to 3.3 mm. CS Cha has similar indications of grain growth in its circumbinary disc.Methods. The T Tauri stars WW Cha and RU Lup were monitored over the course of several years at millimetre and centimetre wavelengths, using the Australia Telescope Compact Array (ATCA). The new ATCA 7 mm system was also used to observe CS Cha at 7 mm.Results. WW Cha was detected on several occasions at 7 and 16 mm. We obtained one detection of WW Cha at 3.5 cm and upper limits only for 6.3 cm. The emission at 16 mm was stable over periods of days, months, and years, whereas the emission at 3.5 cm is found to be variable. A second young stellar object, Ced 112 IRS 4, was found in the field of WW Cha at 16 mm. RU Lup was detected at 7 mm. It was observed at 16 mm three times and at 3 and 6 cm four times and found to be variable in all three wavebands. CS Cha was detected at 7 mm, but the signal-to-noise ratio was not high enough to resolve the gap in the circumbinary disc. The typical resolution of the 7 and 16 mm observations was 5–10 arcsec with rms ~0.2 mJy. Conclusions. The emission at 3, 7, and 16 mm for WW Cha is explained well by thermal emission from millimetre and centimetre-sized “pebbles”. The cm spectral index between 3.5 and 6.3 cm is consistent with the emission from an optically-thick ionised wind, although the high variability of the cm emission points to a non-thermal contribution. The spectral energy distributions of both RU Lup and CS Cha from 1 to 7 mm are consistent with thermal emission from mm-sized grains. The variability of the longer-wavelength emission for RU Lup and the negative spectral index suggests non-thermal emission, arising from an optically-thin plasma.