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Grain growth signatures in the protoplanetary discs of Chamaeleon and Lupus




We present Australia Telescope Compact Array results of a 3 and 7 mm continuum survey of 20 T Tauri stars in the Chamaeleon and Lupus star-forming regions. This survey aims to identify protoplanetary discs with signs of grain growth. We detected 90 per cent of the sources at 3 and 7 mm, and determined the spectral slopes, dust opacity indices and dust disc masses. We also present temporal monitoring results of a small subset of sources at 7, 15 mm and 3+6 cm to investigate grain growth to centimetre (cm) sizes and constrain emission mechanisms in these sources. Additionally, we investigated the potential correlation between grain growth signatures in the infrared (10 μm silicate feature) and millimetre (1–3 mm spectral slope, α).

Eleven sources at 3 and 7 mm have dominant thermal dust emission up to 7 mm, with seven of these having a 1–3 mm dust opacity index less than unity, suggesting grain growth up to at least mm sizes. The Chamaeleon sources observed at 15 mm and beyond show the presence of excess emission from an ionized wind and/or chromospheric emission. Long-time-scale monitoring at 7 mm indicated that cm-sized pebbles are present in at least four sources. Short-time-scale monitoring at 15 mm suggests that the excess emission is from thermal free–free emission. Finally, a weak correlation was found between the strength of the 10 μm feature and α, suggesting simultaneous dust evolution of the inner and outer parts of the disc. This survey shows that grain growth up to cm-sized pebbles and the presence of excess emission at 15 mm and beyond are common in these systems, and that temporal monitoring is required to disentangle these emission mechanisms.