New constraints on the millimetre emission of six debris discs

The presence of dusty debris around main-sequence stars denotes the existence of planetary systems. Such debris discs are often identified by the presence of excess continuum emission at infrared and (sub-)millimetre wavelengths, with measurements at longer wavelengths tracing larger and cooler dust grains. The exponent of the slope of the disc emission at submillimetre wavelengths, ‘q’, defines the size distribution of dust grains in the disc. This size distribution is a function of the rigid strength of the dust producing parent planetesimals. As a part of the survey ‘PLAnetesimals around TYpical Pre-main seqUence Stars’, we observed six debris discs at 9 mm using the Australian Telescope Compact Array. We obtain marginal (∼3σ) detections of three targets: HD 105, HD 61005 and HD 131835. Upper limits for the three remaining discs, HD 20807, HD 109573 and HD 109085 provide further constraint of the (sub-)millimetre slope of their spectral energy distributions. The values of q (or their limits) derived from our observations are all smaller than the oft-assumed steady-state collisional cascade model (q = 3.5), but lie well within the theoretically expected range for debris discs q ∼ 3–4. The measured q values for our targets are all <3.3, consistent with both collisional modelling results and theoretical predictions for parent planetesimal bodies being ‘rubble piles’ held together loosely by their self-gravity.