Relating grain size distributions in circumstellar discs to the spectral index at millimetre wavelengths

Abstract

The excess emission seen in spectral energy distributions (SEDs) is commonly used to infer the properties of the emitting circumstellar dust in protoplanetary and debris discs. Most notably, dust size distributions and details of the collision physics are derived from SED slopes at long wavelengths. This paper reviews the approximations that are commonly used and contrasts them with numerical results for the thermal emission. The inferred size distribution indexes p are shown to be greater and more sensitive to the observed sub(mm) spectral indexes, αmm, than previously considered. This effect results from aspects of the transition from small grains with volumetric absorption to bigger grains that absorb and emit near to their surface, controlled by both the real and the imaginary part of the refractive index. The steeper size distributions indicate stronger size-dependence of material strengths or impact velocities or, otherwise, less efficient transport or erosion processes. Strong uncertainties remain because of insufficient knowledge of the material composition, porosity, and optical properties at long wavelengths.

Four new planetesimals around typical and pre-main-sequence stars (PLATYPUS) debris discs at 8.8 mm

“…Pl and are calculated using equations 1 and 2 respectively. This table has been adapted from Löhne (2020 pared to MacGregor et al (2016) and our correlation decreases, we suggest that as the later star debris disc sample becomes more populated it's likely that the trend between and the spectral type will disappear. We also include -value model predictions from Dohnanyi (1969), Pan & Sari (2005), Pan & Schlichting (2012), Gáspár et al (2012b), andSchüppler et al (2015) indicated by the various hatched regions in the plot.…”

“…Here, we present new results from PLATYPUS, an ongoing survey of debris discs at 8.8 mm with the Australian Telescope Compact Array (ATCA) (Ricci et al 2015b;Marshall et al 2017). The targets in this work are four relatively young debris disc host stars spanning a broad range of stellar luminosities that, including sources from the previously largest disc comparison survey (Löhne 2020), increases the number of systems with measured values to 22.…”

“…This theoretically estimated range of is supported by a number of millimetre wavelength observations (Ricci et al 2012(Ricci et al , 2015bMacGregor et al 2016;Marshall et al 2017;Wilner et al 2018;Moór et al 2020). To date, numerical modelling of observations has constrained the parameter to 3.2 < < 3.8 for various grain materials (Hughes et al 2018;Löhne 2020).…”

Planetesimal/Debris discs

“…Thus, it is in principle possible to constraint the size distribution and test collisional models (Ricci et al, 2015;MacGregor et al, 2016). However, uncertainties about the compo-sition and structure of grains means that size distribution measurements suffer from great systematic uncertainties (Löhne, 2020).…”

Four new PLanetesimals Around TYpical and Pre-main seqUence Stars (PLATYPUS) Debris Discs at 8.8mm

“…This theoretically estimated range of 𝑞 is supported by a number of millimetre wavelength observations (Ricci et al 2012(Ricci et al , 2015bMacGregor et al 2016;Marshall et al 2017;Wilner et al 2018;Moór et al 2020). To date, numerical modelling of observations has constrained the parameter to 3.2 < 𝑞 < 3.8 for various grain materials (Hughes et al 2018;Löhne 2020).…”