The influence of dust grain porosity on the analysis of debris disc observations

TL;DR

This study investigates how dust grain porosity affects debris disc observations and modeling, revealing that porosity significantly influences the estimated minimum grain size and blowout size, with implications for interpreting disc properties.

Contribution

It quantifies the impact of dust grain porosity on debris disc modeling, deriving an analytical formula for blowout size as a function of porosity and stellar luminosity.

Findings

Blowout size increases with porosity up to a factor of two.

Porosity causes overestimation of minimum grain size when using compact grain models.

Porosity partially explains the high ratio of minimum grain size to blowout size in previous studies.

Abstract

Debris discs are often modelled assuming compact dust grains, but more and more evidence for the presence of porous grains is found. We aim at quantifying the systematic errors introduced when modelling debris discs composed of porous dust with a disc model assuming spherical, compact grains. We calculate the optical dust properties derived via the fast, but simple effective medium theory. The theoretical lower boundary of the size distribution -- the so-called 'blowout size' -- is compared in the cases of compact and porous grains. Finally, we simulate observations of hypothetical debris discs with different porosities and feed them into a fitting procedure using only compact grains. The deviations of the results for compact grains from the original model based on porous grains are analysed. We find that the blowout size increases with increasing grain porosity up to a factor of two. An analytical approximation function for the blowout size as a function of porosity and stellar luminosity is derived. The analysis of the geometrical disc set-up, when constrained by radial profiles, are barely affected by the porosity. However, the determined minimum grain size and the slope of the grain size distribution derived using compact grains are significantly overestimated. Thus, the unexpectedly high ratio of minimum grain size to blowout size found by previous studies using compact grains can be partially described by dust grain porosity, although the effect is not strong enough to completely explain the trend.