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Observations of protoplanetary disks (PPDs) at sub-mm wavelengths suggestthat polarisation is caused by scattering of thermal radiation. Most of thedust models that are used to explain these observations have major drawbacks:They either use much smaller grains than expected from dust evolution models,or result in lower polarisations than observed. We investigate the effect ofdust grain porosity on the polarisation due to scattering at sub-mm wavelengthsarising from grains of up to mm sizes, as they are expected to be present inthe midplane of PPDs. Using the effective medium theory, we calculated theoptical properties of porous dust and used them to predict the behaviour of thescattering polarisation. Subsequently, radiative transfer simulations for PPDswith porous dust were performed to analyse the additional effect of the opticaldepth structure, and thus the effect of multiple scattering events andinhomogeneous temperature distributions on the polarisation. We find thatporous dust with moderate filling factors of about 10\% increase thepolarisation compared to compact grains. For higher grain porosities, that is,grains with a filling factor of 1\% or lower, the extinction opacity decreases,as does the optical depth of a disk with constant mass. Consequently, theunpolarised direct radiation dominates the scattered flux, and the polarisationdrops rapidly. Even though the simulated polarisation is higher than in thecase of compact grains, it is still below the typical observed values forface-on disks. However, the polarisation can be increased when crucial modelassumptions derived from disk and dust evolution theories, for instance, dustsettling and mm-sized dust grains, are dropped. In the case of inclined disks,however, our reference model is able to achieve polarisation degrees of about1\%, and using higher disk masses, even higher than this.

Self-scattering on large, porous grains in protoplanetary disks with dust settling