SIMBAD references

Context. Observations of protoplanetary disks in the sub-millimetre wavelength range suggest that polarisation is caused by scattering of thermal re-emission radiation. Most of the dust models that are used to explain these observations have major drawbacks: they either use much smaller grain sizes than expected from dust evolution models, or result in polarisation degrees that are lower than observed.
Aims. We investigate the effect of dust grain porosity on the observable polarisation due to scattering at sub-millimetre wavelengths arising from grain size distributions up to millimetre sizes, as they are expected to be present close to the midplane of protoplanetary disks.
Methods. Using the effective medium theory, we calculated the optical properties of porous dust and used them to predict the behaviour of the observable polarisation degree due to scattering. Subsequently, Monte Carlo radiative transfer simulations for protoplanetary disks with porous dust grains were performed to analyse the additional effect of the optical depth structure, and thus the effect of multiple scattering events and inhomogeneous temperature distributions on the net observable polarisation degree.
Results. We find that porous dust grains with moderate filling factors of about 10% increase the degree of polarisation 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, the unpolarised direct radiation dominates the scattered flux, and the degree of polarisation drops rapidly. Even though the simulated polarisation degree is higher than in the case of compact grains, it is still below the typical observed values for face-on disks. However, the polarisation degree can be increased when crucial model assumptions derived from disk and dust evolution theories, for instance, dust settling and millimetre-sized dust grains, are dropped. In the case of inclined disks, however, our reference model is able to achieve polarisation degrees of about 1%, and using higher disk masses, even higher than this.