  We theoretically analyze protoplanetary disks consisting of porous dust
grains. In the analysis of observations of protoplanetary disks the dust phase
is often assumed to consist of spherical grains, allowing one to apply the Mie
scattering formalism. However, in reality, the shape of dust grains is expected
to deviate strongly from that of a sphere. We investigate the influence of
porous dust grains on the temperature distribution and observable appearance of
protoplanetary disks for dust grain porosities of up to 60 %. We performed
radiative transfer modeling to simulate the temperature distribution, spectral
energy distribution, and spatially resolved intensity and polarization maps.
The optical properties of porous grains were calculated using the method of
discrete dipole approximation. We find that the flux in the optical wavelength
range is for porous grains higher than for compact, spherical grains. The
profile of the silicate peak at 9.7 um strongly depends on the degree of grain
porosity. The temperature distribution shows significant changes in the
direction perpendicular to the midplane. Moreover, simulated polarization maps
reveal an increase of the polarization degree by a factor of about four when
porous grains are considered, regardless of the disk inclination. The
polarization direction is reversed in selected disk regions, depending on the
wavelength, grain porosity, and disk inclination. We discuss several possible
explanations of this effect and find that multiple scattering explains the
effect best. Porosity influences the observable appearance of protoplanetary
disks. In particular, the polarization reversal shows a dependence on grain
porosity. The physical conditions within the disk are altered by porosity,
which might have an effect on the processes of grain growth and disk evolution.
