  Dust coagulation in a protoplanetary disk is the first step of planetesimal
formation. However, a pathway from dust aggregates to planetesimals remains
unclear. Both numerical simulations and laboratory experiments have suggested
the importance of dust structure in planetesimal formation, but it is not well
constrained by observations. We study how dust structure and porosity alters
polarimetric images at millimeter wavelength by performing 3D radiative
transfer simulations. Aggregates with different porosity and fractal dimension
are considered. As a result, we find that dust aggregates with lower porosity
and/or higher fractal dimension are favorable to explain observed
millimeter-wave scattering polarization of disks. Aggregates with extremely
high porosity fail to explain the observations. In addition, we also show that
particles with moderate porosity show weak wavelength dependence of scattering
polarization, indicating that multi-wavelength polarimetry is useful to
constrain dust porosity. Finally, we discuss implications for dust evolution
and planetesimal formation in disks.
