Dust coagulation in a protoplanetary disk is the first step in planetesimal formation. However, the 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 the dust structure and porosity alter polarimetric images at millimeter wavelengths by performing 3D radiative transfer simulations. Aggregates with different porosities and fractal dimensions are considered. As a result, we find that dust aggregates with lower porosity and/or higher fractal dimension are favored to explain the observed millimeter-wave scattering polarization of disks. Although we cannot rule out the presence of aggregates with extremely high porosity, a population of dust particles with relatively compact structure is at least necessary to explain polarized-scattered waves. In addition, we show that particles with moderate porosity show a weak wavelength dependence of the scattering polarization, indicating that multiwavelength polarimetry is useful to constrain dust porosity. Finally, we discuss implications for dust evolution and planetesimal formation in disks.
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