Color measurements of the polarized light scattered by the dust in protoplanetary disks

Abstract

Ground-based high-contrast instruments have yielded reflected light images of protoplanetary disks. Quantitative measurements of the reflected radiation provide strong constraints on the scattering dust which can clarify the dust particle evolution in these disks and the composition of the forming planets. This study aimed to derive the wavelength dependence of polarized reflectivity $(\hat{Q}_{\varphi}/I_\star)_\lambda$ for 11 disks, constraining dust properties and identifying systematic differences. Using ESO archive data from SPHERE/ZIMPOL and SPHERE/IRDIS instruments, we obtained accurate intrinsic polarized reflectivity $\hat{Q}_\varphi/I_\star$ values at wavelengths from 0.62$\mu$m to 2.2$\mu$m. Polarized reflectivities ranged from $Q_\varphi/I_\star\approx 0.1\%$ to 1.0$\%$, with PSF-corrected values averaging 1.6 times higher than observed. Accurate PSF calibrations reduced systematic errors to $\Delta\hat{Q}_\varphi/\hat{Q}_\varphi\approx 10\%$ or less. For each disk, we derived a polarized reflectivity color $\eta_{V/IR}$ between a visible band $\lambda<1~\mu$m and a near-IR band $\lambda>1~\mu$m and other wavelength combinations. Wavelength gradients $\eta$ varied significantly among objects. Disks around Herbig stars (HD 169142, HD 135334B, HD 100453, MWC 758, and HD 142527) showed a red color $\eta_{\rm V/IR}>0.5$, suggesting rather compact dust grains. T-Tauri star disks (PDS 70, TW Hya, RX J1615, and PDS 66) were predominantly gray $-0.5<\eta_{\rm V/IR}<0.5$, with an absence of blue colors incompatible with porous aggregates. Exceptional red colors for LkCa15 and MWC758 were attributed to potential extra reddening from hot dust near the star. Future studies incorporating parameters like fractional polarization $\langle p_\varphi \rangle$ hold promise for advancing our understanding of dust properties within protoplanetary disks.