Quantitative polarimetry for the transition disk in RX J1604.3-213010

The bright disk of RX J1604 has a very simple axisymmetric structure and is well suited as a benchmark object for accurate photo-polarimetric measurements. We used archival data of RX J1604 from the ESO archive and carefully corrected the polarization signal for instrumental effects, also taking the interstellar polarization into account. We derive accurate radial disk profiles for the intrinsic polarized intensity, ${\hat{Q}}_{\varphi}(r)/I_{\star}$, and measure different profile peak radii for different bands because of the wavelength dependence of the dust opacity. The disk-integrated polarization is $\hat{Q}_{\varphi}/I_{\star} = 0.92 \pm 0.04\%$ for the R band and $1.51 \pm 0.11\%$ for the J band, indicating a red color for the polarized reflectivity of the disk. The intensity of the disk is $I_{\rm disk}/I_{\star} = 3.9 \pm 0.5 \%$ in the J band, and the fractional polarization is $\hat{p}_{\varphi} = 38 \pm 4\%$ for the J band and $42 \pm 2\%$ for the H band. The comparison with the IR excess for RX J1604 yields an apparent disk albedo of about $\Lambda_{I} \approx 0.16 \pm 0.08$. We also find that previously described shadows seen in the R band data are likely affected by calibration errors. Using dust scattering models for transition disks, We derive approximate J band values for the scattering albedo $\omega \approx 0.5$, scattering asymmetry $g \approx 0.5$, and scattering polarization $p_{\rm max} \approx 0.7$ for the dust. The positive R to J band color for the polarized reflectivity is mainly a result of the wavelength dependence of dust parameters because the scattering geometry is expected to be very similar for different colors. This work demonstrates the potential of accurate photo-polarimetric measurements of the circumstellar disk RX J1604 for the determination of dust scattering parameters that strongly constrain the physical properties of the dust.