Polarimetry of major Uranian moons at the 6-m telescope

Abstract

We present the results of polarimetric observations of the icymoons of Uranus (Ariel, Titania, Oberon, and Umbriel) performed at the 6-m BTA telescope of the SAO RAS with the SCORPIO-2 focal reducer within the phase angle range of $0_.^ \circ 06 - 2_.^ \circ 37$. The parameters of the negative polarization branch (referred to the scattering plane) are obtained in the V filter: for Ariel the maximum branch depth of P _min ≈ −1.4% is reached at the phase angle of α _min ≈ 1°; for Titania P _min ≈ −1.2%, $\alpha _{\min } \approx 1_.^ \circ 4$; for Oberon P _min ≈ −1.1%, $\alpha _{\min } \approx 1_.^ \circ 8$. For Umbriel the polarization minimum was not reached: for the last measurement point at $\alpha _{\min } \approx 2_.^ \circ 4$, polarization amounts to −1.7%. The declining P _min and shifting α_min towards larger phase angles correlate with a decrease of the geometric albedo of the Uranian moons. There is no longitudinal dependence of polarization for the moons within the observational errors which indicates a similarity in the physical properties of the leading and trailing hemispheres. The phase-angle dependences of polarization for the major moons of Uranus are quite close to those observed in the group of small trans-Neptunian objects (Ixion, Huya, Varuna, 1999 DE9, etc.), which are characterized by a large gradient of negative polarization, about −1% per degree in the phase-angle range of $0_.^ \circ 1 - 1^ \circ$.