2018ApJ...864L..33S

As a function of the solar phase angle, α, the linear polarization degree (referred to the scattering plane, P_r) of solar system objects is a good diagnostic for understanding the scattering properties of their surface materials. We report the P_r of Phaethon over a wide range of α from 19.°1 to 114.°3. The derived phase-polarization curve shows that the maximum of P_r, P_max, is >42.4% at α > 114.°3, a value significantly larger than those of the moderate albedo asteroids (P_max ∼ 9%). The phase-polarization curve classifies Phaethon as B-type as well as M- and K-type asteroids, in the polarimetric taxonomy, being compatible with the spectral property. We compute the geometric albedo, p_v, of 0.14 ± 0.04 independently by using an empirical slope-albedo relation, and the derived p_v is consistent with previous results determined from mid-infrared spectra and thermophysical modeling. We find no periodic variation of P_r in our polarimetric data in the range from 0 up to 7.208 hr (e.g., less than twice the rotational period). We also find significant differences between our P_r during the 2017 approach toward Earth and that in 2016, implying that Phaethon has a region with different properties for light scattering near its rotational pole.