Simultaneous Linear and Circular Optical Polarimetry of Asteroid (4) Vesta

From a single, 3.8-hour observation of asteroid (4) Vesta at $13.7^\circ$ phase angle with the POLISH2 polarimeter at the Lick Observatory Shane 3-m telescope, we confirm rotational modulation of linear polarization in $B$ and $V$ bands. We measure the peak-to-peak modulation in degree of linear polarization to be $\Delta P = (294 \pm 35) \times 10^{-6}$ (ppm) and time-averaged $\Delta P / P = 0.0577 \pm 0.0069$. After rotating the plane of linear polarization to the scattering plane, asteroidal rotational modulation is detected with $12 \sigma$ confidence and observed solely in Stokes $Q/I$. POLISH2 simultaneously measures Stokes $I$, $Q$, $U$ (linear polarization), and $V$ (circular polarization), but we detect no significant circular polarization with a $1 \sigma$ upper limit of 140 ppm in $B$ band. Circular polarization is expected to arise from multiple scattering of sunlight by rough surfaces, and it has previously been detected in nearly all other classes of Solar System bodies save asteroids. Subsequent observations may be compared with surface albedo maps from the Dawn Mission, which may allow identification of compositional variation across the asteroidal surface. These results demonstrate the high accuracy achieved by POLISH2 at the Lick 3-m telescope, which is designed to directly detect scattered light from spatially unresolvable exoplanets.