  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.
