  The investigation of asteroids near the Sun is important for understanding
the final evolutionary stage of primitive solar system objects. A near-Sun
asteroid, (155140) 2005 UD, has orbital elements similar to those of (3200)
Phaethon (the target asteroid for the JAXA's $DESTINY^+$ mission). We conducted
photometric and polarimetric observations of 2005 UD and found that this
asteroid exhibits a polarization phase curve similar to that of Phaethon over a
wide range of observed solar phase angles ($ \alpha = 20 - 105^\circ $) but
different from those of (101955) Bennu and (162173) Ryugu (asteroids composed
of hydrated carbonaceous materials). At a low phase angle ($\alpha \lesssim
30^\circ$), the polarimetric properties of these near-Sun asteroids (2005 UD
and Phaethon) are consistent with anhydrous carbonaceous chondrites, while the
properties of Bennu are consistent with hydrous carbonaceous chondrites. We
derived the geometric albedo, $ p_\mathrm{V} \sim 0.1 $ (in the range of
0.088-0.109); mean $ V $-band absolute magnitude, $ H_\mathrm{V} = 17.54 \pm
0.02 $; synodic rotational period, $ T_\mathrm{rot} = 5.2388 \pm 0.0022 $ hours
(the two-peaked solution is assumed); and effective mean diameter, $
D_\mathrm{eff} = 1.32 \pm 0.06 $ km. At large phase angles ($ \alpha \gtrsim
80^\circ$), the polarization phase curve are likely explained by the dominance
of large grains and the paucity of small micron-sized grains. We conclude that
the polarimetric similarity of these near-Sun asteroids can be attributed to
the intense solar heating of carbonaceous materials around their perihelia,
where large anhydrous particles with small porosity could be produced by
sintering.
