New Evidence for a Physical Link between Asteroids (155140) 2005 UD and (3200) Phaethon

In 2018, the near-Earth object (155140) 2005 UD (hereafter UD) experienced a close fly by of the Earth. We present results from an observational campaign involving photometric, spectroscopic, and polarimetric observations carried out across a wide range of phase angles (0 degrees.7-88 degrees). We also analyze archival NEOWISE observations. We report an absolute magnitude of H-V = 17.51 + 0.02 mag and an albedo of p(V) = 0.10 + 0.02. UD has been dynamically linked to Phaethon due their similar orbital configurations. Assuming similar surface properties, we derived new estimates for the diameters of Phaethon and UD of D = 5.4 + 0.5 km and D = 1.3 + 0.1 km, respectively. Thermophysical modeling of NEOWISE data suggests a surface thermal inertia of G = 300(-110)(+120) and regolith grain size in the range of 0.9-10 mm for UD and grain sizes of 3-30 mm for Phaethon. The light curve of UD displays a symmetric shape with a reduced amplitude of Am(0) = 0.29 mag and increasing at a linear rate of 0.017 mag/degrees between phase angles of 0 degrees and similar to 25 degrees. Little variation in light-curve morphology was observed throughout the apparition. Using light-curve inversion techniques, we obtained a sidereal rotation period P = 5.235 + 0.005 hr. A search for rotational variation in spectroscopic and polarimetric properties yielded negative results within observational uncertainties of similar to 10% mu m(-1)and similar to 16%, respectively. In this work, we present new evidence that Phaethon and UD are similar in composition and surface properties, strengthening the arguments for a genetic relationship between these two objects.