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

<jats:title>Abstract</jats:title> <jats:p>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.°7–88°). We also analyze archival NEOWISE observations. We report an absolute magnitude of <jats:italic>H</jats:italic> <jats:sub>V</jats:sub> = 17.51 ± 0.02 mag and an albedo of <jats:italic>p</jats:italic> <jats:sub>V</jats:sub> = 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 <jats:italic>D</jats:italic> = 5.4 ± 0.5 km and <jats:italic>D</jats:italic> = 1.3 ± 0.1 km, respectively. Thermophysical modeling of NEOWISE data suggests a surface thermal inertia of <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="psjab8e45ieqn1.gif" xlink:type="simple"/> </jats:inline-formula> 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 <jats:italic>Am</jats:italic>(0) = 0.29 mag and increasing at a linear rate of 0.017 mag/° between phase angles of 0° and ∼25°. Little variation in light-curve morphology was observed throughout the apparition. Using light-curve inversion techniques, we obtained a sidereal rotation period <jats:italic>P</jats:italic> = 5.235 ± 0.005 hr. A search for rotational variation in spectroscopic and polarimetric properties yielded negative results within observational uncertainties of ∼10% <jats:italic>μ</jats:italic>m<jats:sup>−1</jats:sup> and ∼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.</jats:p>