Dual broadband phase curves of asteroid (3200) Phaethon

Abstract

In this study, we analyzed photometric data in the o- and c-band of the asteroid (3200) Phaethon from the ATLAS network from 2015 to 2024. Apparent magnitudes and phase angles were used to construct phase curves. Annual data sets were analyzed if the phase angle range was at least 15 degrees. The phase coefficients for 2021 and 2022 were negative and positive, respectively. Negative coefficients are unusual for asteroids and likely result from variations in the observed hemisphere due to changes in the viewing angle relative to Phaethon’s rotation axis. In 2021, the northern hemisphere was observed, while in 2022 the southern hemisphere was observed. In December 2017, negative slopes G were derived from the VRI phase curves, corresponding to a sub-Earth latitude of 46 degrees N. Using the G value in the R-band, we have obtained the approximate parameters of the Hapke model for the northern hemisphere of Phaethon, suggesting that its surface consists of particles with similar porosity or size distribution, a rougher surface, and stronger backward scattering compared to typical low-albedo asteroids, and possibly also compared to the southern region. The northern hemisphere of Phaethon is likely redder than its southern hemisphere, which is supported by the absolute color c-o estimates in 2022 showing that the southern region is bluer than the solar twin YBP 1194, the main-belt comet 107P/Wilson-Harrington, and even than the C-type asteroid (10) Hygiea. In 2022, the relative phase coefficients indicate a phase bluing effect, where Phaethon becomes bluer as the phase angle decreases.