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Umov effect in asteroid (3200) Phaethon

Context. The near-Earth asteroid (3200) Phaethon occasionally reveals a comet-like activity. It is a parent body to the Geminid meteor shower and considered as a target for the space mission called Demonstration and Experiment of Space Technology for Interplanetary Voyage Phaethon Flyby Dust Science, DESTINY+.
Aims. We aim to characterize Phaethon through measurements of the degree of linear polarization P measured on Phaethon at large phase angles on its closest approach to Earth on December 17, 2017. These observations allow a more accurate estimation of the maximum value of the degree of linear polarization Pmax of Phaethon, and therefore, of studying the Umov effect.
Methods. We performed polarimetric measurements of Phaethon at large phase angles α and thus constrained its Pmax. We also estimated the geometric albedo a based on the data available in the literature. The obtained Pmax and A were analysed with the Umov effect previously derived for the Moon that establishes an inverse linear correlation between log(Pmax) and log(A) in the lunar regolith.
Results. Our polarimetric observations of Phaethon in the visible reveal the degree of linear polarization P ≈ (17.23 ± 2.00)% at α ≈ 57.9° and P ≈ (31.86 ± 2.00)% at α ≈ 73.2°, which demonstrates no significant wavelength dependence within the error bars of our measurements (± 2%). These data, when combined with what has previously been reported in the literature, suggests at least three types of polarimetric response on Phaethon. For two of them, we infer the maximum linear polarization to be Pmax ≈ 57.9%, occurring at αmax = 131° and Pmax ≈ 44.5% occurring at αmax = 127°. We estimate the geometric albedo (adjusted to α = 3°) to be AR = 0.075 ± 0.007 in the R filter, which appears to be consistent with dark F-type asteroids, as which Phaethon was first classified. We examine the Umov diagrams previously inferred for lunar regolith and find that they are hardly applicable to Phaethon and therefore not to other asteroids either.