(85989) 1999 JD6 : a first Barbarian asteroid detected by polarimetry in the NEA population

Abstract

Context. To investigate the physical properties of the surface layers of small Solar System objects, we take advantage of the phase-angle dependence of the linear polarization degree, which varies with albedo, composition, and other factors. In particular, the angle at which the positive and negative are reversed is a polarimetric parameter known as inversion angle. A group with large inversion angles (the so-called Barbarians) is unusual and is also noteworthy because of their association with meteorites. Aims. We identified an object with such a large inversion angle in the near-Earth asteroids and derived previously unknown parameters, including the maximum polarization degree. By comparing the polarimetric parameters and spectral properties of meteorites, we inferred the surface conditions based on the degree of polarization of the asteroid. Methods. We carried out multiband polarization imaging observations of an L-type near-Earth asteroid, (85989) 1999 JD6, at the Hokkaido University Observatory in 2015 and 2019, covering a wide range of phase angles 30°–105°. Of the polarimetric parameters, we derived the maximum value, inversion angle, and the slope at that angle from the phase angle-polarization degree curve. Results. We found that the inversion angle and the maximum polarization degree of 1999 JD6 are 27° and 13%. The inversion angle is significantly larger than those of the majority of all asteroids observed before, but is consistent with that of Barbarian asteroids. 1999 JD6 is the first example belonging to this group that is also among the near-Earth asteroids. Conclusions. We claim that 1999 JD6 is a member of the Barbarians and has rougher surface particles than lunar regolith.