September 2019 • 2019A&A...629A.121K
Context. Comets undergo resurfacing due to solar radiation, while their primordial interiors remain unchanged. Multi-epoch observations of comets enable us to characterize a change in sublimation pattern as a function of heliocentric distance, which in turn provides information on the dust environments of comets.
Aims: We aim to constrain the size and porosity of ejected dust particles from comet 252P/LINEAR and their evolution near perihelion via near-infrared (NIR) multiband polarimetry. A close approach of the comet to the Earth in March 2016 ( 0.036 au) provided a rare opportunity for the sampling of the comet at high spatial resolution.
Methods: We made NIR JHKS-band (1.25-2.25 μm) polarimetric observations of the comet for 12 days near perihelion, interspersed between broadband optical (0.48-0.80 μm) imaging observations over four months. In addition, a dynamical simulation of the comet was performed 1000 yr backward in time.
Results: We detected two discontinuous brightness enhancements of 252P/LINEAR. Before the first enhancement, the NIR polarization degrees of the comet were far lower than those of ordinary comets at a given phase angle. Soon after the activation, however, they increased by 13% at most, showing unusual blue polarimetric color over the J and H bands (-2.55% μm-1 on average) and bluing of the dust color in both J-H and H-KS. Throughout the event, the polarization vector was marginally aligned perpendicular to the scattering plane (i.e., θr = 4.6°-10.9°). The subsequent postperihelion reactivation of the comet lasted for approximately 1.5 months, with a factor of 30 times pre-activation dust mass-loss rates in the RC band.
Conclusions: The marked increase in the polarization degree with blue NIR polarimetric color is reminiscent of the behavior of a fragmenting comet D/1999 S4 (LINEAR). The most plausible scenario for the observed polarimetric properties of 252P/LINEAR is an ejection of predominantly large (well within the geometrical optics regime) and compact dust particles from the desiccated surface layer. We conjecture that the more intense solar heating that the comet has received in the near-Earth orbit would cause the paucity of small fluffy dust particles around the nucleus of the comet.