Physical Characterization of Interstellar Comet 2I/2019 Q4 (Borisov)

Abstract

We present a study of interstellar comet 2I/2019 Q4 (Borisov) using both preperihelion and postperihelion observations spanning late September 2019 through late January 2020. The intrinsic brightness of the comet was observed to continuously decline throughout the timespan, likely due to the decreasing effective scattering cross-section as a result of volatile sublimation with a slope of −0.43 ± 0.02 km{sup 2} d{sup −1}. We witnessed no significant change in the slightly reddish color of the comet, with mean values of 〈g−r〉=0.68±0.04, 〈r−i〉=0.23±0.03, and the normalized reflectivity gradient across the g and i bands (S{sup ′})-bar (g,i)=(10.6±1.4)% per 10{sup 3} Å, all unremarkable in the context of solar system comets. Using the available astrometric observations, we confidently detect the nongravitational acceleration of the comet following a shallow heliocentric distance dependence of r{sub H}{sup −1±1}. Accordingly, we estimate that the nucleus is most likely ≲0.4 km in radius, and that a fraction of ≳0.2% of the total mass of the nucleus has been eroded due to the sublimation activity since the earliest observation of the comet in 2018 December by the time of perihelion. Our morphology simulation suggests that the dust ejection speed increased from ∼4 m s{sup −1} in 2019 September to ∼7 m s{sup −1} around perihelion for the optically dominant dust grains with β∼0.01, and that the observable dust grains are no smaller than micron size.