Characterization of the Nucleus, Morphology and Activity of Interstellar Comet 2I/Borisov by Optical and Near-Infrared GROWTH, Apache Point, IRTF, ZTF and Keck Observations


We present visible and near-infrared photometric and spectroscopic observations of interstellar comet 2I/Borisov taken from 2019 September 10 to 2019 November 03 using the GROWTH collaboration, the Apache Point Observatory ARC 3.5 m and the NASA/IRTF 3.0 m combined with post and pre-discovery observations of 2I obtained by the Zwicky Transient Facility from 2019 March 17 to 2019 May 5. Comparison with imaging of distant Solar System comets (Kelly et al. 2013) shows an object very similar to mildly active Solar System comets with an out-gassing rate of $sim$10$^{27}$ mol/sec. The photometry, taken in filters spanning the visible and NIR range shows a gradual brightening trend of $sim0.03$ mags/day since 2019 September 10 UTC for a reddish object becoming neutral in the NIR. The lightcurve from recent and pre-discovery data (Ye et al. 2019) reveals a brightness trend suggesting the recent onset of significant H$_2$O sublimation with the comet being active with super volatiles such as CO at heliocentric distances $>$6 au consistent with its extended morphology. Using the advanced capability to significantly reduce the scattered light from the coma enabled by high-resolution NIR images from Keck adaptive optics taken on 2019 October 04, we estimate a diameter of 2I’s nucleus of $lesssim$3 km, though the true size is likely $sim$2-3 times smaller due to the incomplete removal of dust from the measurement. We use the size estimates of 1I/‘Oumuamua and 2I/Borisov to roughly estimate the slope of the interstellar object cumulative size-distribution resulting in a slope of $gtrsim$-2.9, similar to Solar System comets (Fernandez et al. 2013), though the true slope is likely significantly steeper due to small number statistics and our probable overestimation of the size of 2I.