First VISNIR Photometric Images of Active Transitioning Centaur P/2019 LD₂ (ATLAS) From Spitzer, Hubble, & GROWTH

Comet P/2019 LD2 is an active object currently transitioning from the Centaur region beyond Jupiter into a Jupiter-family comet, and it represents one of the few opportunities for us to observe the effect of this transition on the object’s activity as its thermal environment evolves into a region closer to the Sun. The comet was likely inactive until it came within 5.25 au of the Sun after a particularly close approach to Jupiter (< 0.5 au) in October 2017, evolving onto a temporary co-orbit with the giant planet. We present visible and near-infrared photometric imaging of Jovian co-orbital comet P/2019 LD₂ (ATLAS) taken with the Spitzer Space Telescope/Infrared Array Camera (SST/IRAC) on 2020 January 25 UTC (DDT program 14331, PI Bolin), the Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) on 2020 April 1 (GO program 16077, PI Bolin), and the Global Relay of Telescopes Watching Transients Happen (GROWTH) telescope network (Kasliwal et al. 2019) between 2020 May and July. With our 4.5 micron SST/IRAC observa-tions, we detected an extended gas + dust coma around the comet’s nucleus. Our highest spatial resolution HST observations (~100 km per pixel) indicates that the coma likely consists of micron-scale or larger ejected with some anisotropy towards the anti-Solar direction, and a jet in the near-nucleus coma. Our HST observations also indicate that the comet possesses a nucleus no larger than ~3 km radius, assuming a 0.04 geometric albedo. Multiple bandpass photometry of LD₂ taken by the GROWTH network measured in a 10,000 km radius aperture provides color measurements of g-r = 0.59±0.05, r-i = 0.18±0.04, and i-z = 0.03±0.07, somewhat bluer than other active Centaurs (Jewitt 2015), but with little variation over the several months of the monitoring. Using the improved orbital solution for P/2019 LD₂ provided by the span of our observations and publicly available data from the Minor Planet Center archive, we determine that its trajectory is typical for a Centaur transitioning into the JFC region and will be ejected from the Solar System in < 1 Myr (Levinson & Duncan 1994, Lisse 2002, Horner et al. 2004).