Initial Characterization of Active Transitioning Centaur, P/2019 LD₂ (ATLAS), Using Hubble, Spitzer, ZTF, Keck, Apache Point Observatory, and GROWTH Visible and Infrared Imaging and Spectroscopy

Abstract

We present visible and mid-infrared imagery and photometry of temporary Jovian co-orbital comet P/2019 LD₂ taken with Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3), Spitzer Space Telescope/Infrared Array Camera (Spitzer/IRAC), and the GROWTH telescope network, visible spectroscopy from Keck/Low-Resolution Imaging Spectrometer (LRIS), and archival Zwicky Transient Facility observations taken between 2019 April and 2020 August. Our observations indicate that the nucleus of LD₂ has a radius between 0.2 and 1.8 km assuming a 0.08 albedo and a coma dominated by ~100 μm-scale dust ejected at ~1 m s⁻¹ speeds with a ~1' jet pointing in the southwest direction. LD₂ experienced a total dust mass loss of ~10⁸ kg at a loss rate of ~6 kg s⁻¹ with Afρ/cross section varying between ~85 cm/125 km² and ~200 cm/310 km² from 2019 April 9 to 2019 November 8. If the increase in Afρ/cross section remained constant, it implies LD₂'s activity began ~2018 November when within 4.8 au of the Sun, implying the onset of H₂O sublimation. We measure CO/CO₂ gas production of ≾10²⁷ mol s⁻¹/≾10²⁶ mol s⁻¹ from our 4.5 μm Spitzer observations; g–r = 0.59 ± 0.03, r–i = 0.18 ± 0.05, and i–z = 0.01 ± 0.07 from GROWTH observations; and H₂O gas production of ≾80 kg s⁻¹ scaling from our estimated C₂ production of Q_(C₂) ≾7.5×10²⁴ mol s⁻¹ from Keck/LRIS spectroscopy. We determine that the long-term orbit of LD₂ is similar to Jupiter-family comets having close encounters with Jupiter within ~0.5 Hill radius in the last ~3 y and within 0.8 Hill radius in ~9 y. Additionally, 78.8% of our orbital clones are ejected from the solar system within 1 × 10⁶ yr, having a dynamical half-life of 3.4 × 10⁵ yr.

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