Cometary Activity Discovered on Vacationing Centaur 2019 OE31

Centaurs are icy bodies that orbit in the giant planet region between Jupiter and Neptune. They originate from the Kuiper Belt and are scattered to their current orbits through gravitational interactions with Neptune. Centaurs have short dynamical lifetimes of only a few million years before they migrate to become Jupiter family comets, collide with a giant planet, or are ejected from the solar system (Tiscareno & Malhotra 2003). Observations of Centaurs displaying cometary activity provide key insights into the chemical composition, formation, and dynamical evolution of both outer solar system objects and comets (see, e.g., Jewitt 2009; Chandler et al. 2020; Lisse et al. 2022).

Activity on 2019 OE₃₁ was identified in archival Dark Energy Camera (DECam) data by volunteers of our Active Asteroids Citizen Science project ¹⁶ (Chandler 2022) and independently ¹⁷ through an archival search for activity lead by Sam Deen (e.g., Deen & Hsieh 2023). The Active Asteroids project has over 8500 volunteers who have made over 7 million classifications of DECam images of asteroids and other minor planets, resulting in >20 discoveries of activity (e.g., Chandler et al. 2023; Oldroyd et al. 2023a; Trujillo et al. 2023). Once we identified 2019 OE₃₁ as active, we carried out an extensive archival search for additional signs of cometary activity on 2019 OE₃₁, similar to searches described in Chandler et al. (2019) and Chandler (2022).

We identified three images of 2019 OE₃₁ showing signs of cometary activity (Figure 1) over a four-month span, 2019 June 6 to 2019 September 30 UT, coinciding with its most recent perihelion passage on 2019 July 22 UT. Other archival images of 2019 OE₃₁ either were insufficiently deep to identify the object, or did not show obvious signs of activity.

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Prior to 2013, 2019 OE₃₁ was on a Centaur orbit, entirely exterior to the orbit of Jupiter. ¹⁸ On 2013 October 1, 2019 OE₃₁ passed within ∼0.017 au of Jupiter (JPL Small-Body Database, Giorgini et al. 1996). The gravitational perturbations from this extreme close encounter caused 2019 OE₃₁ to migrate from a Centaur orbit onto an orbit more typical of an asteroid, entirely interior to the orbit of Jupiter. Current orbital parameters for 2019 OE₃₁ include semimajor axis a = 4.375 au, eccentricity e = 0.101, inclination i = 5222, perihelion q = 3.934 au, aphelion Q = 4.817 au, and Tisserand parameter with respect to Jupiter T_J = 3.006 (solution date 2021 October 22 PT, retrieved 2023 November 27 from the JPL Small-Body Database).

To analyze probable dynamical futures for 2019 OE₃₁, we performed a suite of orbital dynamics simulations using the REBOUND N-body integration Python package (Rein & Liu 2012). These simulations are similar to those described in Chandler et al. (2022) and Oldroyd et al. (2023b), utilizing orbital clones to statistically represent probable orbital outcomes following close encounters with Jupiter. We find that 2019 OE₃₁ experiences many close encounters with Jupiter over the next 1,000 yr and, within this time frame, has a ∼60% probability to return to a Centaur orbit, a 22% chance to migrate to a Jupiter family comet orbit (crossing the orbit of Jupiter), and an 18% likelihood to remain on an orbit between the outer main asteroid belt and the orbit of Jupiter. Thus, 2019 OE₃₁ was recently a Centaur and will likely return to a Centaur orbit—potentially within the next 80 yr—from its current "vacation" to the inner solar system. The reduced orbital distances and, consequently, increased temperatures experienced by 2019 OE₃₁ in its current orbit are almost certainly the cause of the observed cometary activity, which is likely driven by volatile sublimation.

We thank Elisabeth Baeten (Belgium) for moderating the Active Asteroids forums and Cliff Johnson (Zooniverse) and Marc Kuchner (NASA) for Citizen Science guidance. Many thanks to Arthur and Jeanie Chandler for their ongoing support. We thank our Active Asteroids volunteers who examined 2019 OE₃₁: Dr. Brian Leonard Goodwin (UK), Graeme Aitken (Australia), H. Franzrahe (Germany), I. Carley (Australia), Michele T. Mazzucato (Italy), Robert Zach Moseley (USA), Tiffany Shaw-Diaz (USA), Virgilio Gonano (Italy), and @WRSunset (UK). We also thank our Superclassifiers: Al Lamperti (USA), Angelina A. Reese (USA), Antonio Pasqua (Italy), C.J.A. Dukes (UK), Carl L. King (USA), Dan Crowson (USA), @EEZuidema (Netherlands), Eric Fabrigat (France), @graham_d (UK), Henryk Krawczyk (Poland), Ivan A. Terentev (Russia), Jose A. da Silva Campos (Portugal), Marvin W. Huddleston (USA), Milton K.D. Bosch MD (USA), Thorsten Eschweiler (Germany), and Washington Kryzanowski (Uruguay).

This work was funded in part by NASA grants 80NSSC21K0114 and 80NSSC19K0869, NSF GRFP grants 2018258765 and 2020303693, and NSF award 1950901. This research received support through the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program. Chandler and Sedaghat acknowledge support from the DiRAC Institute in the Department of Astronomy at the University of Washington. The DiRAC Institute is supported through generous gifts from the Charles and Lisa Simonyi Fund for Arts and Sciences, and the Washington Research Foundation.

Computational analyses were run on Northern Arizona University's Monsoon computing cluster, funded by Arizona's Technology and Research Initiative Fund. This project used data obtained with the Dark Energy Camera, which was constructed by the Dark Energy Survey collaboration, data and/or services provided by the International Astronomical Union's Minor Planet Center, and services or data provided by the Astro Data Archive at NSF's NOIRLab and the CADC Solar System Object Information Search (Gwyn et al. 2012). Based on observations at Cerro Tololo Inter-American Observatory, NSF's NOIRLab (NOIRLab Prop. ID: 2021A-0149, PI: Drlica-Wagner and Prop. ID: 2019B-1014, PI: Olivares).

Facility: CTIO:4m (DECam) - .

Software: astrometry.net (Lang et al. 2010), REBOUND (Rein & Liu 2012), SkyBot (Berthier et al. 2006).