Cometary Activity on Quasi-Hilda Object 2018 CZ16

Active asteroids are rare (∼50) bodies that are found in orbits not normally associated with comets, yet they display comet-like activity such as a tails or comae (Jewitt et al. 2015). A subset (∼20) of the known active asteroids display comet-like activity indicative of volatile sublimation, such as recurrent activity near perihelion (Hsieh et al. 2015). Such bodies in the main asteroid belt are often referred to as Main Belt Comets but other related and more dynamically unstable classes such as the quasi-Hilda objects (QHOs) have also shown similar activity (Oldroyd et al. 2023). Objects with recurrent activity associated with perihelion are plausibly driven by water ice and unveil the present-day volatile distribution in the solar system, potentially yielding clues to the primordial origins of terrestrial water. To date, we have found only one epoch of activity for 2018 CZ₁₆ near perihelion, but we encourage future observations of the object in 2024 to identify whether the activity is recurrent.

Our main thesis is that there are many more known asteroids with heretofore unrecognized tails in public image databases. To find more active objects, we created the Citizen Science project Active Asteroids ¹³ (Chandler et al. 2018, 2019, 2020, 2021, 2022; Chandler 2022) where volunteers examine images of known minor planets we extract from publicly available Dark Energy Camera (DECam) data and classify these images as either active or inactive. To date, about ∼7500 volunteers have made ≳5 million classifications from our project. Volunteers classified images of 2018 CZ₁₆ as active, and we subsequently carried out a more detailed archival investigation to further study this object.

We identified three archival images of 2018 CZ₁₆ (semimajor axis a = 3.54 au, eccentricity e = 0.34, inclination i = 137, perihelion distance q = 2.27 au, aphelion distance Q = 4.64 au; JPL Horizons Solution date 2021 April 15 20:22:55, Giorgini et al. 1996) with unambiguous indications of emission activity in the form of a broad tail (∼60°) about 5'' in length. This tail does not appear to be clearly aligned with either the anti-motion or anti-solar directions (Figure 1). The orbital parameters of 2018 CZ₁₆ suggest that it is a member of the QHO class, which is a dynamical class of minor planet near, but not in, the Hilda asteroid family. We compute that 2018 CZ₁₆ has Jovian Tisserand parameter T_J = 2.994 and an orbital excitation parameter ${ \mathcal E }=0.416$, similar to, but somewhat outside of the QHO definition found in Toth (2006).

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We identified other archival images from 2014 and 2018 showing hints of activity, but these were considerably poorer than the images in Figure 1, so were considered inconclusive. We encourage observers to explore this object in 2024 when it approaches opposition (late February) and as it once again approaches perihelion (late November) to determine if activity is indeed correlated with perihelion. Repeated perihelion activity, especially with a quiescent state near aphelion, would suggest that 2018 CZ₁₆ has thermally driven activity indicative of water-ice sublimation, a mechanism of significant scientific interest (Hsieh & Jewitt 2006).

We thank Dr. Mark Jesus Mendoza Magbanua (UCSF) for feedback. We thank Elizabeth Baeten (Belgium) for moderating the Active Asteroids forums. We thank our NASA Citizen Scientists that examined 2018 CZ₁₆: Dawn Boles (Bakersfield, USA), Milton K.D. Bosch MD (Napa, USA), I. Carley (Gold Coast, Australia), Eric Fabrigat (Velaux, France), Ashok Ghosh (Howrah, India), Virgilio Gonano (Udine, Italy), Al Lamperti (Royersford, USA), Michele T. Mazzucato (Florence, Italy), Angelina A. Reese (Sequim, USA), Adrian Runnicles (London, UK), Tiffany Shaw-Diaz (Dayton, USA), Ivan A. Terentev (Petrozavodsk, Russia), and John M. Trofimuk (South Elgin, USA). We thank super-classifiers C.J.A. Dukes (Oxford, UK) and Marvin W. Huddleston (Mesquite, USA) . Many thanks to Cliff Johnson (Zooniverse) and Marc Kuchner (NASA) for guidance.

This work was supported by the NSF Graduate Research Fellowship Program (grants 2018258765 and 2020303693). C.O.C., H.H.H., and C.A.T. acknowledge support from NASA (grant 80NSSC19K0869). W.J.O. acknowledges support from NASA grant 80NSSC21K0114. This work was supported in part by NSF award 1950901 (NAU REU program). Computational analyses were run on Northern Arizona University's Monsoon computing cluster, funded by Arizona's Technology and Research Initiative Fund.

Astrometry facilitated by Astrometry.net (Lang et al. 2010). This research has made use of NASA's ADS, the Institut de Mécanique Céleste et de Calcul des Éphémérides SkyBoT Virtual Observatory tool (Berthier et al. 2006), and data services provided by the International Astronomical Union's Minor Planet Center, and SAOImageDS9, developed by Smithsonian Astrophysical Observatory (Joye 2006).

This project used data obtained with DECam, which was constructed by the Dark Energy Survey collaboration. This research used the Astro Data Archive at NSF's NOIRLab and is based on observations from CTIO (NOIRLab Prop. ID 2014B-0404; PI Schleigel).

Facility: CTIO:4 m (DECam). -

Software: astropy (Robitaille et al. 2013), Matplotlib (Hunter 2007), NumPy (Harris et al. 2020), pandas (Reback et al. 2022), SAOImageDS9 (Joye 2006), SciPy (Virtanen et al. 2020).