Abstract
We announce the discovery of a main-belt comet (MBC), 2010 LH15 (alternately designated 2010 TJ175). MBCs are a rare type of main-belt asteroid that display comet-like activity, such as tails or comae, caused by sublimation. Consequently, MBCs help us map the location of solar system volatiles, providing insight into the origins of material prerequisite for life as we know it. However, MBCs have proven elusive, with fewer than 20 found among the 1.1 million known main-belt asteroids. This finding derives from Active Asteroids, a NASA Partner Citizen Science program we designed to identify more of these important objects. After volunteers classified an image of 2010 LH15 as showing activity, we carried out a follow-up investigation which revealed evidence of activity from two epochs spanning nearly a decade. This discovery is timely, with 2010 LH15 inbound toward its 2024 March perihelion passage, with potential activity onset as early as late 2023.
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1. Introduction
Main-belt comets (MBCs) are rare, with fewer than 20 found among the 1.1 million known main-belt asteroids. MBCs represent a subpopulation of the active asteroids, which are small solar system bodies that exhibit comet-like activity (i.e., tails, comae) but have asteroidal orbits (Jewitt et al. 2015). The MBCs are active asteroids that are specifically found in the main asteroid belt, and whose activity is attributed to sublimation (Hsieh et al. 2015). Knowledge of these objects and their composition help us map the location of solar system volatiles, thereby improving understanding of the origins of the ingredients for life as we know it.
2. Methods
To find more of these remarkable objects we created the Citizen Science program Active Asteroids, 12 a NASA Partner. Participants classify images of known minor planets, which we extracted from the Dark Energy Camera (DECam) public archive (Chandler et al. 2018, 2019, 2020, 2021, 2022), as either active or inactive. We investigate activity candidates by conducting archival image searches and follow-up telescope observations, then report our confirmed discoveries (e.g., Chandler et al. 2023a, 2023b; Oldroyd et al. 2023).
3. Results
One DECam image (Figure 1) of 2010 LH15 (a = 2.74 au, eccentricity e = 0.35, inclination i = 10
9, perihelion distance q = 1.77 au, aphelion distance Q = 3.72 au, Tisserand's parameter with respect to Jupiter TJ = 3.23, retrieved UT 2023 March 11 from JPL Horizons; Giorgini et al. 1996) originally acquired UT 2019 September 30, was unanimously classified as showing activity by Active Asteroids volunteers. Our archival investigation revealed additional images (examples provided in Figure 1) unambiguously showing activity from two separate orbital epochs: ∼10 images from UT 2010 September 27—October 10; (true anomaly range of 205 < ν < 276) and >10 between UT 2019 August 10—November 3 (−142 < ν < 265). All images of activity were taken when 2010 LH15 was approximately near perihelion passage (ν = 0°). When considered with the recurrent activity, this indicates that sublimation is the probable underlying activity mechanism. Hence, given its main-belt orbit, 2010 LH15 is an MBC.
Figure 1. 2010 LH15 (green dashed arrows) with a tail (white arrows) oriented on sky roughly toward the anti-solar (yellow −⊙) direction, and counter-clockwise from the anti-motion (red −v) vector. The FOV of each image is 126'' × 126''. Left: UT 2020 September 27 45 s i-band GigaPixel1 exposure on the 1.8 m Pan-STARRS 1 (Haleakala). Center: UT 2019 August 31 Zwicky Transient Facility (ZTF) camera on the 48'' Samuel Oschin telescope (Mt. Palomar) 30 s r band exposure. Right: UT 2019 September 30 90 s Dark Energy Camera (DECam) on the 4 m Blanco telescope (Cerro Tololo Inter-American Observatory, Chile); Prop. ID 2019B-1014, PI Olivares, observers F. Olivares, I. Sanchez. This was the image classified as active by Active Asteroids volunteers.
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2010 LH15 is currently observable (as of 2023 March 15; ν ∼ 240°) and currently inbound to its 2024 March 26 perihelion passage, and may become active again as early as 2023 October when it reaches ν = 290°, the earliest activity onset point observed to date for an MBC (Hsieh et al. 2023), just before its current observing window ends. It is also important to observe the target prior to its possible reactivation in order to measure properties of the nucleus (e.g., color, absolute magnitude, rotation rate) in the absence of activity, which will impede these measurements. After 2023 October, the object will next be observable from 2024 July to 2025 May (45° ≲ ν ≲ 130°). Observations of the object during these available windows to characterize its expected activity are highly encouraged.
General: We thank Dr. Mark Jesus Mendoza Magbanua (University of California San Francisco) for his ongoing, timely feedback on the project and observing accompaniment.
Citizen Science: We thank Elizabeth Baeten (Belgium) for moderating the Active Asteroids forums. We thank our NASA Citizen Scientists that examined 2010 LH15: Al Lamperti (Royersford, USA), Angela Hoffa (Greenfield, USA), Carl L. King (Ithaca, USA), Jayanta Ghosh (Purulia, India), Konstantinos Dimitrios Danalis (Athens, Greece), Lydia Yvette Solis (Nuevo, USA), Michele T. Mazzucato (Florence, Italy), Milton K. D. Bosch MD (Napa, USA), Panagiotis J. Ntais (Philothei, Greece), and Virgilio Gonano (Udine, Italy). We also thank super-classifiers Ivan A. Terentev (Petrozavodsk, Russia) and Marvin W. Huddleston (Mesquite, USA) . Many thanks to Cliff Johnson (Zooniverse) and Marc Kuchner (NASA) for their ongoing Citizen Science guidance.
Funding: This material is based upon work supported by the NSF Graduate Research Fellowship Program under grant No. 2018258765 and grant No. 2020303693. C.O.C., H.H.H., and C.A.T. acknowledge support from NASA grant 80NSSC19K0869. W.J.O. and C.A.T. acknowledge support from NASA grant 80NSSC21K0114. This work was supported in part by NSF awards 1950901 (NAU REU program in astronomy and planetary science). Computational analyses were run on Northern Arizona University's Monsoon computing cluster, funded by Arizona's Technology and Research Initiative Fund.
Software and Services: World Coordinate System corrections facilitated by Astrometry.net (Lang et al. 2010). This research has made use of NASA's Astrophysics Data System, the NASA/IPAC Infrared Science Archive, the Institut de Mécanique Céleste et de Calcul des Éphémérides SkyBoT Virtual Observatory tool (Berthier et al. 2006), and data and/or services provided by the International Astronomical Union's Minor Planet Center, SAOImageDS9, developed by Smithsonian Astrophysical Observatory (Joye 2006).
Facilities and Instrumentation: This project used data obtained with the Dark Energy Camera (DECam), which was constructed by the Dark Energy Survey (DES) collaboration. This research uses services or data provided by the Astro Data Archive at NSF's NOIRLab. Based on observations at Cerro Tololo Inter-American Observatory, NSFs NOIRLab (NOIRLab Prop. ID 2019B-1014; PI: F. Olivares), the Pan-STARRS1 Surveys (PS1) and the PS1 public science archive (Chambers et al. 2016), the Zwicky Transient Facility (Bellm et al. 2019), and the CADC Solar System Object Information Search (Gwyn et al. 2012).
Facilities: CTIO:4 m (DECam) - , IRSA - , PO:1.2 m (PTF - , ZTF) - , PS1. -
Software: astropy (Robitaille et al. 2013), 12 Matplotlib (Hunter 2007), NumPy (Harris et al. 2020), pandas (Reback 2022), SAOImageDS9 (Joye 2006), SciPy (Virtanen et al. 2020).