Discovery of Dust Emission Activity Emanating from Main-belt Asteroid 2015 FW412

Active asteroids are rare (∼40) bodies that are found in orbits not normally associated with comets, yet they display comet-like activity such as a tail or coma (Jewitt et al. 2015). A small subset (∼15) of the known active asteroids, the main-belt comets (MBCs), are found to orbit entirely within the main asteroid belt and display comet-like activity indicative of volatile sublimation, such as recurrent activity near perihelion (Hsieh et al. 2015). These objects inform us about, for example, the solar system volatile distribution and, consequently, the origins of terrestrial water.

To find more members of this elusive population we created the Citizen Science project Active Asteroids ¹² (Chandler et al. 2018, 2019, 2020, 2021, 2022; Chandler 2022), a NASA Partner. 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. Volunteers classified images of 2015 FW₄₁₂ as active, and subsequently we carried out an archival investigation and new telescope observations to further study this object.

We identified ∼20 archival images of 2015 FW₄₁₂ (semimajor axis a = 2.76 au, eccentricity e = 0.16, inclination i = 137, perihelion distance q = 2.32 au, aphelion distance Q = 3.21 au, Tisserand parameter with respect to Jupiter T_J = 3.280; retrieved UT 2023 January 25 from JPL Horizons; Giorgini et al. 1996) with clear indications of dust emission activity in the form of a thin tail approximately aligned with the object's orbit plane (Figure 1). Notably, 2015 FW₄₁₂ is a MBC candidate with characteristics consistent with membership of this class, specifically activity occurring near perihelion, and orbiting within the main asteroid belt. On UT 2015 April 13, when 2015 FW₄₁₂ was at a heliocentric distance r_h  = 2.40 au (inbound to perihelion), α = 31 phase angle, and ν = 3201 true anomaly, the visible angular tail length as projected on the sky was ∼44'', corresponding to ∼4.5 × 10³ km at 1.4 au from Earth. We did not detect activity in four 200 s WB4800-7800 filter exposures we acquired with the Inamori-Magellan Areal Camera and Spectrograph on the Magellan 6.5 m Baade telescope on UT 2021 December 12 (PI Trujillo, observer C. Trujillo), when 2015 FW₄₁₂ was at r_h  = 3.10 au, outbound toward aphelion.

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General: We thank Dr. Mark Jesus Mendoza Magbanua (University of California San Francisco) for his continuous and timely feedback on our project.

Citizen Science: We thank Elizabeth Baeten (Belgium) for moderating the Active Asteroids forums. We thank our NASA Citizen Scientists that examined 2015 FW₄₁₂: Al Lamperti (Royersford, USA), Alice Juzumas (São Paulo, Brazil), Bill Shaw (Fort William, Scotland), Brian K. Bernal (Greeley, USA), C. J. A. Dukes (Oxford, UK), Clara Garza (West Covina, USA), Dr. David Collinson (Mentone, Australia), Emilio Jose Rabadan Sevilla (Madrid, Spain), @ graham_d (Hemel Hempstead, UK), Graham Mitchell (Chilliwack, Canada), Ivan A. Terentev (Petrozavodsk, Russia), Jayanta Ghosh (Purulia, India), Leah Mulholland (Peoria IL, USA), Martin Welham, (Yatton, UK), Megan Powell (Cobham, UK), Michael Jason Pearson, (Hattiesburg MS, USA), Michele T. Mazzucato (Florence, Italy), Sergey Y. Tumanov, (Glazov, Russia), Stikhina Olga Sergeevna (Tyumen, Russia), Thorsten Eschweiler (Übach-Palenberg, Germany), Tiffany Shaw-Diaz (Dayton, USA), and Virgilio Gonano (Udine, Italy) . We also thank Marvin W. Huddleston (Mesquite, USA) . Many thanks to Cliff Johnson (Zooniverse) and Marc Kuchner (NASA) for their ongoing 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 the NASA Solar System Observations program (grant 80NSSC19K0869). W.J.O. acknowledges 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 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, NSF's NOIRLab (NOIRLab Prop. ID 2013B-0536; PI: L. Allen; NOIRLab Prop. ID 2015A-0051; PI: S. Sheppard). Magellan observations made use of the Inamori-Magellan Areal Camera and Spectrograph (IMACS) instrument (Dressler et al. 2011); PI C. Trujillo.

Facilities: CTIO:4 m (DECam) - , Magellan:Baade (IMACS). -

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).