Recent results of stellar occultations by (60558) Echeclus

Centaurs are small objects of the Solar System with orbits between Jupiter and Neptune (5.2 AU < q < 30 AU) (Jewitt 2009), being an important population due to the presence of cometary activity (about 13% of Centaurs shows cometary activity) (Bauer et al. 2008). However, after the discovery of ring systems orbiting Chariklo (Braga-Ribas et al. 2013) and Haumea (Ortiz et al. 2017) and the proposition of a ring around Chiron (Ruprecht et al. 2015; Ortiz et al. 2015), we wonder if these structures are common around the small bodies or if specific conditions are necessary for their formation and maintenance (Sicardy et al. 2020). Discovered in March 2000, the active Centaur 174P/Echeclus (60558) has an equivalent diameter estimated in 59 ± 4 km (Bauer et al. 2013) and 64.6 ± 1.6 km (Duffard et al. 2014), and showed cometary activity on different occasions: December 2005 (Choi & Weissman 2006), May 2011 (Jaeger et al. 2011), August 2016 (Miles 2016), and December 2017 Kareta et al. (2019). To determine the main body’s size and shape and investigate whether material ejections during the outbursts could have fed possible rings or a shell of diffuse material around Echeclus, we predicted and observed stellar occultations by this Centaur in 2019, 2020, and 2021.

Stellar occultations by Echeclus were predicted using the Gaia DR2 catalog and NIMA ephemeris (Desmars et al. 2015). The prediction map of the 2019 October 29 event put the shadow’s path over South America, but all the telescopes that participated in this campaign missed the occultation path, providing five negative chords. The 2020 January 22 event is also predicted to pass over South America, resulting in two positive and four negative chords. Finally, we predicted the last event over Japan in 2021 January 19, resulting in one positive and ten negative chords.

With the positive detections of 2020, we fit an ellipse with an equivalent diameter of 59 ± 4 km to the edges of the chords. The resulting ellipse has a semi-major axis a’ = 36 km and oblateness ε′ = 0.325. From the rotational light curves (Rousselot et al., 2021), we note that 2020 stellar occultation occurs near the maximum absolute brightness. Thus, the surface seen during the occultation event was close to the maximum possible. So we were able to compare the ellipse fitted to the chords to the 3D model and pole orientations proposed by (Rousselot et al. 2021). By propagating the Echeclus rotation, we compare the 3D model to the 2021 stellar occultation, where we rule out some of the proposed pole solutions due to the close negative chord. We also fitted the 3D model to the chords, obtaining the triaxial dimensions of Echeclus as a × b × c = 36.5 × 28.0 × 24.5 km, resulting in an area-equivalent diameter of D_equiv = 61.8 ± 0.6 km, which is in agreement with the area-equivalent diameters presented in the literature.

We used all three event data sets to look for sudden drops in flux (evidence of confined material) or shallow and extensive drops (evidence of coma). The best light curves in terms of spatial resolution and SNR were: La Silla/NTT in 2019, which covered about 7,000 km in the sky plane; SOAR in 2020, covering 14,000 km in the sky plane and Okazaki/Japan in 2021, which covered about 9,000 km in the sky plane. With these light curves, we determined lower limits for detection for apparent opacity at the 3σ level as 0.145, 0.189, and 0.258, respectively. In addition, limits for the equivalent width were also determined for these three data sets, with values of 0.19 km for La Silla/NTT, 0.36 km for SOAR, and 0.18 km for Okazaki.

Acknowledgments: C.L.P. is thankful for the support of the CAPES scholarship. The following authors acknowledge the respective CNPq grants: F.B-R 309578/2017-5; J.I.B.C. 308150/2016-3 and 305917/2019-6; F.L.R. CAPES scholarship. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001 and the National Institute of Science and Technology of the e-Universe project (INCT do e-Universo, CNPq grant 465376/2014-2). ARGJr acknowledges FAPESP grant 2018/11239-8.

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