Neptune’s ring arcs from VLT/SPHERE-IRDIS near-infrared observations^★

¹ CRAL, UMR 5574, CNRS, Université de Lyon, Ecole Normale Supérieure de Lyon, 46 allée d’Italie, 69364 Lyon Cedex 07, France
e-mail: damya.souami@obspm.fr; souami@astro.utoronto.ca
² naXys, University of Namur, 8 Rempart de la Vierge, Namur 5000, Belgium
³ Université de Lille, Observatoire de Lille, 1 impasse de l’Observatoire, 59000 Lille, France
⁴ IMCCE, Observatoire de Paris, CNRS UMR 8028, 77 avenue Denfert Rochereau, 75014 Paris, France
⁵ CRAL, UMR 5574, CNRS, Université de Lyon, Ecole Normale Supérieure de Lyon, 46 allée d’Italie, 69364 Lyon Cedex 07, France
⁶ Université de la Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
⁷ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁸ University of Toronto Scarborough, Toronto, ON M1C 1A4, Canada

Received: 20 June 2021
Accepted: 15 October 2021

Abstract

Context. Neptune’s incomplete ring arcs have been stable since their discovery in 1984 by stellar occultation. Although these structures should be destroyed within a few months through differential Keplerian motion, imaging data over the past couple of decades have shown that these structures remain stable.

Aims. We present the first SPHERE near-infrared observations of Neptune’s ring arcs taken at 2.2 μm (broadband Ks) with the IRDIS camera at the Very Large Telescope (VLT) in August 2016.

Methods. The images were aligned using the ephemerides of the satellite Proteus and were suitably co-added to enhance ring and satellite signals.

Results. We analyse high-angular-resolution near-infrared images of Neptune’s ring arcs obtained in 2016 at the ESO VLT-UT3 with the adaptive-optics-fed camera SPHERE-IRDIS. We derive accurate mean motion values for the arcs and the nearby satellite Galatea. The trailing arcs Fraternité and Égalité have been stable since they were last observed in 2007. Furthermore, we confirm the fading away of the leading arcs Courage and Liberté. Finally, we confirm the mismatch between the arcs’ position and the 42:43 inclined and eccentric corotation resonances with Galatea, thus demonstrating that no 42:43 corotation model works to explain the azimuthal confinement of the arcs’ materiel.