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

D Souami,P Golaszewska,M Langlois,P Delorme,S Renner,B Carry,B Sicardy

doi:10.1051/0004-6361/202141598

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.

Highlights

A stellar occultation campaign on July 22, 1984, yielded the first conclusive evidence of an incomplete ring-like structure around Neptune, inside the classical Roche limit, assuming a density of 1 for the ring material (Roques et al 1984; Sicardy et al 1985; Hubbard et al 1986; Covault et al 1986)
In this paper we report on astro-photometric measurements of the arcs that were obtained at the Very Large Telescope (VLT)-UT3 with the SpectroPolarimetric High-contrast Exoplanet REsearch (SPHERE) InfraRed Dual-band Imager and Spectrograph (IRDIS) instrument fed by its extreme adaptive optics (AO) system obtained on August 23, 2016
Observations We used the high-angular-resolution AO SPHERE instrument installed on the VLT-UT3 at the European Southern Observatory (ESO) with IRDIS (Dohlen et al 2008; Beuzit et al 2019) to image the Neptunian system on August 23, 2016

Summary

Introduction

A stellar occultation campaign on July 22, 1984, yielded the first conclusive evidence of an incomplete ring-like structure around Neptune, inside the classical Roche limit, assuming a density of 1 for the ring material (Roques et al 1984; Sicardy et al 1985; Hubbard et al 1986; Covault et al 1986). Voyager 2 data revealed that the arcs are longitudinally confined over a 40◦ azimuthal range They are embedded in the much fainter continuous Adams ring around Neptune (Smith et al 1989), which is the outermost ring of the Neptunian system. Showalter et al (2017) presented a possible three-body resonance mechanism capable of confining ring material within the observed corotation sites This scenario constrains the orbital semi-major axis of the arcs to fall within ∼10 m of a three-body mean motion resonance, which involves the two nearby inner satellites, Galatea and Larissa. Observations We used the high-angular-resolution AO SPHERE instrument installed on the VLT-UT3 at the European Southern Observatory (ESO) with IRDIS (Dohlen et al 2008; Beuzit et al 2019) to image the Neptunian system (ring arcs and moons) on August 23, 2016. The data were acquired using this classical mode of SPHERE in the K s broadband (BB-K s) filter centred at 2.2 μm, which corresponds to a strong absorption in the methane spectrum, reducing the otherwise overwhelming scattered light from Neptune’s atmosphere

Their respective corotation critical arguments write as follows

Data reduction

Astro-photometric analysis of the data

Results

Mean motions We improved the average mean motion of the satellite Galatea: (i)

Conclusions