Integral-field spectroscopy of (90482) Orcus-Vanth

TL;DR

This study used adaptive optics integral-field spectroscopy to analyze the surface composition and orbital parameters of the Trans-Neptunian Object Orcus and its satellite Vanth, revealing crystalline water ice and constraining surface ices.

Contribution

First high signal-to-noise near-infrared spectrum of Orcus confirming crystalline water ice and detecting a 2.2 μm absorption band, with new orbital phase data for Vanth.

Findings

Confirmed presence of crystalline H2O ice on Orcus

Detected a 2.2 μm absorption band suggesting ammonia presence

Set upper limits for methane and ethane on Orcus surface

Abstract

Aims. We seek to constrain the surface composition of the Trans-Neptunian Object (90482) Orcus and its small satellite Vanth, as well as their mass and density. Methods. We acquired near-infrared spectra (1.4-2.4 {\mu}m) of (90482) Orcus and its companion Vanth using the adaptive-optics-fed integral-field spectrograph SINFONI mounted on Yepun/UT4 at the European Southern Observatory Very Large Telescope. We took advantage of a very favorable appulse (separation of only 4") between Orcus and the UCAC2 29643541 star (R = 11.6) to use the adaptive optics mode of SINFONI, allowing both components to be spatially resolved and Vanth colors to be extracted independently from Orcus. Results. The spectrum of Orcus we obtain has the highest signal-to-noise ratio to date, and we confirm the presence of H2O ice in crystalline form, together with the presence of an absorption band at 2.2 {\mu}m. We set an upper limit of about 2% for the presence of methane, and 5% for ethane. Because the methane alone cannot account for the 2.2 {\mu}m band, the presence of ammonia is suggested to the level of a couple of percent. The colors of Vanth are found slightly redder than those of Orcus, but the large measurement uncertainties forbid us from drawing conclusions on the origin of the pair (capture or co-formation). Finally, we reset the orbital phase of Vanth around Orcus, and confirm the orbital parameters derived by Brown et al. (2010, AJ 139).