The first observed stellar occultations by the irregular satellite (Saturn IX) Phoebe and improved rotational period

TL;DR

This paper reports the first stellar occultations by Saturn's irregular satellite Phoebe, improving its rotational period measurement and providing precise astrometric positions, thus enhancing understanding of its shape and rotation.

Contribution

It introduces a novel method to determine Phoebe's rotation period from occultation data and refines its value with unprecedented precision.

Findings

First observed stellar occultations by Phoebe.

Improved rotational period: 9.27365 ± 0.00002 hours.

Six high-precision astrometric positions obtained.

Abstract

We report six stellar occultations by (Saturn IX) Phoebe, an irregular satellite of Saturn, obtained between mid-2017 and mid-2019. The 2017 July 06 event is the first stellar occultation by an irregular satellite ever observed. The occultation chords were compared to a 3D shape model of the satellite obtained from Cassini observations. The rotation period available in the literature led to a sub-observer point at the moment of the observed occultations where the chords could not fit the 3D model. A procedure was developed to identify the correct sub-observer longitude. It allowed us to obtain the rotation period with improved precision over currently known value from literature. We show that the difference between the observed and the predicted sub-observer longitude suggests two possible solutions for the rotation period. By comparing these values with recently observed rotational light curves and single-chord stellar occultations, we can identify the best solution for Phoebe's rotational period as $9.27365 \pm 0.00002$ h. From the stellar occultations, we also obtained 6 geocentric astrometric positions in the ICRS as realised by the Gaia-DR2 with uncertainties at the 1-mas level.