Daily variability of Ceres’ albedo detected by means of radial velocities changes of the reflected sunlight

Abstract

Bright features have been recently discovered by Dawn on Ceres, which extend previous photometric and Space Telescope observations. These features should produce distortions of the line profiles of the reflected solar spectrum and therefore an apparent radial velocity variation modulated by the rotation of the dwarf planet. Here we report on two sequences of observations of Ceres performed in the nights of 2015 July 31, August 26 and 27 by means of the high-precision High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph at the 3.6 m La Silla European Southern Observatory (ESO) telescope. The observations revealed a quite complex behaviour which likely combines a radial velocity modulation due to the rotation with an amplitude of ≈±6 m s−1 and an unexpected diurnal effect. The latter changes imply changes in the albedo of Occator's bright features due to the blaze produced by the exposure to solar radiation. The short-term variability of Ceres’ albedo is on time-scales ranging from hours to months and can both be confirmed and followed by means of dedicated radial velocity observations.