The changing rotational light-curve amplitude of Varuna and evidence for a close-in satellite

TL;DR

This study analyzes 19 years of photometric data of Varuna, revealing significant changes in its rotational light-curve amplitude and shape, and provides evidence for a close-in satellite affecting its rotational properties.

Contribution

It introduces a model explaining light-curve variations through aspect angle changes and proposes the existence of a close-in satellite influencing Varuna's light-curve shape.

Findings

Light-curve amplitude changed from 0.41 to 0.55 mag over 19 years.

Detected an additional periodicity suggesting a satellite.

Estimated satellite orbit at ~1300 km with an 11.98 h period.

Abstract

From CCD observations carried out with different telescopes, we present short-term photometric measurements of the large trans-Neptunian object Varuna in 10 epochs, spanning around 19 years. We observe that the amplitude of the rotational light-curve has changed considerably during this period of time from 0.41 to 0.55 mag. In order to explain this variation, we constructed a model in which Varuna has a simple triaxial shape, assuming that the main effect comes from the change of the aspect angle as seen from Earth, due to Varuna's orbital motion in the 19-year time span. The best fits to the data correspond to a family of solutions with axial ratios b/a between 0.56 and 0.60. This constrains the pole orientation in two different ranges of solutions presented here as maps. Apart from the remarkable variation of the amplitude, we have detected changes in the overall shape of the rotational light-curve over shorter time scales. After the analysis of the periodogram of the residuals to a 6.343572 h double-peaked rotational light-curve fit, we find a clear additional periodicity. We propose that these changes in the rotational light-curve shape are due to a large and close-in satellite whose rotation induces the additional periodicity. The peak-to-valley amplitude of this oscillation is in the order of 0.04 mag. We estimate that the satellite orbits Varuna with a period of 11.9819 h (or 23.9638 h), assuming that the satellite is tidally locked, at a distance of ~ 1300 km (or ~ 2000 km) from Varuna, outside the Roche limit.