Modelling the light curve of ‘Oumuamua: evidence for torque and disc-like shape

Abstract

ABSTRACT We present the first attempt to fit the light curve of the interstellar visitor ‘Oumuamua using a physical model that includes optional torque. We consider both conventional (Lommel–Seeliger triaxial ellipsoid) and alternative (‘black-and-white ball’, ‘solar sail’) brightness models. With all the brightness models, some torque is required to explain the timings of the most conspicuous features – deep minima – of the asteroid’s light curve. Our best-fitting models are a thin disc (aspect ratio 1:6) and a thin cigar (aspect ratio 1:8) that are very close to being axially symmetric. Both models are tumbling and require some torque that has the same amplitude in relation to ‘Oumuamua’s linear non-gravitational acceleration as in Solar system comets whose dynamics is affected by outgassing. Assuming random orientation of the angular momentum vector, we compute probabilities for our best-fitting models. We show that cigar-shaped models suffer from a fine-tuning problem and have only 16 per cent probability to produce light-curve minima as deep as the ones present in ‘Oumuamua’s light curve. Disc-shaped models, on the other hand, are very likely (at 91 per cent) to produce minima of the required depth. From our analysis, the most likely model for ‘Oumuamua is a thin disc (slab) experiencing moderate torque from outgassing.