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

Abstract

We present the first attempt to fit the light curve of the interstellar visitor 'Oumuamua using a physical model which 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) which are very close to being axially symmetric. Both models are tumbling and require some torque which has the same amplitude in relation to 'Oumuamua's linear non-gravitational acceleration as in Solar System comets which 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.

1I/‘Oumuamua as an N2 Ice Fragment of an exo‐Pluto Surface: I. Size and Compositional Constraints

“…the time of observa-tion) 45.4 × 43.9 × 7.50 m. This shape is consistent with the oblate spheroid solution ofMashchenko (2019), re-scaled to our albedo. Nearly pure N 2 ice is observed in abundance on the surfaces of Pluto and Triton, with the surface of Pluto being 98% N 2 ice.…”

“…As such, we also treat the size of 'Oumuamua as a variable. We keep the axis ratios of the Mashchenko (2019) oblate solution (∼6:6:1), but allow the axis lengths to scale as…”

“…Mashchenko (2019) provided two possible solutions for the shape of ‘Oumuamua: their preferred oblate spheroid solution, with axes 115 m × 111 m × 19 m; and a prolate spheroid, with axes 342 m × 42 m × 42 m. Both of these solutions, however, assume that ‘Oumuamua has a geometric albedo, p G , of 0.1. Since the light reflected is proportional to p G , it follows that the axis lengths are proportional to , since the shape is fit to a light curve of known brightness.…”

“…A prolate ellipsoid cannot experience the required acceleration unless it has substantial amounts of H 2 ice, but an oblate spheroid shape is more accommodating of other compositions, further favoring the oblate scenario. We consider the shape favored by Mashchenko (2019); and Seligman and Laughlin (2020): axes (diameters) a = 115 m, b = 111 m, c = 19 m (assuming an albedo 0.1), and assumed average projected-area-to-surface-area ratio ξ ≈ 0.19 (during the times of the light curve observations). Seligman and Laughlin (2020) proposed that 'Oumuamua contained a substantial quantity of H 2 ice, which they showed could provide the necessary non-gravitational acceleration if it covered around 6% of the surface.…”

Bridge to the stars: A mission concept to an interstellar object

“…Although this hypothesis is at odds with the clear cometary behavior of 2I/Borisov, it could explain why this comet outgasses little water (Yang et al, 2020). Finally, 1I/'Oumuamua's disc-like shape with a 6:6:1 aspect ratio and its excited rotational state are unusual when compared to other objects in our solar system (Mashchenko 2019). Taken together, these observations suggest that interstellar objects are compelling objects that hint at both tantalizing similarities and perp lexin g diffe renc es J o u r n a l P r e -p r o o f between our own solar system and exoplanetary systems.…”