Shape and spin-state model of tumbling asteroid (319) Leona

Context. Stellar occultations by asteroids observed from several stations are routinely used to reconstruct the asteroid’s sky-plane projections. Together with the asteroid’s shape model reconstructed from photometry, such occultations enable us to precisely determine its size and reveal details of its shape. When reducing occultation timings, the usual assumption is that the star has a negligible angular diameter compared to the asteroid, so it is modeled as a point source. The occultation of Betelgeuse (α Orionis) by asteroid (319) Leona on 12 December 2023 was a rare exception – the apparent angular diameter of the star was ∼50 mas, about the same as that of the asteroid. Aims. This work aimed to reconstruct the shape and spin model of asteroid Leona. Then, the projection of that model for the time of the occultation can be computed, which is necessary to interpret the occultation timings and infer valuable information about Betelgeuse itself. Methods. We collected available photometric data of Leona, carried out new observations, and reconstructed a unique convex shape model. Using three other occultations observed in 2023, we scaled this convex model. We also reconstructed an alternative nonconvex model with the same spin parameters and size but showing some surface details that explain better one of the occultations. Results. We confirmed the tumbling rotation state of Leona and uniquely determined the rotation period Pψ = 1172.2 ± 0.3 h and the precession period Pϕ = 314.27 ± 0.02 h. The volume-equivalent diameter determined by occultations is 59.1 ± 0.9 km. The reconstructed shape model of Leona enabled us to compute its sky-plane projection for the time of the Betelgeuse occultation. Conclusions. A reliable shape model with accurate dimensions and accurate rotation and precession periods has been reconstructed for slowly tumbling asteroid Leona. It can be used to interpret the observed occultation of Betelgeuse by Leona.