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

¹ Charles University, Faculty of Mathematics and Physics, Institute of Astronomy, V Holešovičkách 2, 180 00 Prague, Czech Republic
² Instituto de Astrofisica de Andalucia-CSIC, Glorieta de la Astronomia sn, 18008 Granada, Spain
³ Florida Space Institute, University of Central Florida, 12354 Research Parkway, Partnership 1 building, Orlando, FL, 32828, USA
⁴ STAR Institute - University of Liège, Allée du 6 Août 19C, 4000 Liège 1, Belgium
⁵ Organ Mesa Observatory, 4438 Organ Mesa Loop, Las Cruces, NM 88011 USA
⁶ Observatorio Nuevos Horizontes, Camas, Sevilla, Spain
⁷ SETI Institute, Carl Sagan Center, 189 Bernado Avenue, Mountain View CA 94043, USA
⁸ Unistellar, 5 allé Marcel Leclerc, bâtiment B, 13008 Marseille, France
⁹ Centro de Estudios de Física del Cosmos de Aragón, Plaza San Juan 1, 44001 Teruel, Spain
¹⁰ Light Bridges S.L, Observatorio del Teide. Carretera del Observatorio s/n Guimar, Tenerife, Spain
¹¹ Instituto de Astrofísica de Canarias (IAC), C/Via Lactea sn, 38205 La Laguna, Canarias, Spain
¹² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Canarias, Spain
¹³ Trans-Tasman Occultation Alliance (TTOA), Wellington, New Zealand
¹⁴ Oukaimeden Observatory, High Energy Physics and Astrophysics Laboratory, Cadi Ayyad University, Marrakech, Morocco
¹⁵ Astronomical Observatory Institute, Faculty of Physics and Astronomy, Adam Mickiewicz University, Słoneczna 36, 60-286 Poznań, Poland

^★ Corresponding author; durech@sirrah.troja.mff.cuni.cz

Received: 3 February 2025
Accepted: 20 March 2025

Abstract

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 P_ψ = 1172.2 ± 0.3 h and the precession period P_ϕ = 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.