The stellar occultation by (319) Leona on 13 September 2023 in preparation for the occultation of Betelgeuse

On 12 December 2023, the star α Orionis will be occulted by asteroid (319) Leona. This represents an extraordinary and unique opportunity to analyze the brightness distribution of Betelgeuse’s photosphere with extreme angular resolution by studying lightcurves from different points on Earth and at different wavelengths. Here we present observations of another occultation by asteroid Leona, on 13 September 2023, whose goal was to determine Leona’s projected shape and size in preparation for the December 12th event and its interpretation. The occultation campaign was highly successful with 25 positive detections from 17 different sites and a near miss. The effective diameter in projected area derived from the positive detections was 66 ± 2 km using an elliptical fit to the instantaneous limb. The body is highly elongated, with dimensions of 79.6 ± 2.2 km x 54.8 ± 1.3 km in its long and short axis, respectively, at occultation time. This result, in combination with dense time series photometry of Leona that we recently obtained, together with archival sparse photometry, allowed us to predict the angular size of the asteroid for the Betelgeuse event and to simulate the expected brightness change. Also, an accurate position coming from the occultation is provided, to improve the orbit of Leona.