Size and Shape of Trojan Asteroid Diomedes from Its Occultation and Photometry

Abstract The first Trojan asteroid occultation, the occultation of HIP 014402A by (1437) Diomedes, was successfully observed from Japan on November 7, 1997. From its occultation timings at six sites including two video observations, an elongated occultation silhouette of (180±28 km)×(96±5 km), at position angle PA=150±4° was revealed. Follow-up photometry of Diomedes obtained at Ondřejov on November 10 and 11 and at Mitaka on November 18, 19, and 24 revealed that its rotation period is 1.019±0.004 days, its amplitude of light variation is 0.70±0.15 mag, and its rotational phase at the occultation was almost at a minimum of the lightcurve. From these observations, a probability distribution ellipsoidal model for Diomedes is derived. It shows that two families of ellipsoidal models are possible. One is a triaxial ellipsoid of b / a ≈0.55; the other is a rather prolate ellipsoid of b / a ≈0.4, c / b ≈1. Possible orientation of the rotation axis is very restricted to two great circles on the celestial sphere. Mean lengths of the three principal axes of the model ellipsoid are (284±61 km)×(126±35 km)×(65±24 km), i.e., approximately a : b : c ≈4:2:1.