Observation of Stellar Occultations by Asteroid (259) Alethea and Comet 21P/Jacobini−Zinner

Abstract

Instrumentation for observing occultations of stars by Solar System bodies and its parameters are described. The complex is designed for observations at the AZT-2 telescope (D = 70 cm, F = 10.5 m) of the Main Astronomical Observatory of the National Academy of Sciences of Ukraine (МAO NASU); it is equipped with an optical reducer of the focal length and may also be mounted at the other telescopes. The observation complex uses the Apogee Alta U47 CCD camera as a light receiver, which operates in the drift scan mode. The results of the study of two phenomena that were observed with this complex are described. During the observations on September 21, 2018, a decrease of approximately 0.04m in the brightness of the star HD 45314 was detected near the ephemeris moment of occultation by comet 21P/Giacobini−Zinner. This value exceeds the level of errors possible in the photometry of a stellar track. A possible explanation of this effect by the attenuation of light in a symmetrical dust coma of the comet was analyzed. It has been shown that this factor cannot yield the required magnitude of the light attenuation. The observed decrease in brightness is explained by the passage of the star through the jet structure with sharp boundaries, the optical thickness of which is 0.035 ± 0.012 and the opening angle is 54.6°. The single scattering albedo of dust particles in this jet is in the range of 0.04−0.06. The occultation of the star UCAC4-475-051755 by asteroid (259) Alethea was recorded on March 31, 2019, and it lasted for 16.7 s. This occultation duration corresponds to the asteroid chord 185 km long, while the expected one is 61 km, and it exceeds a value of 179 km accepted for this asteroid diameter. This difference is explained by the ellipsoidal shape of asteroid (259) Alethea, and the grounds for a value of 1 : 1.19 for its axial ratio are presented.