2021MNRAS.508.2730D

We report on the occultation of the star TYC 1947-290-1 by the asteroid (87) Sylvia. While asteroidal occultations occurring at fixed professional-level locations are relatively rare and are only recently starting to be observed with sufficiently high time resolution and sensitivity, they have the capability to measure sub-milliarcsecond angular diameters. The event described here was especially outstanding because the star was revealed to be a small-separation binary (≃10 mas at discovery), while at the same time the asteroid is not only one of the largest in size but it also has two satellite moons. The observations were carried out at the Russian 6-m telescope in 2019 December, and initially consisted of both a fast photometric series of the occultation itself, as well as of extensive speckle interferometry of the star and asteroid in the time immediately before and after the occultation. Subsequently, we obtained speckle data of TYC 1947-290-1 over a period of 1 yr after the event. We are able to present a detailed study of the binary star including measurements of the angular diameter of the stellar components, their geometry, and relative fluxes over several bandpasses, and to provide an accurate determination of the size of (87) Sylvia. We emphasize that we have been able to obtain the smallest ever directly measured stellar diameter, below the 100 micro-arcsecond level. Our data are also suitable for imaging of the asteroid by speckle holography, a task which we intend to carry out in a separate work.