The Trans-Neptunian Object (19521) Chaos as seen from stellar occultations and photometry observations

<p>Trans-Neptunian Objects (TNOs) are important solar system bodies that carry valuable information on the first stages of our solar system and its evolution. The TNO named (19521) Chaos (formerly known as 1998 WH24) is a large object, presumably in the 600-km size range judging by its approximate absolute magnitude and assuming a typical geometric albedo for a TNO. This is a comparable size to that of the three largest asteroids in the main asteroid belt. Therefore, it is an important body to study and characterize through stellar occultations and through other techniques. On October 20th, 2020, a three-chord stellar occultation was recorded by our team (Vara-Lubiano et al. 2021) within the context of the Lucky Star international collaboration* on stellar occultations by TNOs and other outer solar system bodies, and recently, on January 14th, 2022, another stellar occultation by Chaos has been recorded, whose main preliminary results will be presented. In this case 24 sites participated in the campaign. There were 8 positive detections and 3 near misses. The analysis of this occultation combined with the previous one and with photometry data obtained along 17 years (within our own TNO observing program) will be presented. We expect to derive an accurate size and shape as well as an accurate geometric albedo, which can be compared with radiometric measurements. We also expect to provide constraints on the spin axis orientation. The fact that no satellite around Chaos has been discovered so far means that we do not know the system mass so we cannot infer a bulk density for the body to compare with hydrostatic equilibrium computations, but we can derive other useful constraints.</p> <p>*Lucky Star (LS) is an EU-funded research activity to obtain physical properties of distant Solar System objects using stellar occultations. LS collaboration agglomerates the efforts of the Paris, Granada, and Rio teams. https://lesia.obspm.fr/lucky-star/ </p&gt