DETERMINATION OF PHYSICAL PROPERTIES AND TOPOGRAPHY OF TNOS FROM STELLAR OCCULTATIONS AND ROTATIONAL LIGHT CURVES: THE CASE OF 2002 MS4 OBJECT

Abstract

Forty-five years had elapsed since the discovery of Chiron, three decades since the first observation of a trans-Neptunian object (except Pluto), and currently, thousands of small objects have been discovered in the outer solar system. Due to the significant distances from the Sun, it is thought that their global physical-chemical composition has been unaffected since their formation. Therefore, studies about these objects may reveal valuable information about the solar system’s formation and dynamical evolution. However, mainly due to the faintness and small angular sizes on the sky plane, the knowledge about Centaurs and trans-Neptunian objects is still scarce and fragmented. Direct images of these distant objects acquired from Earth are possible only for a few dozen of them, and space-based telescopes also detected a few hundred objects in the thermal band. Therefore, the physical properties of most objects remain entirely unknown. However, the current stellar catalogs’ accuracy brings to light one of the most accurate technique to determine an object’s size and shape from Earth, the stellar occultations. It consists of acquiring photometric images of a star while a small object passes in front of it (at the sky plane), blocking its flux for a given observer. Occultations have been used successfully in the last ten years for small bodies’ size and shape derivations, but also to discover surrounding structures like rings and jets. This work analyzes nine stellar occultations by 2002 MS4 and one by 2004 XR190. The images were acquired between 2019 and 2022 from telescopes worldwide and came from various instruments. Therefore, standardization was needed before submitting them to the aperture photometry. The aperture photometry provided the stellar flux as a function of time for each observational station, the occultation light curves. Each positive detection of the occultation is a measurement of the object’s limb, and by analyzing all the positive chords at the sky plane, the object’s projected limb is derived with sub-km accuracy. Unlike the expected for large objects, the 2002 MS4 did not present a perfectly rounded limb. Thus a topography search and characterization methodology is developed to measure it adequately. This is the first time that a multichord occultation detected significant topography in a trans-Neptunian object. Direct images of trans-Neptunian objects from public and private repositories are also analyzed through different approaches. Due to the diversity of formats and header keys, they are submitted to a pre-processing step before the aperture photometry. In these images, the aperture photometry gives the object’s flux as a function of time, and the light curves do not provide a limb measurement. However, the variation of the object’s flux as a function of the phase angle may provide the absolute magnitude and some superficial information. A rotational light curve also may be derived if data are precise enough. This work presents estimates of the object’s absolute magnitude and the rotational period from absolute photometry.