Size, shape, and topography of TNOs from stellar occultations: the case of (307261) 2002 MS₄

The small bodies from the outer solar system, apart from Pluto, started to be discovered in the 1990’s, but the knowledge about their physical properties still is scarce and fragmented. Stellar occultation is a powerful technique that allows determining the dimensions and shape of the object’s instantaneous limb from Earth-based observations. In recent years, a great effort has been made inside the Lucky Star collaboration to improve the predictions and observe stellar occultation events by those bodies. However, only the size and shape of the limb are not enough to fully characterize such objects. For example, to derive density and even a 3D shape, the analysis should include rotational information alongside the size and albedo obtained from occultations and thermal observations.

In the context of a combined analysis of data from different techniques and instruments, we started with a particular case, one of the biggest objects (Vilenius et al. 2012) and a dwarf planet candidate, the TNO 2002 MS₄. Our collaboration detected nine stellar occultations by 2002 MS₄ since 2019, and the most successful campaign, with 61 positive detections, allowed us to determine its limb size and shape accurately. Moreover, it also revealed the presence of prominent superficial topography. This is the first time we have acquired enough data to characterize such structures in the object’s limb. In addition, the gradual star’s reappearance on the northernmost light curve also indicates the presence of features on the surface.

Currently, this body is transiting in front of the galactic plane, and the crowded field of view makes it difficult to measure its rotational light curve. The only information about its rotation in the literature was based on a few nights of observations when the object passed in front of a dark cloud (Thirouin et al., 2013). The authors used relative photometry and derived a rotational light curve with an amplitude of 0.05 mag and two possibilities for the rotation period. Here, we tried a different approach: obtaining absolute photometry in every image available in our database and public repositories to get as much information as possible. The data set allowed us to determine a magnitude and provide limits for the rotational parameters. In conclusion, this joint analysis shows that 2002 MS₄ is a body under hydrostatic equilibrium with a Maclaurin shape and prominent superficial topography.