A High Geometric Albedo and Small Size for the Haumea Cluster Member (24835) 1995 Sm55 Determined from a Stellar Occultation and Photometric Observations

Abstract

Context. Trans-Neptunian objects (TNOs) are thought to be some of the most ancient and primitive bodies in our Solar System. Understanding their basic physical properties is crucial to unraveling their origin and the evolution of the outer Solar System beyond Neptune. Stellar occultations are a highly effective and sensitive method of studying these distant and faint objects, allowing us to gather essential information about their physical characteristics. (24835) 1995 SM<inf>55</inf> is one of the few members of the Haumea orbital cluster and, therefore, is an especially relevant body to study within the TNO population. Aims. The main objectives of the present work are to determine the projected size, absolute magnitude, and geometric albedo of 1995 SM<inf>55</inf> and to analyze the results compared to Haumea. Methods. We predicted a stellar occultation by this TNO for February, 25, 2024, carried out a specific campaign to observe the occultation, and derived the projected size and shape from the occultation observations using an elliptical fit to the occultation chords. We also analyzed a large set of photometric observations of (24835) 1995 SM<inf>55</inf> to obtain the absolute magnitude and the rotational period. Finally, we combined these results to derive the geometric albedo of this TNO. Results. The occultation was successfully detected from seven instruments located at five different sites; 33 other sites reported negative detections. Using an elliptical fit to the occultation chords, we determined the size and shape of the limb of (24835) 1995 SM<inf>55</inf> during the occultation: an ellipse with semi-axes (104.3 ± 0.4) × (83.5 ± 0.5) km. The area-equivalent diameter (D<inf>eq,A</inf>) for this ellipse is 186.7 ± 1.8 km. This is consistent with the upper limit of 250 km estimated from Herschel Observatory thermal data. From our photometric observations, we derived an absolute magnitude (H<inf>V</inf>) of 4.55 ± 0.03, a phase slope parameter of 0.04 ± 0.02 mag/deg, and a V − R value of 0.37 ± 0.05. The rotational variability has a maximum peak-to-valley amplitude (∆m) of 0.05 mag, but we could not derive an unambiguous rotational period. Combining the projected size from the occultation with the absolute photometry, we obtained a geometric albedo in the V band (p<inf>V</inf>) of 0.80 ± 0.04 for 1995 SM<inf>55</inf>. This value is remarkably high for a TNO and somewhat higher than that of Haumea, but consistent with the concept that 1995 SM<inf>55</inf> is a member of the orbital cluster of Haumea. © The Authors 2025.