A high geometric albedo and small size of the Haumea cluster member (24835) 1995 SM55 from a stellar occultation and photometric observations

Abstract

Trans-Neptunian objects (TNOs) are among the most ancient bodies of the solar system. Understanding their physical properties is key to constraining their origin and the evolution of the outer regions beyond Neptune. Stellar occultations provide highly accurate size and shape information. (24835) 1995 SM55 is one of the few members of the Haumea cluster and thus of particular interest. We aimed to determine its projected size, absolute magnitude, and geometric albedo, and to compare these with Haumea. A stellar occultation on 25 February 2024 was observed from five sites, with seven positive detections and 33 negative chords. An elliptical fit to the occultation chords yields semi-axes of $(104.3\pm 0.4)\times (83.5\pm 0.5)$ km, giving an area-equivalent diameter of $186.7\pm 1.8$ km, smaller than the 250 km upper limit from Herschel thermal data. Photometry provides an absolute magnitude $H_V=4.55\pm 0.03$, a phase slope of $0.04\pm 0.02$ mag/deg, and a $V-R=0.37\pm 0.05$. The rotational variability has an amplitude $\Delta m=0.05$ mag, but the period remains uncertain. Combining occultation and photometry, we derive a geometric albedo $p_V=0.80\pm 0.04$, one of the highest values measured for a TNO. This value is slightly higher than that of Haumea, consistent with the interpretation that 1995 SM55 belongs to the Haumea cluster.

14 pages, 7 figures, 4 tables, accepted for publication in A&A (in press)

References in corpus (13)

• A ring system detected around the Centaur (10199) Chariklo
• The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation
• Absolute Physical Calibration in the Infrared
• The Creation of Haumea's Collisional Family
• SORA: Stellar Occultation Reduction and Analysis
• The Youthful Appearance of the 2003 EL61 Collisional Family
• "TNOs are Cool": A survey of the trans-Neptunian region XIV. Size/albedo characterization of the Haumea family observed with Herschel and Spitzer
• Stellar occultations enable milliarcsecond astrometry for Trans-Neptunian objects and Centaurs
• Haumea's Shape and Composition
• Occultation Portal: a web-based platform for data collection and analysis of stellar occultations
• Constraints on the structure and seasonal variations of Triton's atmosphere from the 5 October 2017 stellar occultation and previous observations
• Asteroid astrometry by stellar occultations: Accuracy of the existing sample from orbital fitting
• Reaching sub-millisecond accuracy in stellar occultations and artificial satellites tracking