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

¹ Instituto de Astrofisica de Andalucía, IAA-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
² LTE, Observatoire de Paris, Université PSL, Sorbonne Université, Université de Lille, LNE, CNRS 61 Avenue de l’Observatoire, 75014 Paris, France
³ Florida Space Institute (FSI) – University of Central Florida (UCF), Partnership I, Research Parkway, 32826 Orlando, USA
⁴ Federal University of Technology - Paraná (PPGFA/UTFPR-Curitiba), Av. Sete de Setembro, 3165 Curitiba – PR, Brazil
⁵ Laboratório Interinstitucional de e-Astronomia - LIneA, Av. Pastor Martin Luther King Jr 126, 20765-000 Rio de Janeiro, RJ, Brazil
⁶ LIRA, CNRS UMR8254, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, 92190 Meudon, France
⁷ Federal University of Rio de Janeiro – Observatory of Valongo, Rio de Janeiro, Brazil
⁸ Deutsches Zentrum für Astrophysik (DZA), Postplatz 1, 02826 Görlitz, Germany
⁹ Institut Polytechnique des Sciences Avancées IPSA, 63 boulevard de Brandebourg, 94200 Ivry-sur-Seine, France
¹⁰ Türkiye National Observatories, TUG, 07070 Antalya, Türkiye
¹¹ The Scientific and Technological Research Council of Türkiye (TÜBİTAK), 06680 Ankara, Türkiye
¹² School of Physics & Astronomy and the Wise Observatory, Tel-Aviv University, Tel-Aviv 6997801, Israel
¹³ Astronomical Observatory Institute, Faculty of Physics and Astronomy, Adam Mickiewicz University, Słoneczna 36, 60-286 Poznań, Poland
¹⁴ Astronomical Observatory, Cluj-Napoca Branch, Astronomical Institute of the Romanian Academy, Cluj-Napoca, Romania
¹⁵ Astronomical Institute of the Romanian Academy, 5 Cutitul de Argint Street, 040557 Bucharest, Romania
¹⁶ Université de Liège, Space Sciences, Technologies and Astrophysics Research Institute (STAR), COMETA, Belgium
¹⁷ Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Uccle, Belgium
¹⁸ naXys, Department of Mathematics, University of Namur, Rue de Bruxelles 61, Namur 5000, Belgium
¹⁹ Observatório Nacional/MCTI, R. General José Cristino 77, CEP 20921-400 Rio de Janeiro – RJ, Brazil
²⁰ UNESP-São Paulo State University, Grupo de Dinâmica Orbital e Planetologia, CEP 12516-410, Guaratinguetá, SP, Brazil
²¹ University Observatory Munich, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
²² National Research Institute of Astronomy and Geophysics (NRIAG), 11421 Helwan, Cairo, Egypt
²³ Črni Vrh Observatory, Predgriže 29A, 5274 Črni Vrh nad Idrijo, Slovenia
²⁴ University of Ljubljana, Faculty of Mathematics and Physics, Jadranska 19, 1000 Ljubljana, Slovenia
²⁵ Dark Sky Slovenia, Savlje 89, 1000 Ljubljana, Slovenia
²⁶ Société Astronomique de Liège, Belgium
²⁷ Department of Physical Geography and Geoecology, Faculty of Science, Charles University, Prague, Czech Republic
²⁸ Department of Spatial Ecology, Landscape Research Institute, Pruhonice, Czech Republic
²⁹ TÜRKSAT Satellite Communication, Cable TV and Operation Inc., Gölbaşı, Ankara, Türkiye
³⁰ Department of Astronomy and Space Sciences, Faculty of Science, Istanbul University, 34116 Istanbul, Türkiye
³¹ Istanbul University Observatory Research and Application Centre, 34116 Istanbul, Türkiye
³² ISTEK Belde Observatory, Istanbul, Türkiye
³³ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly Thege 15-17, 1121 Budapest, Hungary
³⁴ CSFK, MTA Centre of Excellence, Budapest, Konkoly Thege 15–17, Budapest 1121, Hungary
³⁵ ELTE Eötvös Loránd University, Institute of Physics and Astronomy, Budapest, Hungary
³⁶ Czech Astronomical Society, IOTA-ES
³⁷ Institute of Space Science – INFLPR Subsidiary, Măgurele, Romania
³⁸ Section of Astrophysics, Astronomy and Mechanics, Department of Physics, National and Kapodistrian University of Athens, 15784 Zografos, Athens, Greece
³⁹ Starhoper Observatory, Romania
⁴⁰ Dipartimento di Fisica e Astronomia “Galileo Galilei” – Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁴¹ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴² Observatory Rokycany and Pilsen, Czech Republic
⁴³ Department of Physics, Adiyaman University, Adiyaman 02040, Türkiye
⁴⁴ Department of Physics, The George Washington University, Washington, DC 20052, USA
⁴⁵ University of National Education Commission, Krakow, Poland
⁴⁶ Fondazione GAL Hassin – Centro Internazionale per le Scienze Astronomiche, Via della Fontana Mitri, 90010 Isnello, Italy
⁴⁷ Montarrenti Observatory, Str. di Montarrenti, 2, 53018 Sovicille, Siena, Italy
⁴⁸ Astronomical Observatory, Department of Physical Sciences, Earth and Environment, University of Siena, Via Roma 56, 53100 Siena, Italy
⁴⁹ Department of Mechatronics Engineering, Faculty of Engineering and Natural Sciences, Istanbul Health and Technology University, 34445 Istanbul, Türkiye
⁵⁰ Space Science and Solar Energy Research and Application Center (UZAYMER), University of Çukurova, Adana 01330, Türkiye
⁵¹ Yüregir Science Center Adana 01260, Türkiye
⁵² Mesopotamia Astronomy Association, Batman 72040, Türkiye
⁵³ International Occultation Timing Association/European Section, Am Brombeerhag 13, 30459 Hannover, Germany
⁵⁴ UAI – Unione Astrofili Italiani, GAMP – Gruppo Astrofili Montagna Pistoiese, Italy
⁵⁵ Department of Astronomy, Faculty of Mathematics, University of Belgrade, Serbia
⁵⁶ Astronomical Observatory of Belgrade, Serbia
⁵⁷ Vasile Lucaciu National College, Baia Mare, Romania
⁵⁸ Eskişehir Technical University, Astrophysics Education and Research Unit, Eskişehir, Türkiye
⁵⁹ Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 251 65 Ondřejov, Czech Republic
⁶⁰ Associazione Astronomica Antares APS, Italy
⁶¹ Osservatorio Astronomico di Monte Agliale, Via Cune Motrone, 55023 Borgo a Mozzano, Italy
⁶² ELTE Eötvös Loránd University, Gothard Astrophysical Observatory, 9700 Szombathely, Szent I. h. u 112, Hungary
⁶³ Astronomical Institute, Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovakia
⁶⁴ Valasské Meziříčí Observatory, Czech Republic
⁶⁵ Bavarian Public Observatory, Munich, Germany
⁶⁶ Astroclub Radebeul e.V., Radebeul, Germany
⁶⁷ Harpoint Observatory, Harpoint, Austria
⁶⁸ EUR ING, Hviezdoslavova 1971, 022 01 Cadca, Slovakia
⁶⁹ Ondokuz Mayis University Observatory, Kurupelit Campus, 55139 Atakum, Samsun, Türkiye

^★ Corresponding author: ortiz@iaa.es

Received: 18 July 2025
Accepted: 16 September 2025

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 SM₅₅ 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 SM₅₅ 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 SM₅₅ 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 SM₅₅ during the occultation: an ellipse with semi-axes (104.3 ± 0.4) × (83.5 ± 0.5) km. The area-equivalent diameter (D_eq,A) 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_V) 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_V) of 0.80 ± 0.04 for 1995 SM₅₅. This value is remarkably high for a TNO and somewhat higher than that of Haumea, but consistent with the concept that 1995 SM₅₅ is a member of the orbital cluster of Haumea.