Constraining the size, shape, and albedo of the large trans-Neptunian object (28978) Ixion with multi-chord stellar occultations

Context . Trans-Neptunian objects (TNOs) are among the most primitive remnants of the early Solar System. Determining their sizes, shapes, albedos, and surface properties is essential for understanding their origin and evolution. Stellar occultations provide highly accurate size and shape information for TNOs, while photometry constrains their albedo and surface colours. (28978) Ixion is one of the largest TNOs and a prominent Plutino, making it a key target for comparative studies. Aims . The aim of this work is to constrain Ixion’s projected size, shape, absolute magnitude, geometric albedo, and surface colours, and to search for evidence of an atmosphere or circum-object material. Methods . We conducted a series of campaigns targeting stellar occultations by Ixion between 2020 and 2023 as part of the Lucky Star collaboration, gathering 51 observations from eight events, including 30 positive detections. Five multi-chord events were used for a global limb fit, enabling an accurate reconstruction of Ixion’s projected shape. Calibrated photometric data, including new and archival measurements, were analysed to derive its absolute magnitude, phase-curve parameters, and broadband colours. Results . The multi-chord occultations reveal a slightly elongated limb that is well represented by a single projected ellipse with semi-axes a = 363.42 −3.85 +3.53 km and b = 333.98 −4.96 +7.07 km, yielding R equiv = 348.39 −4.43 +5.37 km ( D equiv = 696.78 −8.87 +10.75 km), and an apparent oblateness ε ′ = 0.081 −0.010 +0.004 . The geometry is consistent with a moderately flattened, nearly spheroidal body that may show slight departures from axial symmetry. The typical radial residuals (~10 km) support a largely stable shape across the observed epochs, with modest epoch-dependent variations. The phase-curve fit gives H V = 3.845 ± 0.006, β = 0.1301 ± 0.0078 mag deg −1 , and p V = 0.106 −0.003 +0.003 . Multi-band photometry yields B − V = 1.06 ± 0.03, V − R = 0.61 ± 0.02, and R − I = 0.54 ± 0.03, which is consistent with moderately red TNO surfaces. No atmosphere or circum-object material was detected down to our sensitivity limits. The best-sampled event (13 October 2020) also allowed us to measure the angular diameter of the occulted star Gaia DR3 4056440205544338944, θ ⋆ = 0.670 ± 0.010 mas, which corresponds to R ⋆ = 128 ± 10 R ⊙ at the Gaia distance.

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Journal
Astronomy and Astrophysics
Published
2026-01-20
DOI
https://doi.org/10.1051/0004-6361/202557970
Primary Topic
Astro and Planetary Science
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article
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article

Constraining the size, shape, and albedo of the large trans-Neptunian object (28978) Ixion with multi-chord stellar occultations

A. Wilberger, E. Jehin, K. Green, M. Assafin et al.
Astronomy and Astrophysics
Astro and Planetary Science
article

Constraining the size, shape, and albedo of the large trans-Neptunian object (28978) Ixion with multi-chord stellar occultations

A. Wilberger, E. Jehin, K. Green, M. Assafin, J. Arcas-Silva, P. Santos-Sanz, V. Kouprianov, J. Dunham, J. Spagnotto, O. Erece, J. Bardecker, T. Blank, D. Herald, L. Vanzi, W. Thomas, N. Morales, G. Isopi, C. Snodgrass, M. Meunier, N. Carlson, Y. Kilic, J. Pollock, G. Margoti, M. Collins, R. Duffard, A. Alvarez-Candal, D. Souami, G. Columba, R. Savalle, M. Melita, A. Stechina, L. Liberato, C. Jacques, T. George, F. Mallia, C. Cebral, B. Christophe, A. Guimaraes, L. Bernasconi, J. Desmars, S. Kerr, M. Emilio, A. Gomes-Junior, F. Rommel, C. Magliano, T. Napoleao, N. Castro, R. Sfair, A. Chapman, M. Konishi, F. Braga-Ribas, Y. Olivera-Cuello, V. Moura, B. Sicardy, A. Zapparata, R. Venable, M. Bonavita, R. Dunford, M. Kretlow, G. Benedetti-Rossi, J. Mattei, R. Leiva, M. Malacarne, B. Morgado, W. Hanna, C. Gao, L. Peiro, R. Lynne Jones, M. Kaplan
article en

Abstract

Context . Trans-Neptunian objects (TNOs) are among the most primitive remnants of the early Solar System. Determining their sizes, shapes, albedos, and surface properties is essential for understanding their origin and evolution. Stellar occultations provide highly accurate size and shape information for TNOs, while photometry constrains their albedo and surface colours. (28978) Ixion is one of the largest TNOs and a prominent Plutino, making it a key target for comparative studies. Aims . The aim of this work is to constrain Ixion’s projected size, shape, absolute magnitude, geometric albedo, and surface colours, and to search for evidence of an atmosphere or circum-object material. Methods . We conducted a series of campaigns targeting stellar occultations by Ixion between 2020 and 2023 as part of the Lucky Star collaboration, gathering 51 observations from eight events, including 30 positive detections. Five multi-chord events were used for a global limb fit, enabling an accurate reconstruction of Ixion’s projected shape. Calibrated photometric data, including new and archival measurements, were analysed to derive its absolute magnitude, phase-curve parameters, and broadband colours. Results . The multi-chord occultations reveal a slightly elongated limb that is well represented by a single projected ellipse with semi-axes a = 363.42 −3.85 +3.53 km and b = 333.98 −4.96 +7.07 km, yielding R equiv = 348.39 −4.43 +5.37 km ( D equiv = 696.78 −8.87 +10.75 km), and an apparent oblateness ε ′ = 0.081 −0.010 +0.004 . The geometry is consistent with a moderately flattened, nearly spheroidal body that may show slight departures from axial symmetry. The typical radial residuals (~10 km) support a largely stable shape across the observed epochs, with modest epoch-dependent variations. The phase-curve fit gives H V = 3.845 ± 0.006, β = 0.1301 ± 0.0078 mag deg −1 , and p V = 0.106 −0.003 +0.003 . Multi-band photometry yields B − V = 1.06 ± 0.03, V − R = 0.61 ± 0.02, and R − I = 0.54 ± 0.03, which is consistent with moderately red TNO surfaces. No atmosphere or circum-object material was detected down to our sensitivity limits. The best-sampled event (13 October 2020) also allowed us to measure the angular diameter of the occulted star Gaia DR3 4056440205544338944, θ ⋆ = 0.670 ± 0.010 mas, which corresponds to R ⋆ = 128 ± 10 R ⊙ at the Gaia distance.

Astronomy and AstrophysicsVol. 707
University of Central Florida (US), Universidade Tecnológica Federal do Paraná (BR), Royal Astronomical Society (GB), Centre National de la Recherche Scientifique (FR), Institute of Astronomy and Astrophysics, Academia Sinica (TW), University of Namur (BE), Pontificia Universidad Católica de Chile (CL), Akdeniz University (TR), Türkiye Bilimsel ve Teknolojik Araştırma Kurumu (TR), Université Côte d'Azur (FR), Université Paris Sciences et Lettres (FR), Lowell Observatory (US), Observatoire de Paris (FR), W.M. Keck Observatory (US), Observer Research Foundation (IN), Sorbonne Université (FR), University of New Haven (US), Hospital San Juan de la Cruz (ES), National Solar Observatory (US), Valongo Observatory (BR), Institute of Astrophysics and Space Sciences (PT), Istituto Nazionale di Fisica Nucleare, Sezione di Roma Tre (IT), Max Planck Institute for Astrophysics (DE), Société Géologique de France (FR), Technology Assessment and Transfer (United States) (US), Observatoire de la Côte d’Azur (FR), Asociación por los Derechos Civiles (AR), Feminist Center for Studies and Advisory Services (BR), Institute of Astronomy and Space Physics (AR), Astronomy and Space (AU), Laboratório Interinstitucional de e-Astronomia (BR), Spanish National Observatory (ES), Instituto de Geofísica y Astronomía (CU), Astronomical Observatory (RS), Istituto Nazionale di Fisica Nucleare, Sezione di Roma I (IT), Johnson & Johnson (Netherlands) (NL), Instituto de Astrofísica de Andalucía (ES), Agrupación Astronómica de la Safor (ES), Observatoire des Engarouines (FR), Oldham Council (GB), National Astronomical Observatories (CN), Complejo Asistencial Universitario de Palencia (ES), Network Group (Czechia) (CZ), Appalachian State University (US), LIRA - Laboratoire d'instrumentation et de recherche en astrophysique (FR), Universidade Federal do Espírito Santo (BR), Institut Polytechnique des Sciences Avancées (FR), Universidade Estadual de Ponta Grossa (BR), Universidade Federal de Uberlândia (BR), University of Tübingen (DE), Sapienza University of Rome (IT), National Institute for Astrophysics (IT), Universidade Estadual Paulista (Unesp) (BR), Centro de Estudios Científicos (CL), University of Bologna (IT), University of Edinburgh (GB)
European Space Agency, Agence Nationale de la Recherche, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Fonds De La Recherche Scientifique - FNRS, Consejo Nacional de Investigaciones Científicas y Técnicas, Conselho Nacional de Desenvolvimento Científico e Tecnológico, Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, European Southern Observatory, Fundação de Amparo à Pesquisa do Estado de Minas Gerais, Junta de Andalucía, Instituto de Astrofísica de Andalucía, Agencia Estatal de Investigación
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