Unveiling the nature of HE 0107–5240: a long period binary CEMP-no star with [Fe/H] of –5.56^★

¹ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92190 Meudon, France
² INAF-OATS, Via G.B.Tiepolo 11, Trieste 34143, Italy
³ Leibniz-Institut für Astrophysik (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Institute for Fundamental Physics of the Universe (IFPU), Via Beirut 2, I34151 Grignano, Trieste, Italy
⁵ Zentrum für Astronomie der Universität Heidelberg, Landesstern-warte, Königstuhl 12, 69117 Heidelberg, Germany
⁶ Instituto de Astrofísica de Canarias, Vía Láctea, 38205 La Laguna, Tenerife, Spain
⁷ Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁸ Centro de Astrobiología (CSIC-INTA), Carretera Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁹ Universidad Andres Bello, Facultad de Ciencias Exactas, Departa-mento de Física y Astronomía – Instituto de Astrofísica, Autopista Concepción-Talcahuano 7100, Talcahuano, Chile
¹⁰ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Roma (INAF – OAR), Via Frascati 33, 00040 Monteporzio Catone, Italy
¹¹ Istituto Nazionale di Fisica Nucleare – Sezione di Perugia (INFN), via A. Pascoli s/n, 06125 Perugia, Italy
¹² Kavli Institute for the Physics and Mathematics of the Universe, Todai Institutes for Advanced Study, University of Tokyo, Kashiwa, 277-8583 (Kavli IPMU, WPI), Japan
¹³ Istituto Nazionale di Astrofisica – Istituto di Astrofisica e Planetolo-gia Spaziali (INAF – IAPS), Via Fosso del Cavaliere 100, 00133, Roma, Italy
¹⁴ Monash Centre for Astrophysics (MoCA), School of Mathematical Sciences, Monash University, Victoria 3800, Australia
¹⁵ Dipartimento di Fisica, Sapienza Università di Roma, P.le A. Moro 5, Roma 00185, Italy
¹⁶ Istituto Nazionale di Fisica Nucleare – Laboratori Nazionali del Sud (INFN – LNS), Via Santa Sofia 62, Catania, Italy
¹⁷ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly Thege Miklós út 15–17, 1121 Budapest, Hungary
¹⁸ CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15–17, 1121 Budapest, Hungary
¹⁹ Goethe University Frankfurt, Institute for Applied Physics (IAP), Max-von-Laue-Str. 12, 60438 Frankfurt am Main, Germany
²⁰ UPJV, Université de Picardie Jules Verne, 33 rue St Leu, 80080 Amiens, France
²¹ European Southern Observatory, Casilla 19001, Santiago, Chile
²² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
²³ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
²⁴ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
²⁵ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciên-cias da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
²⁶ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
²⁷ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51, Sauverny 1290, Switzerland

^★★ Corresponding author.

Received: 18 August 2025
Accepted: 9 October 2025

Abstract

Context. The vast majority of the most iron-poor stars in the Galaxy exhibit strong carbon enhancement, with C/H ratios about two orders of magnitude below solar. This unusual chemical composition likely reflects the properties of the gas cloud from which these stars formed, having been enriched by one, or at most a few, supernovae. A remarkable member of this stellar class, HE 0107 −5240 with [Fe/H]=–5.56, has been identified as part of a binary system.

Aims. We aim to constrain the orbital parameters of HE 0107−5240 through radial velocity monitoring with the ESPRESSO spectrograph.

Methods. We derived radial velocities using cross-correlation with a template, taking advantage of the strong G-band feature. We combined all observations to obtain a high signal-to-noise ratio spectrum, which we used to refine our understanding of the stellar chemical composition. We additionally used a co-added UVES spectrum in the blue to complement the wavelength coverage of ESPRESSO.

Results. Observations of HE 0107−5240 over a span of more than four years yield a revised orbital period of about 29 years. We provide updated elemental abundances for Sc, Cr, Co, and, tentatively, Al, along with a new upper limit for Be. We derive the iron abundance from ionised Fe lines and establish significant upper limits for Li, Si, and Sr.

Conclusions. We confirm that the star is a long-period binary. Iron abundances derived from neutral and ionised lines are consistent with the local thermodynamical equilibrium (LTE) assumption, which casts doubt on published deviations from LTE corrections for Fe in this star. The heavy elements Sr and Ba remain undetected, confirming the classification of HE 0107−5240 as a carbon-enhanced, metal-poor star not enhanced in heavy elements (CEMP-no), and supporting the absence of an n-capture element plateau at the lowest metallicities.