Letter to the Editor

The pristine nature of SMSS 1605−1443 revealed by ESPRESSO^⋆

¹ Dipartimento di Fisica e Astronomia, Universit á degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
² INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Instituto de Astrofísica de Canarias, Vía Láctea, 38205 La Laguna, Tenerife, Spain
e-mail: aguado@iac.es
⁴ GEPI, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France
⁵ INAF-Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
⁶ Institute of Fundamental Physics of the Universe, Via Beirut 2, Miramare, Trieste, Italy
⁷ Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁸ Consejo Superior de Investigaciones Científicas, 28006 Madrid, Spain
⁹ Centro de Astrobiología (CSIC-INTA), Carretera Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁰ Dŕpartement d’astronomie de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹¹ Instituto de Astrofísica e Ciências do Espaço, CAUP, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹² Departamento de Física e Astronomia, Faculdade de Cióncias, Universidade do Porto, Rua Campo Alegre, 4169-007 Porto, Portugal
¹³ Scuola Normale Superiore, P.zza dei Cavalieri, 7, 56126 Pisa, Italy
¹⁴ Instituto de Astrofísica e Cióncias do Espaço, Faculdade de Cincias da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
¹⁵ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁶ INFN, Sezione di Trieste, Via Valerio 2, 34127 Trieste, Italy
¹⁷ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy

Received: 7 November 2022
Accepted: 22 November 2022

Abstract

Context. SMSS J160540.18−144323.1 is the carbon-enhanced metal-poor (CEMP) star with the lowest iron abundance ever measured, [Fe/H] = −6.2, which was first reported with the SkyMapper telescope. The carbon abundance is A(C)≈6.1 in the low-C band, as the majority of the stars in this metallicity range. Yet, constraining the isotopic ratio of key species, such as carbon, sheds light on the properties and origin of these elusive stars.

Aims. We performed high-resolution observations of SMSS 1605−1443 with the ESPRESSO spectrograph to look for variations in the radial velocity (v_rad) with time. These data have been combined with older MIKE and UVES archival observations to enlarge the temporal baseline. The ¹²C/¹³C isotopic ratio is also studied to explore the possibility of mass transfer from a binary companion.

Methods. A cross-correlation function against a natural template was applied to detect v_rad variability and a spectral synthesis technique was used to derive ¹²C/¹³C in the stellar atmosphere.

Results. We confirm previous indications of binarity in SMSS 1605−1443 and measured a lower limit ¹²C/¹³C >  60 at more than a 3σ confidence level, proving that this system is chemically unmixed and that no mass transfer from the unseen companion has happened so far. Thus, we confirm the CEMP-no nature of SMSS 1605−1443 and show that the pristine chemical composition of the cloud from which it formed is currently imprinted in its stellar atmosphere free of contamination.