The ¹²C/¹³C isotopic ratio at the dawn of chemical evolution^★

¹ INAF-Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
e-mail: paolo.molaro@inaf.it
² Institute of Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy
³ Instituto de Astrofísica de Canarias, Vía Láctea, 38205 La Laguna, Tenerife, Spain
⁴ GEPI, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France
⁵ Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁶ Consejo Superior de Investigaciones Científicas, C. De Serrano 117, 28006 Madrid, Spain
⁷ Centro de Astrobiología (CSIC-INTA), Carretera Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁸ Département d’astronomie de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁹ Instituto de Astrofísica e Ciências do Espaço, CAUP, Universidade do Porto, Rua das Estrelas, 4150-762, Porto, Portugal
¹⁰ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua Campo Alegre, 4169-007 Porto, Portugal
¹¹ Scuola Normale Superiore P.zza dei Cavalieri 7, 56126 Pisa, Italy
¹² Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
¹³ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁴ INFN, Sezione di Trieste, Via Valerio 2, 34127 Trieste, Italy
¹⁵ INAF - Osservatorio Astrofísico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
¹⁶ Physics Institute of University of Bern, Gesellschafts strasse 6, 3012 Bern, Switzerland
¹⁷ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, 13 John St, Hawthorn, Victoria 3122, Australia

Received: 8 August 2023
Accepted: 20 September 2023

Abstract

Context. The known mega metal-poor (MMP) and hyper metal-poor (HMP) stars, with [Fe/H] < −6.0 and < −5.0, respectively, likely belong to the CEMP-no class, namely, carbon-enhanced stars with little or no second peak neutron-capture elements. They are likely second-generation stars, and the few elements measurable in their atmospheres are used to infer the properties of a single or very few progenitors.

Aims. The high carbon abundance in the CEMP-no stars offers a unique opportunity to measure the carbon isotopic ratio, which directly indicates the presence of mixing between the He- and H-burning layers either within the star or in the progenitor(s). By means of high-resolution spectra acquired with the ESPRESSO spectrograph at the VLT, we aim to derive values for the ¹²C/¹³C ratio at the lowest metallicities.

Methods. We used a spectral synthesis technique based on the SYNTHE code and on ATLAS models within a Markov chain Monte Carlo methodology to derive ¹²C/¹³C in the stellar atmospheres of four of the most metal-poor stars known: the MMP giant SMSS J0313–6708 ([Fe/H] < −7.1), the HMP dwarf HE 1327–2326 ([Fe/H] = −5.8), the HMP giant SDSS J1313–0019 ([Fe/H] = −5.0), and the ultra metal-poor subgiant HE0233 -0343 ([Fe/H] = −4.7). We also revised a previous value for the MMP giant SMSS J1605–1443 ([Fe/H] = −6.2).

Results. In four stars we derive an isotopic value while for HE 1327–2326 we provide a lower limit. All measurements are in the range 39 < ¹²C/¹³C < 100, showing that the He- and H-burning layers underwent partial mixing either in the stars or, more likely, in their progenitors. This provides evidence of a primary production of ¹³C at the dawn of chemical evolution. CEMP-no dwarf stars with slightly higher metallicities show lower isotopic values, <30 and even approaching the CNO cycle equilibrium value. Thus, extant data suggest the presence of a discontinuity in the ¹²C/¹³C ratio at around [Fe/H] ≈ − 4, which could mark a real difference between the progenitor pollution captured by stars with different metallicities. We also note that some MMP and HMP stars with high ¹²C/¹³C show low ⁷Li values, providing an indication that mixing in the CEMP-no progenitors is not responsible for the observed Li depletion.