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

• Published in: Astronomy and astrophysics (Berlin) 2023-11, Vol.679, p.A72
• Main Authors: Molaro, P., Aguado, D. S., Caffau, E., Allende Prieto, C., Bonifacio, P., González Hernández, J. I., Rebolo, R., Zapatero Osorio, M. R., Cristiani, S., Pepe, F., Santos, N. C., Alibert, Y., Cupani, G., Di Marcantonio, P., D’Odorico, V., Lovis, C., Martins, C. J. A. P., Milaković, D., Murphy, M. T., Nunes, N. J., Schmidt, T. M., Sousa, S., Sozzetti, A., Suárez Mascareño, A.
• Format: Article
• Language: English
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202347676

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 12 C/ 13 C 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 12 C/ 13 C 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 < 12 C/ 13 C < 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 13 C at the dawn of chemical evolution. CEMP-no dwarf stars with slightly higher metallicities show lower isotopic values,