Extragalactic ⁸⁵Rb/⁸⁷Rb and ⁶Li/⁷Li ratios in the Small Magellanic Cloud

¹ INAF-OAT, Via G.B.Tiepolo 11, Trieste 34143, Italy
² Institute of Fundamental Physics of the Universe, IFPU, Via Beirut, 2, Trieste 34151, Italy
³ GEPI, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France
⁴ Department of Physics and Astronomy, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556 Indiana, USA

Received: 7 February 2024
Accepted: 3 July 2024

Abstract

Aims. The line of sight toward Sk 143 (AzV 456), an O9.5 Ib star in the Small Magellanic Cloud (SMC), shows significant absorption from neutral atoms and molecules. We report a new study of this line of sight by means of high-resolution spectra obtained with the ESPRESSO spectrograph at the VLT of ESO.

Methods. The absorption from neutral and ionized species is well characterized by a single component at v_hel ≈ +132 km s⁻¹ that was modeled with the ASTROCOOK code.

Results. The rubidium Rb I 780.0 nm line is detected for the first time outside the Galaxy, and we derive [Rb/H] = −1.86 ± 0.09. As a result of the high resolution, the ⁸⁵Rb and ⁸⁷Rb isotope lines are also exceptionally well resolved. The ⁸⁵Rb/⁸⁷Rb isotope ratio is 0.46, which is opposite of the meteoritic value of 2.43. This implies that Rb is made through a dominant contribution of the r-process, which is dominant for the ⁸⁷Rb isotope. We also confirm the presence of ⁷Li I 670.7 nm and set a limit on the isotopic ratio of ⁶Li/⁷Li < 0.1.

Conclusions. The dominance of the ⁸⁷Rb isotope implies that Rb is made through a dominant contribution of the r-process. At the low metallicity of the cloud of [Zn/H] = −1.28 ± 0.09, neutron rich material may have occurred in rotating metal-poor massive stars. Moreover, the low metallicity of the cloud leads to an absolute Li abundance of A(⁷Li) ≈ 2.2, which differs from the expectation from big bang nucleosynthesis. Because the gas-phase abundance is not affected by stellar depletion, the burning of Li inside the halo stars is probably not the solution for the cosmological ⁷Li problem.