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

Abstract

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. The absorption from neutral and ionized species is well characterized by a single component at v$_ hel $ approx +132 that was modeled with the ASTROCOOK code. The rubidium Rb I 780.0 nm line is detected for the first time outside the Galaxy, and we derive Rb/H = -1.86 pm 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 670.7 nm and set a limit on the isotopic ratio of $^6$Li/$^7Li$ < 0.1. 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 pm 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($^7$Li) approx 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 $^7$Li problem.