Rubidium abundances in solar metallicity stars

Abstract

Context. Rubidium is one of the few elements produced by the neutron capture sand r-processes almost in equally proportions. Recently, a Rb deficiency ([Rb/Fe] < 0.0) by a factor of about two with respect to the Sun has been found in M dwarfs of near solar metallicity. This contrasts with the close to solar [Sr,Zr/Fe] ratios derived in the same stars. This deficiency is difficult to understand from both the observational and nucleosynthesis point of views. Aims. To test the reliability of this Rb deficiency, we study the Rb and Zr abundances in a sample of KM-type giant stars in a similar metallicity range extracted from the AMBRE Project. Methods. We use high resolution and high signal-to-noise spectra to derive Rb and Zr abundances in a sample of 54 bright giant stars with metallicity in the range −0.6 .[Fe/H]. +0.4 dex by spectral synthesis in both local and non-local thermodynamic equilibrium (LTE and NLTE, respectively). The impact of the Zeeman broadening in the profile of the Rb i at λ7800 Å line is also studied. Results. The LTE analysis also results in a Rb deficiency in giant stars although considerably lower than that obtained in M dwarfs. However, when NLTE corrections are done the [Rb/Fe] ratios are very close to solar (average −0.01 ± 0.09 dex) in the full metallicity range studied. This contrasts with the figure found in M dwarfs. The [Zr/Fe] ratios derived are in excellent agreement with those obtained in previous studies in FGK dwarf stars with a similar metallicity. We investigate the effect of gravitational settling and magnetic activity as possible causes of the Rb deficiency found in M dwarfs. While, the former phenomenon has a negligible impact Article number, page 1 of 23 ar X iv :2 10 2. 02 49 8v 1 [ as tr oph .S R ] 4 F eb 2 02 1 C. Abia et al.: Rubidium abundances in solar metallicity stars on the surface Rb abundance, the existence of an average magnetic field with intensity typical of that observed in M dwarfs may result in systematic Rb abundance underestimations if the Zeeman broadening is not considered in the spectral synthesis. This can explain the Rb deficiency in M dwarfs, but not completely. On the other hand, the new [Rb/Fe] and [Rb/Zr] vs. [Fe/H] relationships can be explained when the Rb production by rotating massive stars and low-andintermediate mass stars (these later also producing Zr) are considered, without the need of any deviation from the standard s-process nucleosynthesis in AGB stars as suggested previously.