The CARMENES search for exoplanets around M dwarfs: Rubidium abundances in nearby cool stars

DOI: 
10.1051/0004-6361/202039032
Publication date: 
20/11/2020
Main author: 
Abia C.
IAA authors: 
Amado, P.J.;Rodríguez López, C.
Authors: 
Abia C., Tabernero H.M., Korotin S.A., Montes D., Marfil E., Caballero J.A., Straniero O., Prantzos N., Ribas I., Reiners A., Quirrenbach A., Amado P.J., Béjar V.J.S., Cortés-Contreras M., Dreizler S., Henning T., Jeffers S.V., Kaminski A., Kürster M., Lafarga M., López-Gallifa Á., Morales J.C., Nagel E., Passegger V.M., Pedraz S., Rodríguez López C., Schweitzer A., Zechmeister M.
Journal: 
Astronomy and Astrophysics
Refereed: 
Yes
Publication type: 
Article
Volume: 
642.0
Pages: 
A227
Number: 
A227
Abstract: 
Due to their ubiquity and very long main-sequence lifetimes, abundance determinations in M dwarfs provide a powerful and alternative tool to GK dwarfs to study the formation and chemical enrichment history of our Galaxy. In this study, abundances of the neutron-capture elements Rb, Sr, and Zr are derived, for the first time, in a sample of nearby M dwarfs. We focus on stars in the metallicity range-0.5 ≲ [Fe/H] ≲ +0.3, an interval poorly explored for Rb abundances in previous analyses. To do this we use high-resolution, high-signal-to-noise-ratio, optical and near-infrared spectra of 57 M dwarfs observed with CARMENES. The resulting [Sr/Fe] and [Zr/Fe] ratios for most M dwarfs are almost constant at about the solar value, and are identical to those found in GK dwarfs of the same metallicity. However, for Rb we find systematic underabundances ([Rb/Fe] < 0.0) by a factor two on average. Furthermore, a tendency is found for Rb-but not for other heavy elements (Sr, Zr)-to increase with increasing metallicity such that [Rb/Fe] ≳ 0.0 is attained at metallicities higher than solar. These are surprising results, never seen for any other heavy element, and are difficult to understand within the formulation of the s-and r-processes, both contributing sources to the Galactic Rb abundance. We discuss the reliability of these findings for Rb in terms of non-LTE (local thermodynamic equilibrium) effects, stellar activity, or an anomalous Rb abundance in the Solar System, but no explanation is found. We then interpret the full observed [Rb/Fe] versus [Fe/H] trend within the framework of theoretical predictions from state-of-the-art chemical evolution models for heavy elements, but a simple interpretation is not found either. In particular, the possible secondary behaviour of the [Rb/Fe] ratio at super-solar metallicities would require a much larger production of Rb than currently predicted in AGB stars through the s-process without overproducing Sr and Zr. © 2020 ESO.
Database: 
SCOPUS
ADS
URL: 
https://ui.adsabs.harvard.edu/#abs/2020A&A...642A.227A/abstract
ADS Bibcode: 
2020A&A...642A.227A
Keywords: 
Nuclear reactions, nucleosynthesis, abundances; Stars: abundances; Stars: late-type