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

AutorAbia, C. CSIC; Tabernero, H.M.; Korotin, S. A.; Montes, D.; Marfil, E.; Caballero, J. A. CSIC ORCID; Straniero, O.; Prantzos, N.; Ribas, Ignasi CSIC ORCID; Reiners, Ansgar; Quirrenbach, Andreas; Amado, Pedro J. CSIC ORCID; Béjar, Victor J. S.; Cortés-Contreras, M. CSIC ORCID; Dreizler, S.; Henning, Thomas; Jeffers, Sandra V.; Kaminski, Adrian; Kürster, M.; Lafarga, M. CSIC ORCID; López-Gallifa, Á.; Morales, Juan Carlos CSIC ORCID; Nagel, Evangelos; Passegger, V.M.; Pedraz, S.; Rodríguez-López, Cristina CSIC ORCID; Schweitzer, Andreas; Zechmeister, Mathias
Palabras claveNuclear reactions
Stars: late-type
Stars: abundances
Nucleosynthesis
Fecha de publicación23-oct-2020
EditorEDP Sciences
CitaciónAstronomy & Astrophysics 642: A227 (2020)
ResumenDue 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.
Versión del editorhttp://dx.doi.org/10.1051/0004-6361/202039032
URIhttp://hdl.handle.net/10261/223731
DOI10.1051/0004-6361/202039032
ISSN0004-6361
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