The magnetic strip(s) in the advanced phases of stellar evolution

Theoretical convective turnover timescale and Rossby number for low- and intermediate-mass stars up to the AGB at various metallicities^⋆

¹ Department of Astronomy, University of Geneva, Chemin des Maillettes 51, 1290 Versoix, Switzerland
e-mail: Corinne.Charbonnel@unige.ch
² IRAP, UMR 5277, CNRS and Université de Toulouse, 14 av. E. Belin, 31400 Toulouse, France
³ INAF–Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio, Italy
⁴ Institut Utinam, CNRS UMR 6213, Univ. Bourgogne Franche-Comté, OSU THETA Franche-Comté-Bourgogne, Observatoire de Besançon, BP 1615, 25010 Besançon Cedex, France
⁵ LUPM, UMR5 299, Université de Montpellier, CNRS, place Eugène Bataillon, 34095 Montpellier, France
⁶ IRAP, UMR 5277, CNRS and Université de Toulouse, 57 av. d’Azereix, BP 826, 65008 Tarbes Cedex, France
⁷ Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 72 Tsarigradsko shose, 1784 Sofia, Bulgaria
⁸ Laboratoire AIM Paris-Saclay, CEA/Irfu Université Paris-Diderot CNRS/INSU, 91191 Gif-sur-Yvette, France
⁹ Physics and Astronomy Department, Johns Hopkins University, Baltimore, MD 21218, USA

Received: 12 June 2015
Accepted: 23 March 2017

Abstract

Context. Recent spectropolarimetric observations of otherwise ordinary (in terms e.g. of surface rotation and chemical properties) G, K, and M giants have revealed localized magnetic strips in the Hertzsprung-Russell diagram coincident with the regions where the first dredge-up and core helium burning occur.

Aims. We seek to understand the origin of magnetic fields in such late-type giant stars, which is currently unexplained. In analogy with late-type dwarf stars, we focus primarily on parameters known to influence the generation of magnetic fields in the outer convective envelope.

Methods. We compute the classical dynamo parameters along the evolutionary tracks of low- and intermediate-mass stars at various metallicities using stellar models that have been extensively tested by spectroscopic and asteroseismic observations. Specifically, these include convective turnover timescales and convective Rossby numbers, computed from the pre-main sequence (PMS) to the tip of the red giant branch (RGB) or the early asymptotic giant branch (AGB) phase. To investigate the effects of the very extended outer convective envelope, we compute these parameters both for the entire convective envelope and locally, that is, at different depths within the envelope. We also compute the turnover timescales and corresponding Rossby numbers for the convective cores of intermediate-mass stars on the main sequence.

Results. Our models show that the Rossby number of the convective envelope becomes lower than unity in the well-delimited locations of the Hertzsprung-Russell diagram where magnetic fields have indeed been detected.

Conclusions. We show that α − Ω dynamo processes might not be continuously operating, but that they are favored in the stellar convective envelope at two specific moments along the evolution tracks, that is, during the first dredge-up at the base of the RGB and during central helium burning in the helium-burning phase and early-AGB. This general behavior can explain the so-called magnetic strips recently discovered by dedicated spectropolarimetric surveys of evolved stars.