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Recent spectropolarimetric observations of otherwise ordinary G, K, and Mgiants revealed localized magnetic strips in the HRD coincident with theregions where the first dredge-up and core He-burning occur. We seek tounderstand the origin of magnetic fields in such late-type giant stars. Inanalogy with late-type dwarf stars, we focus primarily on parameters known toinfluence the generation of magnetic fields in the outer convective envelope.We compute the classical dynamo parameters along the evolutionary tracks oflow- and intermediate-mass stars at various metallicities using stellar modelsthat have been extensively tested by spectroscopic and asteroseismicobservations. These include convective turnover timescales and convectiveRossby numbers, computed from the PMS to the tip of the RGB or the early AGB.To investigate the effects of the very extended outer convective envelope, wecompute these parameters both for the entire convective envelope and locally,that is, at different depths within the envelope. We also compute the turnovertimescales and corresponding Rossby numbers for the convective cores ofintermediate-mass stars on the main sequence.  Our models show that the Rossby number of the convective envelope becomeslower than unity in the well-delimited locations of the Hertzsprung-Russelldiagram where magnetic fields have indeed been detected. We show thatalpha-omega dynamo processes might not be continuously operating, but that theyare favored in the stellar convective envelope at two specific moments alongthe evolution tracks, that is, during the first dredge-up at the base of theRGB and during central helium burning in the helium-burning phase andearly-AGB. This general behavior can explain the so-called magnetic stripsrecently discovered by dedicated spectropolarimetric surveys of evolved stars.

astronomy and astrophysics

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