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Can Extra Mixing in RGB and AGB Stars Be Attributed to Magnetic Mechanisms?

Busso, Maurizio and Wasserburg, Gerald J. and Nollett, Kenneth M. and Calandra, Andrea (2007) Can Extra Mixing in RGB and AGB Stars Be Attributed to Magnetic Mechanisms? Astrophysical Journal, 671 (1). pp. 802-810. ISSN 0004-637X. doi:10.1086/522616.

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It is known that there must be some weak form of transport (called cool bottom processing, or CBP) acting in low-mass red giant branch (RGB) and asymptotic giant branch (AGB) stars, adding nuclei, newly produced near the hydrogen-burning shell, to the convective envelope. We assume that this extra mixing originates in a stellar dynamo operated by the differential rotation below the envelope, maintaining toroidal magnetic fields near the hydrogen-burning shell. We use a phenomenological approach to the buoyancy of magnetic flux tubes, assuming that they induce matter circulation as needed by CBP models. This establishes requirements on the fields necessary to transport material from zones where some nuclear burning takes place, through the radiative layer and into the convective envelope. Magnetic field strengths are determined by the transport rates needed by CBP for the model stellar structure of a star of initially 1.5 M_⊙, in both the AGB and RGB phases. The field required for the AGB star in the processing zone is B_0 ~ 5 × 10^6 G; at the base of the convective envelope this yields an intensity B_E ≾ 10^4 G. For the RGB case, B_0 ~ 5 × 10^4 – 4 × 10^5 G, and the corresponding B_E are ~450-3500 G. These results are consistent with existing observations on AGB stars. They also hint at the basis for high field sources in some planetary nebulae, and the very large fields found in some white dwarfs. It is concluded that transport by magnetic buoyancy should be considered as a possible mechanism for extra mixing through the radiative zone, as is required by both stellar observations and the extensive isotopic data on circumstellar condensates found in meteorites.

Item Type:Article
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URLURL TypeDescription Paper
Wasserburg, Gerald J.0000-0002-7957-8029
Additional Information:© 2007 The American Astronomical Society. Received 2007 May 31; accepted 2007 August 20. The authors would like to thank Hans Hornung for views on the possible values of C_D. M.B. and A. C. acknowledge support from MURST under contract PRIN2006-022731. G. J. W. acknowledges the support of DOE-FG03-88ER13851 and the generosity of the Epsilon foundation and Caltech’s contribution 9178(1123). K. M. N. is supported by the US Department of Energy, Office of Nuclear Physics under contract DE-AC02-06CH11357. The authors would like to thank the reviewer (clearly an expert on MHD) for a most constructive and useful critical review. We would like to dedicate this paper to Gene Parker and Bob Leighton for their pioneering solar activities.
Funding AgencyGrant Number
Ministero per l'Università e per la Ricerca Scientifica e Tecnologica (MURST)PRIN2006-022731
Department of Energy (DOE)FG03-88ER13851
Epsilon FoundationUNSPECIFIED
Department of Energy (DOE) Office of Nuclear PhysicsDE-AC02-06CH11357
Subject Keywords:MHD; stars : AGB and port-AGB; stars : evolution; stellar dynamics
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Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences9178
Lunatic Asylum Lab1123
Issue or Number:1
Record Number:CaltechAUTHORS:20091026-115718450
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Official Citation:Can Extra Mixing in RGB and AGB Stars Be Attributed to Magnetic Mechanisms? Maurizio Busso, Gerald J. Wasserburg, Kenneth M. Nollett, and Andrea Calandra 2007 ApJ 671 802-810 doi: 10.1086/522616
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16478
Deposited By: Tony Diaz
Deposited On:29 Oct 2009 19:46
Last Modified:08 Nov 2021 23:27

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