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  • A prevalence of dynamo-generated magnetic fields in the cores of intermediate-mass stars

    Dennis Stello, 1 University of Sydney
    Matteo Cantiello, 2 Kavli Institute for Theoretical Physics & UCSB
    Jim Fuller, 3 California Institute of Technology & Kavli Institute for Theoretical Physics
    Daniel Huber University of Sydney
    Rafael A. García, Service d’Astrophysique, IRFU/DSM/CEA Saclay
    Timothy R. Bedding, University of Sydney
    Lars Bildsten Kavli Institute for Theoretical Physics & UCSB
    Victor Silva Aguirre Department of Physics and Astronomy, Aarhus University


    1. dennis.stello@sydney.edu.au

    2. matteo@kitp.ucsb.edu

    3. jfuller@caltech.edu

    Abstract

    This is the author’s version of the work. It is posted here for personal use, not for redistribution. The definitive version was published in Nature on 04 January 2016, DOI:10.1038/nature16171

    Magnetic fields play a role in almost all stages of stellar evolution (Landstreet, 1992). Most low-mass stars, including the Sun, show surface fields that are generated by dynamo processes in their convective envelopes (Parker, 1955; Donati et al., 2009). Intermediate-mass stars do not have deep convective envelopes (Kippenhahn et al., 1990), although 10% exhibit strong surface fields that are presumed to be residuals from the stellar formation process (Power et al., 2008). These stars do have convective cores that might produce internal magnetic fields (Brun et al., 2005), and these might even survive into later stages of stellar evolution, but information has been limited by our inability to measure the fields below the stellar surface (Aurière et al., 2015). Here we use asteroseismology to study the occurrence of strong magnetic fields in the cores of low- and intermediate-mass stars. We have measured the strength of dipolar oscillation modes, which can be suppressed by a strong magnetic field in the core (Fuller et al., 2015), in over 3,600 red giant stars observed by Kepler. About 20% of our sample show mode suppression but this fraction is a strong function of mass. Strong core fields only occur in red giants above 1.1 solar masses (1.1\(\mathrm{M}_\odot\)), and the occurrence rate is at least 60% for intermediate-mass stars (1.6–2.0\(\mathrm{M}_\odot\)), indicating that powerful dynamos were very common in the convective cores of these stars.

    Main

    Red giants are formed when a low- or intermediate-mass star has finished burning the hydrogen in its core. This leaves an inert helium core surrounded by a thin hydrogen-burning shell and a very thick outer convective envelope. Like the Sun, red giants oscillate in a broad comb-like frequency spectrum of radial and non-radial acoustic modes that are excited by the turbulent surface convection (Ridder et al., 2009). The observed power spectrum has a roughly Gaussian envelope whose central frequency, \(\nu_\mathrm{max}\), decreases as a star expands during the red giant phase (Stello et al., 2008). The comb structure of the spectrum arises from a series of overtone modes separated by the so-called large frequency separation, \(\Delta_{\nu}\). One of these overtone sequences is seen for each spherical degree, \(\ell\). For observations of unresolved distant stars, geometric cancellation prevents detection of modes with \(\ell>3\). Their spectra are characterised by a pattern of radial (\(\ell=0\)) and quadrupolar (\(\ell=2\)) modes that form close pairs, interspersed with dipolar (\(\ell=1\)) modes located roughly halfway between successive radial-quadrupolar pairs. The octupolar modes (\(\ell=3\)) are weak or undetectable. The dipolar modes have turned out to be particularly useful probes of internal structure (García et al., 2015). They have been used to distinguish between hydrogen-shell and helium-core burning stars (Bedding et al., 2011; Stello et al., 2013; Mosser et al., 2014) and to measure radial differential rotation (Beck et al., 2011; Mosser et al., 2012). This usefulness arises because each acoustic non-radial mode in the envelope couples to multiple gravity modes in the core, forming several observable mixed modes with frequencies in the vicinity of the acoustic mode (Beck et al., 2011). This coupling is strongest for dipole modes, making them the most useful probes of the core (Dupret et al., 2009).