10/23/2015

# Jim Fuller$$^\star$$, 1 California Institute of Technology & Kavli Institute for Theoretical Physics Matteo Cantiello$$^\star$$, 2 Kavli Institute for Theoretical Physics & UCSB Dennis Stello, University of Sydney Rafael A. García, Service d’Astrophysique, IRFU/DSM/CEA Saclay Lars Bildsten Kavli Institute for Theoretical Physics & UCSB $$\star$$ The first and second authors contributed equally to this work jfuller@caltech.edu↩ matteo@kitp.ucsb.edu↩

Abstract

This is the author’s version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science on Vol. 350 no. 6259 pp. 423-426 , DOI: 10.1126/science.aac6933

Internal stellar magnetic fields are inaccessible to direct observations and little is known about their amplitude, geometry and evolution. We demonstrate that strong magnetic fields in the cores of red giant stars can be identified with asteroseismology. The fields can manifest themselves via depressed dipole stellar oscillation modes, which arises from a magnetic greenhouse effect that scatters and traps oscillation mode energy within the core of the star. The Kepler satellite has observed a few dozen red giants with depressed dipole modes which we interpret as stars with strongly magnetized cores. We find field strengths larger than $$\sim\! 10^5 \,{\rm G}$$ may produce the observed depression, and in one case we infer a minimum core field strength of $$\approx \! \! 10^7 \,{\rm G}$$.

# Main Text

\label{main}

Despite rapid progress in the discovery and characterization of magnetic fields at the surfaces of stars, very little is known about internal stellar magnetic fields. This has prevented the development of a coherent picture of stellar magnetism and the evolution of magnetic fields within stellar interiors.

After exhausting hydrogen in their cores, most main sequence stars evolve up the red giant branch (RGB). During this phase, the stellar structure is characterized by an expanding convective envelope and a contracting radiative core. Acoustic waves (p modes) in the envelope can couple to gravity waves (g modes) in the core (Bedding 2014). Consequently, non-radial stellar oscillation modes become mixed modes that probe both the envelope (the p mode cavity) and the core (the g mode cavity), as illustrated in Fig. \ref{fig:cartoon}. Mixed modes (Beck 2011) have made it possible to distinguish between hydrogen and helium-burning red giants (Bedding 2011, Mosser 2014) and have been used to measure the rotation rate of red giant cores (Beck 2012, Mosser 2012).

A group of red giants with depressed dipole modes were identified using Kepler observations (Mosser 2012a), see also Fig. \ref{fig:moneyplot}. These stars show normal radial modes (spherical harmonic degree $$\ell=0$$), but exhibit dipole ($$\ell=1$$) modes whose amplitude is much lower than usual. Until now, the suppression mechanism was unknown (García 2014). Below, we demonstrate that dipole mode suppression may result from strong magnetic fields within the cores of these red giants.

\label{fig:cartoon} Wave propagation in red giants with magnetized cores. Acoustic waves excited in the envelope couple to gravity waves in the radiative core. In the presence of a magnetic field in the core, the gravity waves are scattered at regions of high field strength. Since the field cannot be spherically symmetric, the waves are scattered to high angular degree $$\ell$$ and become trapped within the core where they eventually dissipate (dashed wave with arrow). We refer to this as the magnetic greenhouse effect.