\label{Fig:Struc}
Structure of the $M \! = \! 1.6 \, M_\odot$ stellar model shown in Fig. \ref{fig:Prop}. ({\bf A}) At this stage of evolution, the star has a $\sim \! 0.2 \, M_\odot$ helium core, surrounded by a $\sim \! 0.1 \, M_\odot$ radiative outer core. The vertical purple band shows the location of the hydrogen-burning shell. The bulk of the mass and radial extent of the star is comprised by the thick convective envelope. ({\bf B}) Core magnetic field. The field strength $B$ is calculated by assuming magnetic flux conservation from a dipole field on the main sequence as described in the supplementary material. We also plot the critical field $B_c$ from Eqn. \ref{eqn:Bc} as in Fig. \ref{fig:Prop}. Because $B \! > \! B_c$ in the core of this stellar model, the magnetic greenhouse effect may occur.