Confirming the oblique rotator model for the extremely slowly rotating O8f?p star HD 108

Abstract: The O8f?p star HD 108 is implied to have experienced the most extreme rotational braking of any magnetic, massive star, with a rotational period $P_{\rm rot}$ of at least 55 years, but the upper limit on its spindown timescale is over twice the age estimated from the Hertzsprung-Russell diagram. HD 108's observed X-ray luminosity is also much higher than predicted by the XADM model, a unique discrepancy amongst magnetic O-type stars. Previously reported magnetic data cover only a small fraction ($\sim$3.5\%) of $P_{\rm rot}$, and were furthermore acquired when the star was in a photometric and spectroscopic `low state' at which the longitudinal magnetic field $\langle B_z\rangle$~was likely at a minimum. We have obtained a new ESPaDOnS magnetic measurement of HD 108, 6 years after the last reported measurement. The star is returning to a spectroscopic high state, although its emission lines are still below their maximum observed strength, consistent with the proposed 55-year period. We measured $\langle B_z\rangle=-325 \pm 45$ G, twice the strength of the 2007-2009 observations, raising the lower limit of the dipole surface magnetic field strength to $B_{\rm d} \ge 1$ kG. The simultaneous increase in $\langle B_z\rangle$~and emission strength is consistent with the oblique rotator model. Extrapolation of the $\langle B_z\rangle$~maximum via comparison of HD 108's spectroscopic and magnetic data with the similar Of?p star HD 191612 suggests that $B_{\rm d} > 2$~kG, yielding $t_{\rm S, max}<3$~Myr, compatible with the stellar age. These results also yield a better agreement between the observed X-ray luminosity and that predicted by the XADM model.