2017MNRAS.468.3985S

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_rot of at least 55 yr, but the upper limit on its spin-down time-scale 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 X-ray Analytical Dynamical Magnetosphere (XADM) model, a unique discrepancy amongst magnetic O-type stars. Previously reported magnetic data cover only a small fraction (∼3.5 per cent) of P_rot, and were furthermore acquired when the star was in a photometric and spectroscopic 'low state' at which the longitudinal magnetic field <B_z> was likely at a minimum. We have obtained a new ESPaDOnS magnetic measurement of HD 108, 6 yr 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-yr period. We measured <B_z> = -325 ± 45 G, twice the strength of the 2007-2009 observations, raising the lower limit of the dipole surface magnetic field strength to B_d >= 1 kG. The simultaneous increase in <B_z> and emission strength is consistent with the oblique rotator model. Extrapolation of the <B_z> maximum via comparison of HD 108's spectroscopic and magnetic data with the similar Of?p star HD 191612 suggests that B_d > 2 kG, yielding t_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.