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 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 _ { 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 _ { d } \geq 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 _ { 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 .