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 $P_{\mathrm{rot}}$ 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 $P_{\mathrm{rot}}$, 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 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 $⟨B_z⟩=-325\pm 45$ G, twice the strength of the 2007-2009 observations, raising the lower limit of the dipole surface magnetic field strength to $B_{\mathrm{d}}\ge 1$ 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 $B_{\mathrm{d}}>2$~kG, yielding $t_{\mathrm{S},\max }<3$~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.

9 pages, 7 figures, accepted for publication in MNRAS (MN-17-0243-MJ.R2)

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