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Confirming the oblique rotator model for the extremely slowly rotating O8f?p star HD 108
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy. Queens Univ, Dept Phys Engn Phys & Astron, Canada.;Royal Mil Coll Canada, Dept Phys, Canada..
Royal Mil Coll Canada, Dept Phys, POB 17000 Stn Forces, Kingston, ON K7K 0C6, Canada..
2017 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 468, no 4, p. 3985-3992Article in journal (Refereed) Published
Abstract [en]

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 (similar to 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.

Place, publisher, year, edition, pages
2017. Vol. 468, no 4, p. 3985-3992
Keywords [en]
stars: individual: HD108, stars: massive, stars: rotation, stars: winstars: rotationds, outflows
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-327362DOI: 10.1093/mnras/stx759ISI: 000402819700016OAI: oai:DiVA.org:uu-327362DiVA, id: diva2:1135355
Available from: 2017-08-23 Created: 2017-08-23 Last updated: 2017-08-23Bibliographically approved

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