Abstract
We report the detection of a magnetic field on the Of?p star HD108. Spectropolarimetric observations conducted in 2007, 2008 and 2009 respectively with NARVAL@TBL and ESPaDOnS@CFHT reveal a clear Zeeman signature in the average Stokes V profile, stable on timescales of days to months and slowly increasing in amplitude on timescales of years. We speculate that this timescale is the same as that on which Ha emission is varying and is equal to the rotation period of the star. The corresponding longitudinal magnetic field, measured during each of the three seasons, increases slowly from 100 to 150G, implying that the polar strength of the putatively-dipolar large-scale magnetic field of HD108 is at least 0.5kG and most likely of the order of 1-2 kG. The stellar and wind properties are derived through a quantitative spectroscopic analysis with the code CMFGEN. The effective temperature is difficult to constrain because of the unusually strong HeI4471 and HeI5876 lines. Values in the range 33000-37000 K are preferred. A mass loss rate of about 1e-7 Msun/yr (with a clumping factor f=0.01) and a wind terminal velocity of 2000 km/s are derived. The wind confinement parameter eta_star is larger than 100, implying that the wind of HD108 is magnetically confined. Stochastic short-term variability is observed in the wind-sensitive lines but not in the photospheric lines, excluding the presence of pulsations. Material infall in the confined wind is the most likely origin for lines formed in the inner wind. Wind-clumping also probably causes part of the Ha variability. The projected rotational velocity of HD108 is lower than 50 km/s, consistent with the spectroscopic and photometric variation timescales of a few decades. Overall, HD108 is very similar to the magnetic O star HD191612 except for an even slower rotation.
Highlights
Very little is known about magnetic fields of O stars and very few O stars are yet known as magnetic; preliminary results (e.g. Donati et al 2002, 2006; Bouret et al 2008; Petit et al 2008) suggest that most magnetic O stars could be the high-mass equivalent of magnetic chemically peculiar A and B stars with fields of primordial origin, i.e. fossil remnants from the formation stage (Donati & Landstreet 2009)
We have presented spectropolarimetric observations of the Of?p star HD 108 conducted with the NARVAL and ESPaDOnS instruments. 110 circularly polarized spectral sequences have been collected between 2007 and 2009
We report the clear detection of a Stokes V Zeeman signature stable on timescales of days to months, but likely slowly increasing in amplitude on time-scales of years
Summary
Very little is known about magnetic fields of O stars and very few O stars are yet known as magnetic; preliminary results (e.g. Donati et al 2002, 2006; Bouret et al 2008; Petit et al 2008) suggest that most magnetic O stars could be the high-mass equivalent of magnetic chemically peculiar A and B stars (the so-called Ap and Bp stars) with fields of primordial origin, i.e. fossil remnants from the formation stage (Donati & Landstreet 2009). The presence of magnetic fields and the very similar Hα modulation reported for HD 191612 suggests that this star is analogous to θ 1 Ori C (Donati et al 2006), but more evolved and much more slowly rotating (538 d versus 15.4 d for θ 1 Ori C) as a likely result of wind magnetic braking. As θ 1 Ori C and HD 191612, HD 108 shows spectral-type variations (from 04 to 08) due to changes in the strength of its He I lines They (and the Balmer lines too) fluctuate between pure absorption and P Cygni profiles. We discuss briefly the implications of our results for understanding how magnetic fields affect the lives of massive stars
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