Discovery of a magnetic field in the O9 sub-giant star HD 57682 by the MiMeS Collaboration

J.H. Grunhut$^{1,2}$, G.A. Wade$^2$, W.L.F. Marcolino$^3$, V. Petit$^{4}$, H.F. Henrichs$^5$, D.H. Cohen$^6$, E. Alecian$^7$, D. Bohlender$^8$, J.-C. Bouret$^3$, O. Kochukhov$^9$, and the MiMeS collaboration

1-Queen's University, Canada; 2-Royal Military College of Canada; 3-LAM-UMR, France; 4- Universit{' e} Laval, Canada; 5-University of Amsterdam; 6-Swarthmore College, USA; 7-LESIA, France; 8-Herzberg Institute of Astrophysics, Canada; 9-Uppsala University, Sweden; 10-GEPI, France; 11-Universit{' e} de Montr{' e}al, Canada; 12-University of Wisconsin, USA

We report the detection of a strong, organised magnetic field in the O9IV star HD~57682, using spectropolarimetric observations obtained with ESPaDOnS at the 3.6-m Canada-France-Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Program. From the fitting of our spectra using NLTE model atmospheres we determined that HD~57682 is a $17^{+19}_{-9}$~M$_{odot}$ star with a radius of $7.0^{+2.4}_{-1.8}$~R$_odot$, and a relatively low mass-loss rate of $1.4^{+3.1}_{-0.95}times10^{-9}$~M$_{odot}$,yr$^{-1}$. The photospheric absorption lines are narrow, and we use the Fourier transform technique to infer $vsin i=15pm3$~km,s$^{-1}$. This $vsin i$ implies a maximum rotational period of 31.5 d, a value qualitatively consistent with the observed variability of the optical absorption and emission lines, as well as the Stokes~$V$ profiles and longitudinal field. Using a Bayesian analysis of the velocity-resolved Stokes~$V$ profiles to infer the magnetic field characteristics, we tentatively derive a dipole field strength of $1680^{+134}_{-356}$~G. The derived field strength and wind characteristics imply a wind that is strongly confined by the magnetic field.

Reference: Accepted for publication in MNRAS Letters
Status: Manuscript has been accepted


Comments: 6 pages, 5 figures