Magnetic field measurements of O stars with FORS1 at the VLT.pdf

a r X i v : 0 8 0 8 .2 0 3 9 v 1 [ a s t r o - p h ] 1 4 A u g 2 0 0 8 Astronomy Astrophysics manuscript no. ostars c? ESO 2008 August 14, 2008 Magnetic field measurements of O stars with FORS1 at the VLT? S. Hubrig1, M. Scho?ller1, R.S. Schnerr2,3, J. F. Gonza?lez4, R. Ignace5, and H. F. Henrichs2 1 European Southern Observatory, Casilla 19001, Santiago 19, Chile 2 Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands 3 SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands 4 Complejo Astrono?mico El Leoncito, Casilla 467, 5400 San Juan, Argentina 5 Department of Physics, Astronomy, Geology, East Tennessee State University, Johnson City, TN 37614, USA Received date / Accepted date ABSTRACT Context. The presence of magnetic fields in O-type stars has been suspected for a long time. The discovery of such fields would explain a wide range of well documented enigmatic phenomena in massive stars, in particular cyclical wind variability, Hα emission variations, chemical peculiarity, narrow X-ray emission lines and non-thermal radio/X-ray emission. Aims. To investigate the incidence of magnetic fields in O stars, we acquired 38 new spectropolarimetric observations with FORS1 (FOcal Reducer low dispersion Spectrograph) mounted on the 8-m Kueyen telescope of the VLT. Methods. Spectropolarimetric observations have been obtained at different phases for a sample of 13 O stars. 10 stars were observed in the spectral range 348–589 nm, HD36879 and HD148937 were observed in the spectral region 325–621 nm, and HD155806 was observed in both settings. To prove the feasibility of the FORS1 spectropolarimetric mode for the measurements of magnetic fields in hot stars, we present in addition 12 FORS1 observations of the mean longitudinal magnetic field in θ1 OriC and compare them with measurements obtained with the MuSiCoS, ESPaDOnS and Narval spectropolarimeters. Results.