Magnetic field morphology of Orion IRc2 from 86 GHz SiO maser polarization images.
PLAMBECK R.L., WRIGHT M.C.H. and RAO R.
Abstract (from CDS):
In an attempt to probe the magnetic field morphology near the massive young star Orion IRc2, we mapped the linear polarization of its J=2-1 SiO masers, in both the v=0 and v=1 vibrational levels, with 0".5 resolution. The intense v=1 masers are confined to a narrow zone 40 AU from the star. Their polarization position angles vary significantly on timescales of years. For the v=1 masers the stimulated emission rate R is likely to exceed the Zeeman splitting gΩ caused by any plausible magnetic field; in this case, the maser polarization need not correlate with the field direction. The much weaker v=0 masers in the ground vibrational level lie 100-700 AU from IRc2, in what appears to be a flared disk. Their fractional polarizations are as high as 50%. The polarization position angles vary little across the line profile or the emission region and appear to be stable in time. The position angle (P.A.=80°) we measure for the J=2-1 masers differs by 70° from that measured for the J=1-0 SiO transition, possibly because of Faraday rotation in the foreground, Orion A, H II region. A rotation measure 3.3x104 rad/m2 is required to bring the J=2-1 and J=1-0 position angles into concordance. The intrinsic polarization position angle for both transitions is then 57°, parallel to the plane of the putative disk. The magnetic field probably threads the disk poloidally. There is little evidence for a pinched or twisted field near the star.