Magnetic Field Morphology of Orion IRc2 from 86 GHz SiO Maser Polarization Images

Abstract

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 05 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 3.3 × 104 rad m-2 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.