VLA OBSERVATIONS OF 1612 AND 1665 MHZ OH MASERS ASSOCIATED WITH IRC+10420

Simultaneous spectral line VLA observations at 1612 and 1665 MHz in right and left circular polarizations have been obtained for the OH masers associated with the highly luminous supergiant IRC +10420. Comparison with previous data indicates that the 1612 MHz emission has increased by at least a factor of 3 at all velocities over the past 14 yr and by a factor of 60 at 55 km s-1. Meanwhile, the 1665 MHz emission has varied erratically, and the 1667 MHz emission has declined by a factor of 3. The unusual temporal behavior is similar to that observed for the Mira variable U Orionis. Most of the 1665 MHz maser components are distributed in a shell with a radius of 1".3, marginally smaller than the value of 1."5 for the main shell at 1612 MHz. However, there are indications of clumps or portions of shells at much smaller and larger radii. At some velocities, the angular structure of the 1612 MHz emission has changed significantly with time. It is possible to parameterize the majority of maser components by an oblate spheroid (axial ratio approximately 1.5) for which the near side of the equatorial plane is tilted almost-equal-to 25-degrees above the line of sight and the outflow velocity increases from almost-equal-to 28 km s-1 in the plane to almost-equal-to 63 km s-1 along the polar axis. The increases in maser intensity from widely separated portions of the shell over a time scale of a few years support a radiative pumping model, while the positions of the masers are consistent with the regions of highest expected OH density. At the radius of the maser shells (approximately 8000 AU), the magnetic field is estimated to be 15 mG greater than or similar to B greater than or similar to 1/6 mG.