IRC +10420: A cool hypergiant near the top of the H-R diagram

New data are reported for the OH/IR star IRC +10420, including optical/infrared imaging, spectroscopy, polarimetry, and photometry. We conclude the following: 1. The optical spectrum is that of a very luminous F supergiant (F Ia+) with a very strong O I blend at 7774 Å. Hα is strongly in emission and shows a double-peaked profile similar to the emission seen in stars with rotating equatorial disks. 2. The optical image taken through polarizing filters is elongated, and shows that the star must be intrinsically polarized at a position angle near 90°. The 8.7 μm image is also elongated, but at a position angle near 150°. 3. The optical/infrared polarization is composed of a strong interstellar component and a complex intrinsic component. The intrinsic position angle in the optical is about 100°, but rotates to about 140° in the infrared. 4. Analysis of the infrared polarization, the reddening of the optical energy distribution, and the stellar radial velocity determined from the optical spectrum indicates that IRC +10420 is at a distance of 4-6 kpc. The resulting luminosity of the star is M_bol = -9.6 ± 0.5 mag (5 kpc), near the top of the H-R diagram at or just below the Humphreys-Davidson limit. 5. There has been a decline in the flux from the 1-7 μm continuum from the mid-1970s to the mid-1980s, whereas the optical energy distribution and the 10 μm silicate feature have remained largely unchanged over the same time period. We propose a model for IRC +10420 in which a true core-burning supergiant is surrounded by a rotating equatorial disk of gas and dust that is viewed from about midway above the plane. Dissipation of dust near the star is responsible for the drop in the infrared continuum. The polarization and OH emission are caused by a complex distribution of gas and dust that surrounds the star and its disk from an earlier phase of evolution. IRC +10420 may be evolving from a red supergiant to the blue side of the H-R diagram, perhaps to become a Wolf-Rayet star. We argue that it is now in a phase of evolution that is analogous to that of degenerate core giants evolving through the proto-planetary nebula phase, but at a much higher luminosity.