IRC - A COOL HYPERGIANT NEAR THE TOP OF THE H-R DIAGRAM(10420 )

New data are reported for the OH/IR star IRC + 10420, including optica l/infrared imaging, spectroscopy, polarimetry, and photometry. We conc lude 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 angstrom. H alpha is strongly in emission and shows a double-peaked profile simila r to the emission seen in stars with rotating equatorial disks. 2. The optical image taken through polarizing filters is elongated, and show s that the star must be intrinsically polarized at a position angle ne ar 90-degrees. The 8.7 mum image is also elongated, but at a position angle near 150-degrees. 3. The optical/infrared polarization is compos ed of a strong interstellar component and a complex intrinsic componen t. The intrinsic position angle in the optical is about 100-degrees, b ut rotates to about 140-degrees in the infrared. 4. Analysis of the in frared 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 l uminosity of the star is M(bol) = -9.6 +/- 0.5 mag (5 kpc), near the t op 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 mum continuum from the mid-1970s to the mid-1980s, whereas the optical energy distributi on and the 10 mum silicate feature have remained largely unchanged ove r the same time period. We propose a model for IRC +10420 in which a t rue core-burning supergiant is surrounded by a rotating equatorial dis k of gas and dust that is viewed from about midway above the plane. Di ssipation of dust near the star is responsible for the drop in the inf rared continuum. The polarization and OH emission are caused by a comp lex 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 tha t is analogous to that of degenerate core giants evolving through the proto-planetary nebula phase, but at a much higher luminosity.