NONSPHERICAL STRUCTURES AND TEMPORAL VARIATIONS IN THE DUST SHELL OF O-CETI OBSERVED WITH A LONG-BASE-LINE INTERFEROMETER AT 11 MICRONS

Visibility observations at 11 mu m of o Ceti have been made with the U niversity of California (Berkeley) Infrared Spatial Interferometer dur ing the time period 1988-1995. The observed visibilities change dra ma tically from one epoch to another and are not consistent with simple h eating or cooling of the dust with change in luminosity as a function of stellar phase. Instead, large temporal variations in the density of dust within a few stellar radii of the photosphere of o Ceti have occ urred. Spherically symmetric models of the dust distribution with two dust shells, one within three stellar radii of the photosphere of the star, the other approximately 10 stellar radii from the star, can acco unt reasonably well for the observed changes. Four types of axially sy mmetric radiative transfer models were also compared with the data-a s pherical shell with an ellipsoidal inner cavity, a disk, a spherical s hell. with one or two inhomogeneities or clumps, and a set of thin par tial shells with a fixed distance between them. Of the four models, on ly the one with the ellipsoidal inner cavity is excluded. The data wer e best-fitted with the last two models, which emphasize inhomogeneitie s or clumps. To fit the observed temporal changes in the visibility da ta, all models must include a change in the density-increasing and dec reasing-of dust close to the photosphere of the star. The axially symm etric models had clumps placed at distances from the star in agreement with distances of the spherical models. Good fits to the observed bro adband spectrum of the star were also obtained with these models.