Deep 10 and 18 micron imaging of the HR 4796A circumstellar disk: Transient dust particles and tentative evidence for a brightness asymmetry

We present new 10.8 and 18.2 mu m images of HR 4796A, a young AO V star tha t was recently discovered to have a spectacular, nearly edge-on, circumstel lar disk prominent at similar to 20 mu m (Jayawardhana and coworkers; Koern er and coworkers). These new images, obtained with OSCIR (the University of Florida Observatory Spectrometer/Camera for the Infrared) at Keck. II, sho w that the disk's size at 10 mu m is comparable to its size at 18 mu m. The refore, the 18 mu m-emitting dust may also emit some, or ail, of the 10 mu m radiation. Using these multiwavelength images, we determine a "characteri stic" diameter of 2-3 mu m for the mid-infrared-emitting dust particles if they are spherical and composed of astronomical silicates. Particles this s mall, are expected to be blown out of the system by radiation pressure in a few hundred years, and therefore these particles are unlikely to be primor dial. Rather, as inferred in a companion paper (Wyatt and coworkers), they are probably products of collisions that dominate both the creation and the destruction of dust in the HR 4796A disk. Dynamical modeling of the disk, the details of which are presented in the companion paper, indicates that t he disk surface density is relatively sharply peaked near 70 AU, which agre es with the mean annular radius deduced by Schneider and coworkers from the ir NICMOS images. Interior to 70 AU, the model density drops steeply by a f actor of 2 between 70 and 60 AU, falling to zero by 45 AU, which correspond s to the edge of the previously discovered central hole; in the context of the dynamical models, this "soft" edge for the central hole occurs because the dust particle orbits are noncircular. The optical depth of mid-infrared -emitting dust in the hole is similar to 3% of the optical depth in the dis k, and the hole is therefore relatively very empty. We present evidence (si milar to 1.8 sigma significance) for a brightness asymmetry that may result from the presence of the hole and the gravitational perturbation of the di sk particle orbits by the low-mass stellar companion or a planet. This "per icenter glow," which must still be confirmed, results from a very small (a few AU) shift of the disk's center of symmetry relative to the central star HR 7496A; one side of the inner boundary of the annulus is shifted toward HR 4796A, thereby becoming warmer and more infrared-emitting. The possible detection of pericenter glow implies that the detection of even complex dyn amical effects of planets on disks is within reach.