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R. F. Knacke (Penn State Erie), C.M. Telesco, R. S. Fisher, R. K. Pina (Univ. Florida), S. F. Dermott, M. C. Wyatt, K. Grogan, E. K. Holmes (Univ. Florida), A. M. Ghez, L. Prato (U.C.L.A.), L. W. Hartmann, R. Jayawardhana (Harvard-Smithsonian CFA)
We present new 10.8 and 18.2 micron images of HR 4796A, a young A0V star that was recently discovered to have a spectacular, nearly edge-on, circumstellar disk prominent at ~20 mic. These new images, obtained with OSCIR at Keck II, show that the disk's size at 10 mic is comparable to its size at 18 mic. Therefore, the 18 mic-emitting dust may also emit some, or all, of the 10 mic radiation. Using these multi-wavelength images, we determine a ``characteristic" diameter of 2 mic (Mie radius ~1 mic) for the infrared-emitting dust particles if they are spherical and composed of astronomical silicates. Because the collision timescale for these particles is so short (~104 yr) compared to the inferred stellar lifetime, these particles are unlikely to be primordial. Rather, they are relatively recent products of collisions that dominate both the creation and destruction of dust in the HR 4796A disk. Collision fragments this small are expected to be blown out of the system by radiation pressure in a few hundred years. Dynamical modeling of the disk indicates that the disk surface density is relatively sharply peaked near 70 AU. Interior to 70 AU, the model density drops steeply by a factor of two between 70 and 60 AU, falling to zero by 45 AU, which corresponds to the edge of the previously discovered central hole; in the context of the dynamical models, this ``soft" edge for the central hole results from the fact that the dust particle orbits are non-circular. The optical depth of mid-infrared-emitting dust in the hole is ~3 therefore relatively very empty. We present evidence (~1.8 sigma significance) for a brightness asymmetry that may result from the presence of the hole and the gravitational perturbation of the disk particle orbits by the low-mass stellar companion or a planet. This ``pericenter glow," which must still be confirmed, results from a very small (a few AU) shift of the disk's apparent center of symmetry relative to the central star HR 4796A; one side of the inner boundary of the annulus is shifted towards HR 4796A, thereby becoming warmer and more infrared-emitting. The possible detection of pericenter glow implies that the detection of even complex dynamical effects of planets on disks is within reach. This research was supported by NSF and NASA grants to the University of Florida and Penn State Erie.
The author(s) of this abstract have provided an email address for comments about the abstract: rfk2@psu.edu