HST IMAGERY OF THE NONEXPANDING, CLUMPED SHELL OF THE RECURRENT NOVA T-PYXIDIS

The ''shell'' of the recurrent nova T Pyxidis has been resolved with H ST imagery into more than two thousand individual knots. Hydrogen and [N II] emission have very similar spatial distributions. Four epochs o f observation allow us to place an upper limit of 40 km/s on the syste matic expansion velocity of the knots. This is in contrast to our 1985 long slip spectra which show differential velocities of 350 km/s, and post-outburst (1966) spectra which show velocities almost three times larger. The azimuthally averaged surface brightness radial distributi on exhibits nine distinct peaks. A multiple shell model is required to fit the data. Our best estimate of the shell mass is approximately M- sh = 1.3 X 10(-6) M., with our inability to determine the electron den sity limiting any attempt to better constrain the shell mass, A few kn ots are observed to fade or brighten significantly on a time scale of months. We outline a model in which successive eruptions of T Pyx give rise to collisions and shocking of successive generations of ejecta. This leads to the clumping, emission line ratios, and knot variability that we observe. We also note that other nova ''shells'' might be as highly structured as T Pyxidis; and that the standard picture of unifo rm, monolithic nova shells is probably a great oversimplification. (C) 1997 American Astronomical Society.