HST Imagery of the Non-Expanding, Clumped "Shell" of the Recurrent Nova T Pyxidis

The "shell" of the recurrent nova T Pyxidis has been resolved with HST imagery into more than two thousand individual knots. Hydrogen and [N II] emission have very similar spatial distributions. Four epochs of observation allow us to place an upper limit of 40 km/s on the systematic 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 distribution 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 = 1.3×10^-6M_sun, with our inability to determine the electron density limiting any attempt to better constrain the shell mass. A few knots 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 uniform, monolithic nova shells is probably a great oversimplification.