Hydrodynamical modelling of t pyxidis

The Hubble Space Telescope observations showed that some nova outbursts may not produce smooth shells of gas, but thousands of gaseous blobs. In the case of T Pyx the blobs are distributed in eight concentric circles around the exploding star, producing a pattern similar to tree rings. We explore here the formation of the tree rings by hydrodynamical simulations. The simulations have been performed using the magnetohydrodynamic code ZEUS-3D (version 3.4), developed by M. L. Norman and the Laboratory for Computational Astrophysics (Stone & Norman 1992). We perform the simulations in spherical coordinates (r, θ, φ) with periodic boundary conditions in φ, and rotational symmetry assumed with respect to θ. Thus, our models are effectively two-dimensional, and the simulations are carried out in the equatorial (r, φ) plane. The models include the Raymond & Smith (1977) cooling curve above 10 K. For temperatures below 10 K, the shocked gas region is allowed to cool down with the radiative cooling curves given by Dalgarno & McCray (1972) and MacDonald & Bailey (1981). As initial conditions for the computations, we assume that there is no background wind from the red dwarf in T Pyx, but only material from the previous nova outbursts. Our best guess as to the mass loss is that it dies away as either 1/t or 1/t down to extremely low values within 0.5 yr after the explosion. The velocity also dies away, though, from 2000 down to 10 km/s over the same time period, so the next blast finds a stretched out wind to run in to. The functional dependence of Ṁ and v∞ versus time it is a simple linear model: Ṁ(t) = 2×10M yr −1 ×[0.5−t]fort(yr) ≤ 0.5. v∞(t) = 4000 km s −1 × [0.5 − t]fort(yr) ≤ 0.5. Then a repetition each 30 yr. We did first a run, in order to get the densities and pressures of the large hot bubble. After that, we did a run (Figure 1) inside the bubble, with density of 5 × 10 g cm and energy density of 2× 10, keeping pressurized the outer boundary. As one can notice, the clumps form in the ejecta,