THE SHELL OF RECURRENT NOVA T-PYXIDIS - A MODEL-BASED ON SHOCKS

A model for the shell of the recurrent nova T Pyxidis is presented, co nsistent with the bulk of observational data on this object. Evolution ary calculations of thermonuclear runaways on the surface of an accret ing white dwarf that simulate the observed outburst characteristics of this nova-recurrence time, rise time, time of decline, luminosity-pro vide the mass of the ejected shell, its composition, and the effective temperature and luminosity of the nova in quiescence (for a white dwa rf mass of 1.25 M(.), core temperature of 10(7) K, and an accretion ra te of 10(-7) M(.) yr(-1)). These results are used as input parameters for the SUMA code, which simulates the physical conditions of an emitt ing gaseous cloud under the combined effects of photoionization and sh ocks. The shell model involves two emitting regions besides the centra l star (which is the main UV radiation source), both resulting from co mpressed gas behind a shock front. The shocks are assumed to be produc ed by the collision between two extended shells ejected at consecutive outbursts. As a result of the collision, a secondary blast wave propa gates outward into the older ejecta, while a reflected (reverse) shock wave propagates back through the new ejecta. Applying the SUMA code t o each zone, we find that different lines are emitted in the two zones , and combined, they account for the entire observed line spectrum. Th e absolute fluxes in different spectral ranges agree with those derive d from observations, assuming a distance of similar to 2 kpc. The deri ved abundances imply that mixing has taken place between the nova ejec ta and interstellar material. Nevertheless, helium is overabundant and N/O is high. The model accounts for the observed structure of the neb ula surrounding T Pyx, including the inner shell, the stagnant outer s hell, and the extended faint region beyond it.