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, consistent with the bulk of observational data on this object. Evolutionary calculations of thermonuclear runaways on the surface of an accreting white dwarf that simulate the observed outburst characteristics of this nova - recurrence time, rise time, time of decline, luminosity - provide the mass of the ejected shell, its composition, and the effective temperature and luminosity of the nova in quiescence (for a white dwarf mass of 1.25 M⊙, a core temperature of 10⁷ K, and an accretion rate of 10^-7 M⊙ yr^-1). These results are used as input parameters for the SUMA code, which simulates the physical conditions of an emitting gaseous cloud under the combined effects of photoionization and shocks. The shell model involves two emitting regions besides the central star (which is the main UV radiation source), both resulting from compressed gas behind a shock front. The shocks are assumed to be produced by the collision between two extended shells ejected at consecutive outbursts. As a result of the collision, a secondary blast wave propagates outward into the older ejecta, while a reflected (reverse) shock wave propagates back through the new ejecta. Applying the SUMA code to each zone, we find that different lines are emitted in the two zones, and combined, they account for the entire observed line spectrum. The absolute fluxes in different spectral ranges agree with those derived from observations, assuming a distance of ∼2 kpc. The derived abundances imply that mixing has taken place between the nova ejecta and interstellar material. Nevertheless, helium is overabundant and N/O is high. The model accounts for the observed structure of the nebula surrounding T Pyx, including the inner shell, the stagnant outer shell, and the extended faint region beyond it.