A thousand and one nova outbursts

A full nova cycle includes mass accretion, thermonuclear runaway resulting in outburst and mass-loss, and finally, decline. Resumed accretion starts a new cycle, leading to another outburst. Multicycle nova evolution models have been calculated over the past twenty years, the number being limited by numerical constraints. Here we present a long-term evolution code that enables a continuous calculation through an unlimited number of nova cycles for an unlimited evolution time, even up to 1.5 × 10¹⁰ yr. Starting with two sets of the three independent nova parameters - the white dwarf (WD) mass, the temperature of its isothermal core, and the rate of mass transfer on to it - we have followed the evolution of two models, with initial masses of 1 M_⊙ and 0.65 M_⊙ through over 1000 and over 3000 cycles, respectively. The accretion rate was assumed constant throughout each calculation: 10^-11 M_⊙ yr^-1 for the 1 M_⊙ WD, and 10^-9 M_⊙ yr^-1 for the 0.65 M_⊙ one. The initial temperatures were taken to be relatively high: 30 × 10⁶ and 50 × 10⁶ K, respectively, as they are likely to be at the onset of the outburst phase. The results show that although on the short-term consecutive outbursts are almost identical, on the long-term scale the characteristics change. This is mainly due to the changing core temperature, which decreases very similarly to that of a cooling WD for a time, but at a slower rate thereafter. As the WD's mass continually decreases, since both models lose more mass than they accrete, the central pressure decreases accordingly. The outbursts on the massive WD change gradually from fast to moderately fast, and the other characteristics (velocity, abundance ratios, isotopic ratios) change, too. Very slowly, a steady state is reached, where all characteristics, both in quiescence and in outburst, remain almost constant. For the less massive WD accreting at a high rate, outbursts are similar throughout the evolution.