A universal decline law of classical novae

We calculate many different nova light curves for a variety of white dwarf masses and chemical compositions, with the assumption that free-free emission from optically thin ejecta dominates the continuum flux. We show that all these light curves are homologous and a universal law can be derived by introducing a “time scaling factor.” The template light curve for the universal law has a slope of the flux, F ∝ t, in the middle part (from ∼ 2 to∼ 6 magnitude below the optical maximum) but it declines more steeply, F ∝ t, in the later part (from ∼ 6 to ∼ 10 mag), where t is the time from the outburst in units of days. This break on the light curve is due to a quick decrease in the wind mass loss rate. The nova evolutions are approximately scaled by the time of break. Once the time of break is observationally determined, we can derive the period of a UV burst phase, the duration of optically thick wind phase, and the turnoff date of hydrogen shell-burning. An empirical observational formula, t3 = (1.68 ± 0.08) t2 + (1.9 ± 1.5) days, is derived from the relation of F ∝ t, where t2 and t3 are the days during which a nova decays by 2 and 3 magnitude from the optical maximum, respectively. We have applied our template light curve model to the three well-observed novae, V1500 Cyg, V1668 Cyg, and V1974 Cyg. Our theoretical light curves