A Universal Decline Law of Classical Novae

Abstract

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 scaling factor. The template light curve for the universal law has a slope of flux F ∝ t-1.75 in the middle part (from ~2 to ~6 mag below the optical maximum), but it declines more steeply, F ∝ t-3.5, 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-1.75, where t2 and t3 are the times in days during which a nova decays by 2 and 3 mag 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 show excellent agreement with the optical y and infrared J, H, K light curves. The continuum UV 1455 A light curves observed with IUE are well reproduced. The turn-on and turn-off of supersoft X-ray observed with ROSAT are also explained simultaneously by our model. The WD mass is estimated, from the light curve fitting, to be MWD ≈ 1.15 M☉ for V1500 Cyg, MWD ≈ 0.95 M☉ for V1668 Cyg, and MWD ≈ 0.95-1.05 M☉ for V1974 Cyg, together with the appropriate chemical compositions of the ejecta.