Light curve analysis of classical novae: free-free emission vs photospheric emission

Abstract

We analyzed light curves of relatively slower novae, based on an optically thick wind theory of nova outbursts. In slower novae, we must take into account photospheric emission as well as free-free emission because their wind mass loss rates are smaller and the brightness of freefree emission is as faint as photospheric emission. We calculated model light curves of free-free plus photospheric emission for various WD masses and various chemical compositions of the envelopes, and fitted them reasonably with observational data of optical, near-infrared (NIR), and UV bands. From light curve fittings, we estimated their absolute magnitudes, distances, and WD masses. In the fast novae V1668 Cyg and V1974 Cyg, free-free emission dominates the spectrum in the optical and NIR regions, so the optical and NIR light curves follow the universal decline law. In slower novae PW Vul and V705 Cas, free-free emission still dominates the spectrum in the optical and NIR bands but photospheric emission contributes significantly to the spectrum in the later phase. In very slow novae such as RR Pic and V723 Cas, photospheric emission dominates the spectrum rather than free-free emission. These photospheric emission effect makes a deviation from the universal decline law and therefore a systematic deviation from the proposed maximum magnitude vs rate of decline (MMRD) relation.