Mixing fraction in classical novae

Abstract

Context. Classical novae are powered by thermonuclear runaways occurring on the surface of accreting white dwarfs (WDs). In the observations, the enrichments of heavy elements in nova ejecta have been detected, indicating the existence of a mixing process between the accreted matter and the matter from the outer layers of the underlying WDs prior to nova outbursts. However, the mixing fraction in classical novae is still uncertain.Aims. The purpose of this article is to investigate some elemental abundance ratios during nova outbursts, which can be used to estimate the WD-mixing fraction in classical novae. Methods. By considering different WD-mixing fractions with the stellar evolution code Modulesfor Experiments in Stellar Astrophysics (MESA), we carried out a series of simulations of nova outbursts, in which the initial CO WD masses range from 0.7 − 1.0 M ⊙ .Results. We identified four elemental abundance ratios (i.e. (H + He)/ CNO, (H + He)/Ne, CNO/Mg and CNO/Si) that satisfy the conditions for determining the WD-mixing fraction, in which (H + He)/ CNO is the most suitable mixing meter. We also estimated the WD-mixing fraction in some representative classical novae. Additionally, we found that a higher metallicity (i.e. higher WD-mixing fraction) prefers to be accompanied by a longer t 2 (the time of decline by two magnitudes from peak luminosity) during nova outbursts. The present results can be used to constrain the mixing process in classical novae.