Helium enrichment during classical nova outbursts

Abstract

ABSTRACT Nova outbursts play an important role in the chemical evolution of Galaxy, especially they are the main source of synthetic $^{13}\rm C$, $^{15}\rm N$, $^{17}\rm O$, and some radioactive isotopes like $^{22}\rm Na$ and $^{26}\rm Al$. The enrichment of He in nova ejecta indicates that the accreted material may mix with the He-shell (He-mixing). The purpose of this work is to investigate how the He-mixing affects the nova outbursts in a systematic way. We evolved a series of accreting WD models, and found that the mass fraction of H and He in nova ejecta can be influenced by different He-mixing fractions significantly. We also found that both the nova cycle duration and ejected mass increase with the He-mixing fractions. Meanwhile, the nuclear energy generation rate of p–p chains decreases with the He-mixing fraction during the nova outbursts, whereas the CNO-cycle increases. This work can reproduce the chemical abundances in the ejecta of some representative novae, such as GQ Mus, ASASSN-18fv, HR Del, T Aur, and V443 Sct. This implies that the He-mixing process cannot be neglected when studying nova outbursts. This study also develops a He-mixing meter (i.e. $\rm He/H$) that can be used to estimate the He-mixing fraction in classical nova systems.