Helium enrichment during classical nova outbursts

TL;DR

This study systematically investigates how helium mixing affects nova outbursts, revealing significant impacts on ejecta composition, cycle duration, and nuclear processes, and introduces a method to estimate He-mixing in nova systems.

Contribution

It provides a comprehensive analysis of He-mixing effects on nova outbursts and develops a He-mixing meter based on ejecta composition.

Findings

He and H mass fractions in ejecta are significantly affected by He-mixing.

Nova cycle duration and ejected mass increase with He-mixing fraction.

The nuclear energy production shifts from p-p chains to CNO cycle with more He-mixing.

Abstract

Nova outbursts play an important role in the chemical evolution of galaxies, 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 production from $p$-$p$ chains decreases with the He-mixing fraction during the nova outbursts, whereas the CNO-cycle increases. The present work can reproduce the chemical abundances in the ejecta of some 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.