Recently, Ph.D. candidate GUO Yunlang and Prof. WANG Bo from Yunnan Observatories of the Chinese Academy of Sciences investigated the white dwarf mixing fraction in classical novae.

This work was published in Astronomy & Astrophysics.

Classical novae are powered by thermonuclear runaways occurring on the surface of accreting white-dwarfs (WDs) in close binaries. In close binaries, the white dwarf accrete matter from its companion and the thermonuclear runaway can be triggered once the accreted mass reaches a critical value.

During the novae outbursts, the luminosity increases dramatically, and most of the accreted envelope is ejected. Observationally, the enrichments of intermediate-mass elements (e.g., C, N, O, Ne, Na, Mg, and Al) have been detected in nova ejecta, indicating that the accreted shell is mixed with the outer layers of the underlying white dwarf, called white dwarf mixing.

In this study, the researchers investigated the white dwarf mixing fraction and the ejected material systematically. By considering different WD masses and mixing fractions, they carried out a series of simulations of nova outbursts. They adopted the pre-mixed model in the simulations, in which the material accreted by WDs is assumed to be a mixture of the companion star material (solar abundances) and the outermost layers of the underlying WDs.

The elemental abundance ratios that can be used to estimate the WD mixing fraction should be sensitive to the WD mixing degree but not affected by the WD mass. In addition, it is worth noting that these ratios cannot be affected by the He mixing that may occur in classical novae because the abundances of H and He in nova ejecta are sensitive to the He mixing fractions.

After the researchers performed a series of simulations for nova models, they identified four elemental abundance ratios that can be used to estimate the white dwarf mixing fraction in classical novae. They estimated the WD mixing fraction in some representative classical novae by comparing the element abundance ratios in the observations and the simulations, i.e. GQ Mus, PW Vul and V842 Cen, etc. Their simulations indicate that the WD mixing fraction in classical novae spans a relatively wide range.

In addition, they suggested that higher white dwarf mixing fraction may exist in nova systems with less massive white dwarfs. This work can be used to constrain the mixing process in classical novae.

Contact:
GUO Yunlang
Yunnan Observatories, CAS
E-mail: yunlang@ynao.ac.cn