# The Formation of Bimodal Dust Species in Nova Ejecta I: Chemical Conditions

## TL;DR

This study investigates how chemical conditions, influenced by white dwarf properties and mixing depth, can lead to the formation of bimodal dust species in nova ejecta, focusing on the evolution of the C/O ratio during eruptions.

## Contribution

It introduces a new parameter, mixing depth, and demonstrates its effect on the chemical conditions conducive to bimodal dust formation in nova ejecta.

## Key findings

- C/O ratio varies with mixing depth during eruptions
- Small mixing depth can cause C/O to shift from >1 to <1
- Chemical conditions for bimodal dust formation are identified

## Abstract

The formation of bimodal dust species (that is, both of silicate and amorphous carbon dust grains are observed in a nova eruption) in nova ejecta is still debated. Using the Modules for Experiments in Stellar Astrophysics code and considering the effects of WD's mass, mass-accretion rate and the chemical profiles of WD which are described by new parameter --- mixing depth on the chemical abundances of nova ejecta, we investigate the possibility that bimodal dust species are produced in a nova eruption. We find that $C/O$ (the ratio of carbon number density to oxygen number density) of nova ejecta is affected by the mixing depth. For the model with a small mixing depth, the $C/O$ of nova ejecta can evolute from lager than 1.0 to smaller than 1.0 in a whole eruption, which provides the chemical condition for the formation of bimodal dust species.