The Formation of Bimodal Dust Species in Nova Ejecta

TL;DR

This study investigates the formation of bimodal dust species, specifically silicate and amorphous carbon grains, in nova ejecta using simulations of different models, revealing conditions conducive to dust formation and growth timelines.

Contribution

It introduces detailed simulations of dust formation in nova ejecta, highlighting the radiative shock model as a key environment for bimodal dust species creation.

Findings

Dust grains form within days to weeks after eruption.

Radiative shock model enables efficient dust production.

Predicted dust-to-gas ratio is approximately 1%.

Abstract

The formation of bimodal dust species (namely the silicate and amorphous carbon dust grains coexistent) in a nova eruption is an open problem. According to the nova model simulated by Modules for Experiments in Stellar Astrophysics code, we calculate the formation and growth of carbon (C) and forsterite (Mg2SiO4) dust grains purely in nova ejecta for the fee-expansion model and the radiative shock model by assuming spherical geometry of the nova ejecta. In these models, the chemical properties of pre-existing circumstellar medium are not taken into account. In the free-expansion model, the nova ejecta is not an idea environment for dust nucleation. However, it can efficiently produce dust in the radiative shock model. We estimate that every nova can produce C grains with an average mass of about $10^{-9}$ and $10^{-8}$ ${\rm M_\odot}$, and Mg$_2$SiO$_4$ grains with an average mass of about $10^{-8}$ and $10^{-7}$ ${\rm M_\odot}$. Based on the mass of ejected gas, the ratio of dust to gas is about 1\%. The C grains form first after several or tens of days of nova eruption. After that, the Mg$_2$SiO$_4$ grains begin to grow in tens of days, which is consistent with observations.