Dust formation during the interaction of binary stars by common envelope

TL;DR

The paper investigates dust formation during common-envelope evolution in binaries with intermediate-mass AGB donors by performing SPH simulations that explicitly model carbon dust nucleation and growth. Using two donor masses (1.7 and 3.7 M$_{\odot}$) and a 0.6 M$_{\odot}$ companion, the study finds dust appears within 1–3 years, forming a high-opacity shell that shifts the CE photosphere outward by about an order of magnitude. Dust grains start small (∼0.02–0.04 μm) and grow to ∼0.03–1 μm, with total dust masses plateauing at roughly 0.008–0.022 M$_{\odot}$ depending on the donor mass. Although dust strongly affects the optical appearance, it does not substantially increase mass unbinding or drastically alter orbital evolution, highlighting that dust’s observational imprint can be significant even when dynamical impact is limited.

Abstract

We performed numerical simulations of the common envelope (CE) interaction between two intermediate-mass asymptotic giant branch (AGB) stars and their low-mass companions. For the first time, formation and growth of dust in the envelope is calculated explicitly. We find that the first dust grains appear as early as $\sim$1-3 yrs after the onset of the CE, and are smaller than grains formed later. As the simulations progress, a high-opacity dusty shell forms, resulting in the CE photosphere being up to an order of magnitude larger than it would be without the inclusion of dust. At the end of the simulations, the total dust yield is $0.0082~M_{\odot}$ ($0.022~M_{\odot}$) for a CE with a $1.7~M_{\odot}$ ($3.7~M_{\odot}$) AGB star. Dust formation does not substantially lead to more mass unbinding or substantially alter the orbital evolution.

Figures (5)

• Figure 1: Dust opacity versus distance from the centre of mass for the 1.7 M$_{\odot}$ (left) and 3.7 M$_{\odot}$ (right) models after 20 years of simulation. The color bar indicates $t_{\rm nuc}$, the time in years after the start of the simulations, when dust nucleation occurred for each SPH particle.
• Figure 2: Average dust grains size versus distance for the 1.7 M$_{\odot}$ (top) and 3.7 M$_{\odot}$ (bottom) model, after 44 years of simulations. Same color bar as in Figure \ref{['fig:opacity-versus-distance']}.
• Figure 3: Dashed lines show the total dust mass over time in CE simulations. Horizontal lines represent the maximum carbon available in the envelope for 1.7 M$_{\odot}$ (black solid) and 3.7 M$_{\odot}$ (red solid) AGB stars.
• Figure 4: Left panel: orbital separation over time in CE simulations with a 1.7 M$_{\odot}$ AGB star. Dust formation is explicitly calculated (blue lines), or assumed with a maximum dust opacity of $5~\mathrm{cm^2\,g^{-1}}$ (red lines) or $15~\mathrm{cm^2\,g^{-1}}$ (green lines) as in GonzalezBolivar2023. Also shown is a CE simulation without dust (yellow lines). The inset displays the orbital separation at the end of the in-spiral phase. Right panel: Mechanical bound mass (solid lines) and thermal bound mass (dashed lines) over time.
• Figure 5: Photosphere size over time in the orbital (solid lines) and the "polar" plane (dashed lines) for the 3.7 M$_{\odot}$ model with and without dust (blue and orange, respectively). Shaded areas depict uncertainty.