[Paper Review] Exploring the origin of clumpy dust clouds around cool giants. A global 3D RHD model of a dust-forming M-type AGB star

S. Höfner, B. Freytag|arXiv (Cornell University)|Feb 11, 2019

Astrophysics and Star Formation Studies58 references4 citations

TL;DR

This study presents a 3D radiation-hydrodynamics model of an M-type AGB star that self-consistently simulates dust formation and grain growth, revealing that non-spherical shock waves from convection and pulsations naturally produce clumpy dust clouds in the inner, gravitationally bound atmosphere. The mechanism explains observed mid-IR variability and spatially resolved dust morphology without requiring wind-driven acceleration, with corundum forming first near 2 stellar radii and silicates marking the outer edge of bound layers.

ABSTRACT

Dust grains forming in the extended atmospheres of AGB stars are critical for the heavy mass loss of these cool luminous giants, as they provide radiative acceleration for the stellar winds. Characteristic mid-IR spectral features indicate that the grains consist mainly of silicates and corundum. The latter species seems to form in a narrow zone within about 2 stellar radii, preceding the condensation of silicate dust, which triggers the outflow. Recent high-angular-resolution observations show clumpy, variable dust clouds at these distances. We explore possible causes for the formation of inhomogeneous dust layers, using 3D dynamical simulations. We modeled the outer convective envelope and the dust-forming atmosphere of an M-type AGB star with the CO5BOLD radiation-hydrodynamics code. The simulations account for frequency-dependent gas opacities, and include a time-dependent description of grain growth and evaporation for corundum and olivine-type silicates. In the inner, gravitationally bound, and corundum-dominated layers of the circumstellar envelope, a patchy distribution of the dust emerges naturally, due to atmospheric shock waves that are generated by large-scale convective flows and pulsations. The formation of silicate dust at somewhat larger distances probably indicates the outer limit of the gravitationally bound layers. The current models do not describe wind acceleration, but the cloud formation mechanism should also work for stars with outflows. Timescales of atmospheric dynamics and grain growth are similar to observed values. In spherical averages of dust densities the variable 3D morphology manifests itself as cycle-to-cycle variations. Grain growth in the wake of large-scale non-spherical shock waves, generated by convection and pulsations, is a likely mechanism for producing the observed clumpy dust clouds, and for explaining their physical and dynamical properties.

Motivation & Objective

• To understand the origin of clumpy, variable dust clouds observed in high-angular-resolution images of M-type AGB stars.
• To investigate whether atmospheric dynamics such as convection and pulsations can naturally produce inhomogeneous dust distributions without assuming pre-existing inhomogeneities.
• To determine the role of grain growth in shock wakes for forming the observed patchy dust structures.
• To assess whether the observed dust morphology and variability can be explained by 3D dynamics alone, prior to wind acceleration.
• To compare 3D model results with unresolved observations and 1D models by analyzing spherical averages of dust density and other quantities.

Proposed method

• Simulates the outer convective envelope and dust-forming atmosphere of an M-type AGB star using the CO5BOLD radiation-hydrodynamics code.
• Incorporates frequency-dependent gas opacities to accurately model radiative transfer and heating in the dynamic atmosphere.
• Includes time-dependent treatment of grain nucleation, growth, and evaporation for corundum (Al₂O₃) and olivine-type silicates (Mg₂SiO₄).
• Tracks the evolution of dust density and temperature in response to pulsations and convective flows, focusing on shock wave dynamics.
• Analyzes both 3D spatial structures and spherical averages of dust density to compare with unresolved observations and 1D models.
• Uses a global 3D setup covering the stellar atmosphere up to ~2–3 stellar radii, focusing on the region where dust forms before wind acceleration.

Experimental results

Research questions

• RQ1 Can large-scale shock waves generated by convection and pulsations naturally produce clumpy dust distributions in the atmospheres of M-type AGB stars?
• RQ2 What is the role of grain growth in the wakes of non-spherical shock fronts in shaping the observed dust morphology?
• RQ3 How do the dynamical timescales of atmospheric shocks and grain growth rates compare to observed variability timescales?
• RQ4 Where in the circumstellar envelope do corundum and silicate dust form, and what does this imply about the structure of the gravitationally bound layer?
• RQ5 To what extent do cycle-to-cycle variations in spherical averages of dust density reflect the underlying 3D, time-dependent morphology?

Key findings

• Clumpy dust distributions emerge naturally in the 3D simulations due to non-spherical shock waves from convection and pulsations, without requiring external inhomogeneities.
• Corundum (Al₂O₃) forms in a narrow zone within ~2 stellar radii, consistent with observations indicating its presence in the innermost dust-forming region.
• Silicate dust (Mg₂SiO₄) forms at slightly larger radii, marking the outer edge of the gravitationally bound layer, where radiation pressure may later trigger wind acceleration.
• Grain growth occurs predominantly in the wakes of outward-propagating shocks, which enhances local dust density and creates the observed patchy morphology.
• Timescales of atmospheric dynamics and grain growth in the model are consistent with observed variability periods and cycle-to-cycle variations.
• Spherical averages of dust density show cycle-to-cycle variations that reflect both radial pulsation and the 3D, time-dependent morphology, linking unresolved observations to 3D dynamics.