[Paper Review] Wind acceleration in AGB stars: Solid ground and loose ends

S. Höfner|arXiv (Cornell University)|May 27, 2015

Stellar, planetary, and galactic studies1 references3 citations

TL;DR

This paper reviews the Pulsation-Enhanced Dust-Driven Outflow (PEDDRO) model, which explains mass loss in AGB stars via radiative acceleration of dust grains formed in pulsation-driven shock waves. State-of-the-art models successfully reproduce wind properties, photometry, and time-dependent color variations, indicating that dust-driven winds are a robust mechanism supported by multi-wavelength observations.

ABSTRACT

The winds of cool luminous AGB stars are commonly assumed to be driven by radiative acceleration of dust grains which form in the extended atmospheres produced by pulsation-induced shock waves. The dust particles gain momentum by absorption or scattering of stellar photons, and they drag along the surrounding gas particles through collisions, triggering an outflow. This scenario, here referred to as Pulsation-Enhanced Dust-DRiven Outflow (PEDDRO), has passed a range of critical observational tests as models have developed from empirical and qualitative to increasingly self-consistent and quantitative. A reliable theory of mass loss is an essential piece in the bigger picture of stellar and galactic chemical evolution, and central for determining the contribution of AGB stars to the dust budget of galaxies. In this review, I discuss the current understanding of wind acceleration and indicate areas where further efforts by theorists and observers are needed.

Motivation & Objective

• To assess the current theoretical and observational status of dust-driven wind acceleration in asymptotic giant branch (AGB) stars.
• To evaluate the viability of the PEDDRO model—where pulsation triggers shocks, enabling dust formation and radiative acceleration of gas via dust-gas collisions.
• To identify open questions in microphysics (e.g., dust opacity, non-equilibrium chemistry) and macrophysics (e.g., 3D convection, pulsation dynamics) limiting predictive power.
• To guide future theoretical and observational efforts toward a quantitative, predictive theory of AGB mass loss.
• To reconcile multi-wavelength, time-resolved observations (e.g., interferometry, polarimetry) with self-consistent wind models.

Proposed method

• Modeling wind acceleration using the PEDDRO framework, where dust grains form in shock-heated regions and gain momentum via photon scattering or absorption.
• Employing time-dependent radiative transfer and hydrodynamics to simulate dust formation, wind acceleration, and spectral energy distributions.
• Using the critical opacity condition $\Gamma = \frac{\langle\kappa\rangle L_\ast}{4\pi c G M_\ast} \geq 1$ to determine whether radiative forces can overcome gravity.
• Applying flux-mean opacity $\langle\kappa\rangle$ to assess the radiative force relative to gravity, with $\langle\kappa\rangle_{\rm crit} \approx 2.6 \left(\frac{M_\ast}{M_\odot}\right)\left(\frac{L_\ast}{5000L_\odot}\right)^{-1}$ cm²/g.
• Constraining dust properties via synthetic photometry and spectro-photometric modeling, comparing with observed (J-K) and (V-K) colors.
• Using interferometric and polarimetric data (e.g., from Norris et al. 2012) to infer grain sizes (~0.3 μm) and spatial distribution in the wind acceleration zone.

Experimental results

Research questions

• RQ1 Can the PEDDRO model quantitatively reproduce observed wind velocities, mass loss rates, and photometric variability in AGB stars?
• RQ2 What is the role of Fe-free silicate grains in driving winds in M-type AGB stars, and how does their opacity compare to the critical value required for wind acceleration?
• RQ3 How do atmospheric shocks from pulsation influence dust condensation distances and the efficiency of momentum transfer to gas?
• RQ4 Why do some AGB stars show silicate features in mid-IR spectra at a few stellar radii while others do not, and what does this imply about grain composition or formation conditions?
• RQ5 To what extent do 3D convection and pulsation dynamics affect dust formation and wind acceleration, and how can these be better modeled?

Key findings

• PEDDRO models with amorphous carbon grains successfully reproduce wind properties and near-IR photometry in carbon-rich AGB stars, with synthetic (J-K) and (V-K) colors matching observations.
• For M-type AGB stars, photon scattering on Fe-free silicate grains provides a viable mechanism for wind acceleration, with models reproducing observed wind velocities and photometric variability.
• Time-dependent models show flat loops in the (J-K) vs. (V-K) diagram due to pulsation-driven variations in molecular features (e.g., TiO, H₂O), not dust opacity, indicating high transparency in the visual and near-IR.
• Interferometric studies resolve dust grain sizes of ~0.3 μm at ~2 R⋆, supporting the presence of small, efficient scatterers in the wind acceleration region.
• Mid-IR spectro-interferometry detects silicate features at a few stellar radii in some stars (e.g., Sacuto et al. 2013), but Al-bearing grains complicate modeling due to uncertain optical properties.
• Despite progress, current models rely on simplified piston-driven pulsation and lack full 3D convection, highlighting the need for 3D star-and-wind-in-a-box simulations to capture realistic dynamics.