[Paper Review] Dust formation in winds of long-period variables. V. The influence of micro-physical dust properties in carbon stars

Anja C. Andersen, S. Höfner+1 more|ArXiv.org|Oct 13, 2002

Astrophysics and Star Formation Studies30 references20 citations

TL;DR

This study presents self-consistent, frequency-dependent radiation hydrodynamics models for dust-driven winds in carbon-rich AGB stars, explicitly investigating how micro-physical dust properties—such as opacity, intrinsic density, sticking coefficients, and surface tension—affect mass-loss rates, outflow velocities, and dust condensation. The key finding is that variations in these parameters can alter mass-loss rates by up to a factor of four and outflow velocities by up to a factor of ten, highlighting critical uncertainties in theoretical predictions of AGB mass loss.

ABSTRACT

We present self-consistent dynamical models for dust-driven winds of carbon-rich AGB stars. The models are based on the coupled system of frequency-dependent radiation hydrodynamics and time-dependent dust formation. We investigate in detail how the wind properties of the models are influenced by the micro-physical properties of the dust grains that are required by the description of grain formation. The choice of dust parameters is significant for the derived outflow velocity, the degree of condensation and the resulting mass loss rates of the models. In the transition region between models with and without mass loss the choice ofmicro-physical parameters turns out to be very significant for whether a particular set of stellar parameters will give rise to a dust-driven mass loss or not. We also calculate near-infrared colors to test how the dust parameters influence the observable properties of the models, however, at this point we do not attempt to fit particular stars.

Motivation & Objective

• To investigate the impact of micro-physical dust parameters on dust-driven wind properties in carbon-rich AGB stars.
• To assess how uncertainties in dust opacity, density, sticking coefficients, and surface tension affect predicted mass-loss rates and outflow velocities.
• To evaluate the influence of these parameters on synthetic near-infrared colors and observable wind characteristics.
• To determine whether a given set of stellar parameters leads to dust-driven mass loss, depending on the choice of dust properties.

Proposed method

• The study employs a self-consistent, spherically symmetric radiation hydrodynamics model coupled with time-dependent dust formation.
• Frequency-dependent radiative transfer is used for both gas and dust, based on 51-wavelength opacity sampling data from the SCAN database.
• The model solves a system of nonlinear partial differential equations for gas, dust, and radiation, including momentum and energy conservation.
• Grain formation and growth are modeled using nucleation and accretion processes governed by sticking coefficients and surface tension.
• An adaptive grid and implicit numerical methods are used to manage computational complexity.
• Synthetic near-infrared colors are calculated to compare model outputs with observational trends.

Experimental results

Research questions

• RQ1 How do variations in dust opacity and extinction efficiency affect the predicted mass-loss rates in carbon-rich AGB stars?
• RQ2 To what extent do the intrinsic density and surface tension of dust grains influence the efficiency of dust formation and wind acceleration?
• RQ3 How do changes in sticking coefficients impact the growth rate of dust grains and the resulting wind properties?
• RQ4 What is the role of wavelength-dependent radiation pressure in determining the outflow velocity and dust condensation degree?
• RQ5 How do model-predicted near-infrared colors compare with observed colors for stars with similar mass-loss rates and velocities?

Key findings

• Varying micro-physical dust parameters can change predicted mass-loss rates by a factor of four and outflow velocities by a factor of ten.
• The choice of dust opacity, especially around 1 μm, can vary by nearly an order of magnitude between different amorphous carbon types, significantly affecting radiation pressure.
• The intrinsic dust density must be consistent with the extinction efficiency to ensure physical self-consistency in the models.
• Sticking coefficients and surface tension strongly influence nucleation and grain growth rates, with moderate variations producing noticeable changes in wind properties.
• The transition region between models with and without mass loss is highly sensitive to dust parameter choices, determining whether mass loss occurs at all.
• Synthetic near-infrared colors of models are similar to those of observed stars with comparable mass-loss rates and velocities, though model color variations are smaller than observed temporal variations.