[Paper Review] Three-component modeling of C-rich AGB star winds III. Micro-physics of drift-dependent dust formation

C. Sandín, S. Höfner|arXiv (Cornell University)|Sep 30, 2003

Astrophysics and Star Formation Studies11 references14 citations

TL;DR

This paper develops a three-component hydrodynamic model of C-rich AGB star winds that self-consistently includes drift-dependent dust formation physics, showing that grain drift significantly enhances dust production—particularly via changes in dominant growth species like C₂H and C₂H₂—resulting in up to several times more dust than in non-drift models, without altering bulk wind properties such as mass loss rate.

ABSTRACT

A proper treatment of the non-equilibrium dust formation process is crucial in models of AGB star winds. In this paper the micro-physics of this process is treated in detail, with an emphasis on the effects of drift (drift models). We summarize the description of the dust formation process and make a few additions to previous work. A detailed study shows that different growth species dominate the grain growth rates at different drift velocities. The new models show that the net effect of drift is to significantly increase the amounts of dust, seemingly without affecting the mean wind properties, such as e.g., the mass loss rate. In some cases there is several times more dust in drift models, compared to the values in the corresponding non-drift models. We study the formation of a dust shell in the inner parts of the wind and find that drift plays an active role in accumulating dust to certain narrow regions. In view of the results presented here it is questionable if drift -- under the current assumptions -- can be ignored in the grain growth rates.

Motivation & Objective

• To address the lack of self-consistent modeling of grain drift in dust formation processes within time-dependent AGB wind models.
• To investigate how drift velocity influences the micro-physics of dust grain growth, particularly the dominance of different carbon-bearing molecules in accretion.
• To assess whether drift effects on dust formation can be neglected in wind modeling, given their potential to alter dust mass and distribution.
• To examine the role of drift in shaping dust shell structures in the inner wind region.
• To improve the moment method for grain size distribution by incorporating drift-dependent growth rates and sticking coefficients.

Proposed method

• Adopts a three-component model framework: gas, dust, and radiation, coupled via mass, momentum, and energy exchange.
• Uses the moment method (GGS90) with four moment equations to describe grain size distribution evolution, including nucleation and growth.
• Incorporates drift by allowing the dust velocity $v$ to vary in the moment equations, affecting growth rates through relative drift velocity.
• Models grain growth using species-specific sticking coefficients and growth rates for C₂H, C₂H₂, and other radicals, with dependence on drift velocity.
• Applies classical nucleation theory with stationary assumptions, though acknowledges uncertainties in nucleation rate calculations.
• Solves the system numerically using a time-dependent hydrodynamic scheme, with improved treatment of gas opacity and pulsations from prior models.

Experimental results

Research questions

• RQ1 How does grain drift influence the dominant growth species in dust formation processes in C-rich AGB winds?
• RQ2 To what extent does drift enhance the total dust mass compared to non-drift models, and does this occur without altering bulk wind properties like mass loss rate?
• RQ3 What is the role of drift in the formation of localized dust shells in the inner wind region?
• RQ4 How do changes in drift velocity affect the efficiency of grain growth, particularly for weakly bound species like C₂H?
• RQ5 Can the assumption of chemical equilibrium in the gas phase be trusted when modeling drift-dependent dust formation?

Key findings

• Drift increases grain growth efficiency significantly even at low velocities (e.g., >10 km s⁻¹), leading to several times more dust in drift models compared to non-drift models.
• The dominant growth species shift with drift velocity: C₂H becomes increasingly important at higher drift speeds (above ~10 km s⁻¹), while C₂H₂ remains dominant overall.
• At drift velocities of 30–35 km s⁻¹, radical-mediated growth (e.g., C₂H) becomes prominent in low-dust-density regions, altering local grain size evolution.
• The average grain radius decreases by up to two orders of magnitude in high-drift regions due to changes in growth mechanisms.
• Dust destruction via non-thermal sputtering is found to be negligible under typical drift conditions.
• The micro-physical treatment of drift-dependent dust formation cannot be ignored, as it significantly alters dust mass and distribution without changing mean wind properties such as mass loss rate.