[Paper Review] Three-component modeling of C-rich AGB star winds. II. The effects of drift in long-period variables

C. Sandín, S. Höfner|arXiv (Cornell University)|Apr 15, 2003

Astrophysics and Star Formation Studies17 references15 citations

TL;DR

This study develops three-component hydrodynamic models for C-rich AGB stars, explicitly including gas-dust drift and pulsation effects, to investigate how drift alters wind properties. It finds that drift significantly reduces mass loss rates and terminal velocities compared to non-drift models, especially in low-density Planck mean opacity models, and that drift effects cannot be reliably predicted by non-drift models, rendering existing fit formulas inapplicable to these advanced models.

ABSTRACT

We present three-component wind models for carbon rich pulsating AGB stars. In particular we study the effects of drift in models of long-period variables, meaning that the dust is allowed to move relative to the gas (drift models). In addition we investigate the importance of the degree of variability of the wind structures. The wind model contains separate conservation laws for each of the three components of gas, dust and the radiation field. We use two different representations for the gas opacity, resulting in models with different gas densities in the wind. The effects which we investigate here are important for the understanding of the wind mechanism and mass loss of AGB stars. This study is hereby a necessary step towards more reliable interpretations of observations. We find that the effects of drift generally are significant. They cannot be predicted from models calculated without drift. Moreover, the non-drift models showing the lowest mass loss rates, outflow velocities, and the smallest variability in the degree of condensation do not form drift model winds. The wind formation in drift models is, except for a few cases, generally less efficient and the mass loss consequently lower than in the corresponding non-drift models. The effects of drift are generally larger in the more realistic models using that representation of the gas opacity which results in lower densities. The outflow properties of these models are also -- for all cases we have studied -- sensitive to the period of the stellar pulsations. A check of the mass loss rates against a (recent) fit formula shows systematically lower values, in particular in the more realistic models with a low density. The fit is in its current form inapplicable to the new models presented here.

Motivation & Objective

• To investigate the impact of gas-dust drift on mass loss and wind structure in C-rich AGB stars.
• To assess how stellar pulsations in long-period variables affect wind formation when drift is included.
• To evaluate whether non-drift models can reliably reproduce the effects seen in drift models.
• To test the applicability of existing mass loss rate fit formulas to new, more realistic wind models with drift.
• To determine if drift-induced changes in wind properties are predictable without explicitly modeling drift.

Proposed method

• Three-component hydrodynamic equations are solved for gas, dust, and radiation, with separate conservation laws for each component.
• Drift is modeled via a second-order-accurate numerical scheme for momentum transfer between gas and dust, improving on first-order accuracy in prior work.
• Two gas opacity representations are used: constant opacity (Bowen 1988) and Planck mean molecular opacities (HJLA98), to compare high- and low-density wind structures.
• Stellar pulsations are included via time-dependent boundary conditions to simulate long-period variables.
• Wind properties such as mass loss rate, terminal velocity, and condensation degree are monitored and averaged over time to compare drift and non-drift models.
• The models are benchmarked against the Wachter et al. (2002) fit formula for mass loss rates to assess its validity.

Experimental results

Research questions

• RQ1 How does gas-dust drift affect the terminal velocity and mass loss rate in C-rich AGB star winds?
• RQ2 To what extent do pulsation periods influence the wind structure and drift effects in long-period variables?
• RQ3 Can non-drift models accurately predict the wind properties observed in drift models?
• RQ4 Why do some Planck mean PC models fail to form corresponding drift model winds?
• RQ5 Is the Wachter et al. (2002) fit formula for mass loss rates applicable to models that include drift and realistic gas opacities?

Key findings

• Drift models consistently show lower mass loss rates than their non-drift counterparts, with the reduction being most pronounced in Planck mean opacity models due to weaker gas-dust coupling.
• The inclusion of drift leads to stronger temporal variations in dust-related quantities and more irregular variability in wind properties, especially in Planck mean models.
• In some cases, drift increases mass loss rates, particularly in constant-opacity models where the non-drift counterpart has low terminal velocity and low mass loss.
• No drift model wind forms for certain Planck mean PC models, especially those with low variability in condensation degree, indicating that drift can prevent wind formation where non-drift models succeed.
• The Wachter et al. (2002) fit formula overestimates mass loss rates by a factor of ten for Planck mean models, rendering it inapplicable to the new drift-inclusive models presented here.
• Planck mean drift models are more sensitive to pulsation period than constant-opacity models, highlighting the importance of pulsation in shaping drift-driven wind dynamics.