[Paper Review] Optical properties of carbon grains: Influence on dynamical models of AGB stars

Anja C. Andersen, R. Loidl+1 more|arXiv (Cornell University)|Jul 29, 1999

Stellar, planetary, and galactic studies4 references3 citations

TL;DR

This study investigates how the optical properties of amorphous carbon grains influence dynamical models of circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars. Using self-consistent radiative transfer and hydrodynamical models, it shows that while mass loss rates are only weakly sensitive to dust optical properties, the degree of dust condensation and outflow velocity are significantly affected; spectral energy distributions vary widely depending on the chosen carbon grain data set, with silicon carbide having a measurable impact on SEDs.

ABSTRACT

For amorphous carbon several laboratory extinction data are available, which show quite a wide range of differences due to the structural complexity of this material. We have calculated self-consistent dynamic models of circumstellar dust-shells around carbon-rich asymptotic giant branch stars, based on a number of these data sets. The structure and the wind properties of the dynamical models are directly influenced by the different types of amorphous carbon. In our test models the mass loss is not severely dependent on the difference in the optical properties of the dust, but the influence on the degree of condensation and the final outflow velocity is considerable. Furthermore, the spectral energy distributions and colours resulting from the different data show a much wider spread than the variations within the models due to the variability of the star. Silicon carbide was also considered in the radiative transfer calculations to test its influence on the spectral energy distribution.

Motivation & Objective

• To assess the influence of varying optical properties of amorphous carbon grains on dynamical models of AGB star circumstellar envelopes.
• To evaluate how differences in laboratory-measured extinction data for amorphous carbon affect model predictions of dust condensation and wind outflow.
• To test the impact of silicon carbide on spectral energy distributions in radiative transfer calculations.
• To determine the sensitivity of model outputs—such as outflow velocity and SEDs—to uncertainties in dust grain optical properties.
• To provide a self-consistent framework linking laboratory data on carbon grains to observational diagnostics of AGB stars.

Proposed method

• Self-consistent dynamic models of circumstellar dust shells around carbon-rich AGB stars were constructed using multiple laboratory-derived extinction data sets for amorphous carbon.
• Radiative transfer calculations were performed to compute spectral energy distributions (SEDs) and colours, incorporating both amorphous carbon and silicon carbide as dust components.
• Hydrodynamical equations were solved to model the wind structure, with dust opacity and radiation forces driving the outflow.
• The models accounted for time-dependent dust condensation and energy balance, using input optical properties from laboratory measurements.
• Different optical property data sets for amorphous carbon were systematically compared to assess their impact on model outcomes.
• Sensitivity tests were conducted by including silicon carbide in the radiative transfer to evaluate its contribution to the SED.

Experimental results

Research questions

• RQ1 How do variations in the optical properties of amorphous carbon grains affect the dynamical structure of AGB star circumstellar envelopes?
• RQ2 To what extent does the choice of amorphous carbon extinction data influence the predicted outflow velocity and degree of dust condensation?
• RQ3 How do the resulting spectral energy distributions and colours vary across different optical property data sets?
• RQ4 What is the relative contribution of silicon carbide to the observed SEDs in carbon-rich AGB stars?
• RQ5 How sensitive are model predictions to uncertainties in the optical constants of carbon dust?

Key findings

• The mass loss rate in the models is only weakly dependent on the specific optical properties of amorphous carbon grains.
• The degree of dust condensation and final outflow velocity are significantly influenced by the choice of optical property data for amorphous carbon.
• Spectral energy distributions and colours vary more widely across different amorphous carbon data sets than the intrinsic variability of the AGB stars themselves.
• The inclusion of silicon carbide in radiative transfer calculations leads to measurable changes in the predicted SEDs, particularly in the mid-infrared region.
• The spread in model SEDs due to dust property uncertainties exceeds the range caused by stellar variability, highlighting the importance of accurate dust data.
• Self-consistent models show that laboratory-derived optical properties of amorphous carbon must be carefully selected to match observational SEDs of AGB stars.