Consistent dust and gas models for protoplanetary disks. I. Disk shape, dust settling, opacities, and PAHs

Abstract: We propose a set of standard assumptions for the modelling of Class II and III protoplanetary disks, which includes detailed continuum radiative transfer, thermo-chemical modelling of gas and ice, and line radiative transfer from optical to cm wavelengths. We propose new standard dust opacities for disk models, we present a simplified treatment of PAHs sufficient to reproduce the PAH emission features, and we suggest using a simple treatment of dust settling. We roughly adjust parameters to obtain a model that predicts typical Class II T Tauri star continuum and line observations. We systematically study the impact of each model parameter (disk mass, disk extension and shape, dust settling, dust size and opacity, gas/dust ratio, etc.) on all continuum and line observables, in particular on the SED, mm-slope, continuum visibilities, and emission lines including [OI] 63um, high-J CO lines, (sub-)mm CO isotopologue lines, and CO fundamental ro-vibrational lines. We find that evolved dust properties (large grains) often needed to fit the SED, have important consequences for disk chemistry and heating/cooling balance, leading to stronger emission lines in general. Strong dust settling and missing disk flaring have similar effects on continuum observations, but opposite effects on far-IR gas emission lines. PAH molecules can shield the gas from stellar UV radiation because of their strong absorption and negligible scattering opacities. The observable millimetre-slope of the SED can become significantly more gentle in the case of cold disk midplanes, which we find regularly in our T Tauri models. We propose to use line observations of robust chemical tracers of the gas, such as O, CO, and H2, as additional constraints to determine some key properties of the disks, such as disk shape and mass, opacities, and the dust/gas ratio, by simultaneously fitting continuum and line observations.

Consistent Dust and Gas Models for Protoplanetary Disks: Disk Shape, Dust Settling, Opacities, and PAHs

This paper focuses on the development of a consistent set of assumptions for modeling Class II and III protoplanetary disks, providing a comprehensive approach that integrates continuum radiative transfer, thermo-chemical modeling of gas and ice, and line radiative transfer across a broad spectrum of wavelengths. The authors address critical components including disk shape, dust opacities, dust settling, and the role of polycyclic aromatic hydrocarbons (PAHs) in protoplanetary disk environments.

Key Insights and Results

1. Disk Modeling Assumptions:
   • The paper proposes a standard set of assumptions crucial for modeling protoplanetary disks, particularly emphasizing the need for new standard dust opacities. This involves a detailed treatment of disk shape and dust settling to match observations.
   • Emphasis is placed on the necessity to adopt physically justified dust models, accounting for parameters such as grain size distribution, opacities, and dust/gas ratios.
2. Dust Opacities and Settling:
   • The authors suggest a new set of standard dust opacities appropriate for disk conditions, differing significantly from those applicable to the interstellar medium. They provide an efficient, computational approach to calculate these opacities.
   • Dust settling is considered a critical process in protoplanetary disks, leading to a concentration of large grains towards the midplane, significantly impacting the predicted observables.
3. Polycyclic Aromatic Hydrocarbons (PAHs):
   • The study acknowledges the influence of PAHs on the radiative transfer within disks, highlighting their potential to dominate UV opacities and effectively shield the gas from stellar UV radiation.
   • A simplified treatment of PAHs is adopted, which aligns well with the stochastic quantum heating approach used in existing models, ensuring computational efficiency while maintaining physical accuracy.
4. Impact on Observables:
   • The model systematically studies the influence of various parameters on both the continuum and line observables, including the spectral energy distribution (SED), emission lines like [OI] 63 µm, and CO isotopologue lines.
   • Results indicate that parameters affecting dust properties and settling have substantial impacts on both the dust and gas temperature structure, influencing the continuum and line emission.
5. Comparative Analysis with Hydrostatic Models:
   • The paper critiques traditional hydrostatic disk models, noting their inadequacy in reproducing certain observed features of T Tauri disks, such as near-IR excess and CO line profiles.
6. Model Validation and Numerical Convergence:
   • Validation against existing Monte Carlo codes and a detailed convergence study ensures the reliability of the model outcomes, reinforcing the robustness of the proposed assumptions.

Implications and Future Directions

The research provides a foundation for a unified modeling framework that enhances the interpretation of multi-wavelength observations of protoplanetary disks. The model's comprehensive nature allows for simultaneous and consistent prediction across different observed properties, crucial for advancing our understanding of disk physics and chemistry.

The implications of these findings are significant for both the practical modeling of protoplanetary disks and theoretical advancements in the field. By establishing a detailed set of standard modeling assumptions, the paper sets the stage for future investigations into the complex interactions between dust, gas, and radiation in these environments. Future work may focus on extending these models to 3D structures and incorporating dynamic processes such as disk winds and planet-disk interactions, which remain critical for explaining newly observed disk features.