Sensitivity of PDR Calculations to Microphysical Details

TL;DR

This study investigates how variations in microphysical processes affect the spectral predictions of PDR models, highlighting the importance of detailed physics in accurately reproducing observed infrared emission lines.

Contribution

The paper demonstrates the sensitivity of PDR spectral models to microphysical details using the Cloudy code, emphasizing the impact of grain physics and chemical processes on emission line predictions.

Findings

Emission line intensities vary significantly with grain physics treatments.

Different physical process combinations can produce similar chemical structures.

Cloudy effectively models molecular environments and their spectra.

Abstract

Our understanding of physical processes in Photodissociation regions or Photon Dominated Regions (PDRs) largely depends on the ability of spectral synthesis codes to reproduce the observed infrared emission-line spectrum. In this paper, we explore the sensitivity of a single PDR model to microphysical details. Our calculations use the Cloudy spectral synthesis code, recently modified to include a wealth of PDR physical processes. We show how the chemical/thermal structure of a PDR, along with the calculated spectrum, changes when the treatment of physical processes such as grain physics and atomic/molecular rates are varied. We find a significant variation in the intensities of PDR emission lines, depending on different treatments of the grain physics. We also show how different combinations of the cosmic-ray ionization rate, inclusion of grain-atom/ion charge transfer, and the grain size distribution can lead to very similar results for the chemical structure. Additionally, our results show the utility of Cloudy for the spectral modeling of molecular environments.