Bayesian Analysis of Molecular Emission and Dust Continuum of Protoplanetary Disks

TL;DR

This paper introduces a Bayesian framework and a new model, DuCKLinG, for analyzing JWST MIRI spectra of protoplanetary disks, enabling detailed inference of dust and molecular gas properties with validated accuracy.

Contribution

It presents the DuCKLinG model for simultaneous dust and molecular emission analysis and demonstrates its effectiveness through benchmarking against complex thermo-chemical models.

Findings

The model accurately recovers molecular conditions within true values.

Radial gradients in temperature and column density are statistically supported.

Molecules like HCN and C2H2 originate from small, uniform regions.

Abstract

The Mid-InfraRed Instrument (MIRI) on board the James Webb Space Telescope (JWST) probes the chemistry and dust mineralogy of the inner regions of protoplanetary disks. The observed spectra are unprecedented in their detail, complicating interpretations which are mainly based on manual continuum subtraction and 0D slab models. We investigate the physical conditions under which the gas emits in protoplanetary disks. Based on MIRI spectra, we apply a full Bayesian analysis that provides the posterior distributions of dust and molecular properties. For doing so, we introduce the Dust Continuum Kit with Line emission from Gas (DuCKLinG), a model describing the molecular line emission and the dust continuum simultaneously without large computational cost. The dust model is based on work by Juhasz et al. (2009, 2010). The molecular emission is based on LTE slab models, but with radial gradients in column densities and temperatures. The model is compared to observations using Bayesian analysis. We benchmark this model to a complex thermo-chemical ProDiMo model and fit the MIRI spectrum of GWLup. We find that the retrieved molecular conditions from DuCKLinG fall within the true values from ProDiMo. The column densities retrieved by Grant et al. (2023) fall within the retrieved ranges in this study for all examined molecules (CO2, H2O, HCN, and C2H2). Similar overlap is found for the temperatures with only the temperature range of HCN not including the previously found value. This discrepancy may be due to the simultaneous fitting of all molecules compared to the step-by-step fitting of the previous study. There is statistically significant evidence for radial temperature and column density gradients for H2O and CO2 compared to the constant temperature and column density assumed in the 0D slab models. Additionally, HCN and C2H2 emit from a small region with near constant conditions.