A parameterised approach to disequilibrium retrievals in the JWST era: Application to NIRCam observations of HD 189733b

Atmospheric retrievals are a widely used technique for inferring the physical and chemical properties of exoplanetary atmospheres from observed spectra. A common simplifying assumption in such analyses is that the atmosphere is in thermochemical equilibrium, which allows the use of precomputed chemical abundance grids as a function of pressure, temperature, metallicity ([M/H]), and carbon-to-oxygen ratio (C/O). However, exoplanet atmospheres often deviate from equilibrium, particularly at lower temperatures or in the presence of strong vertical mixing. In this work, we investigate the impact of disequilibrium chemistry on retrieval outcomes by generating synthetic James Webb Space Telescope (JWST) observations of HD,189733,b with varying strengths of vertical mixing. We demonstrate that assuming thermochemical equilibrium can lead to significant biases in the retrieved atmospheric parameters, including incorrect estimates of C/O and [M/H]. To address this, we incorporate transport-induced quenching of carbon and nitrogen-bearing species into the retrieval framework by allowing the quench pressures to be free parameters. We show that this approach recovers the correct bulk atmospheric properties in most cases. Finally, we apply our disequilibrium retrieval model to published JWST/NIRCam transmission observations of HD,189733,b and find tentative evidence for quenching. We also find tentative evidence for the photochemically active region of the atmosphere via a newly developed H$_2$S parameterisation, this is the first time this has been constrained in a hot Jupiter atmosphere.

Source: arXiv:2607.06491v1 - http://arxiv.org/abs/2607.06491v1 PDF: https://arxiv.org/pdf/2607.06491v1 Original Link: http://arxiv.org/abs/2607.06491v1