Towards non-LTE retrieval of molecules in Exoplanet atmospheres

As next generation observation facilities become available for the characterisation of Exoplanet atmospheres, the capacity to move beyond the assumption of molecules existing in local thermodynamic equilibrium becomes more pressing. While atmospheric retrieval of Exoplanet atmospheres has operated under this assumption, it is known that many effects within planetary atmospheres drive molecules away from this equilibrium - into a state of non-local thermodynamic equilibrium, or non-LTE.  Here we revisit the bi-temperature model for approximating non-LTE molecule populations which uses a two temperature parameterisation: distinct rotational and vibrational temperatures. Here the implementation of this model is used in the molecular cross section generation code Exocross (Yurchenko et al. 2018) to further explore the differences arising in forward modelled spectra due to molecules existing in non-LTE.

Now we include an exploration of non-LTE in the the optical with the additional consideration of Titanium Oxide (TiO) - this diatomic exhibits bands of accentuated intensity when in non-LTE. This can be seen in the figure shown here - forward model examples for TiO in the atmosphere of WASP-76b with 3 transit observations simulated for JWST's NIRSpec instrument set to use its G140H grating. Here the divergence between spectra for the LTE and non-LTE cases can be seen to be particularly pronounced around 0.85 micron. 

In addition we show that the bi-temperature paramterisation is tractable for atmospheric retrieval. Here modifications were made to the publicly available atmospheric retrieval code TauREx 3 to include the additional temperature degree of freedom and non-LTE cross section data were generated using the Exocross code. This enables us to demonstrate the retrieval of both rotational and vibrational temperature parameters from JWST data simulated with under 10 transits.