Combining spectral databases to simulate molecular absorption in diverse exoplanetary atmospheres

Molecular features in the spectra of exoplanet atmospheres are diagnostics for the physical and chemical conditions present and are essential elements in understanding planet formation and evolution. The Planetary Spectrum Generator contains all of the elements required these spectra. Aside from the modulations by the atmosphere, this includes stellar fluxes, geometrical considerations, as well as telescope and detector characteristics to assess the noise level of the observations. Molecular contributions in atmospheric emission or transmission spectra, are calculated using molecular energy levels, transition strengths, and line broadening parameters. Combining spectroscopic information from several databases is required to enable accurate calculations across the parameter space where exoplanetary atmospheres are found. 

Over the recent years, we have collected line broadening parameters, state and transitions energies, and line intensities from HITRAN, HITEMP and ExoMol to enable the calculation of absorption spectra in all types of environments, ranging from N₂-broadened such as the Earth, to CO₂-broadened Mars, and hydrogen/helium broadened hot-Jupiters and brown dwarfs. The combination of databases allows calculations of molecular absorption at high temperatures and at high energies (i.e., into the near-infrared and visible).

In our previous work [Kofman and Villanueva 2021], the ExoMol line lists of H₂O and HDO were ingested. The need of using more the more complete theoretical databases was demonstrated in high-temperature exoplanet atmospheres, as well as in the detectability of HDO, which absorbs in an absorbance minimum of H₂O. Figure 1 shows the absorbance of water at 880 K. Absorption was calculated using both HITRAN and the variational Pokazatel/VTT databases at the natural abundances of the isotopes. Note that the HDO absorbance maximum coincides with a minimum in H₂O, which is a region where HITRAN is relatively incomplete for higher temperatures.  

In atmospheres where water has a significant presence an important component of the absorbance is the water continuum, described by the MT_CKD formalism. Although not as strong as the typical ro-vibrational features, the continuum adds a significant amount of opacity between the main features, which is especially relevant when considering that this is where one often searches for other molecules. We show that the opacity can be well described using typical collision induced absorption parameterization and demonstrate its importance. Figure 1 shows the contrast ratio of GJ 1214b transiting in front of its host star, with MT_CKD-based H₂O-H₂O collisional induced absorption neglected/included. For this simulation, the atmosphere consists of 83% He, 16% H2, 670 ppm H₂O, and 485 ppm CH₄.

In this talk, in addition to the cases described above, we will highlight where in the parameters space of exoplanet atmospheres the new additions are particularly relevant. We will look at warm transiting exoplanets, water worlds, and hot-Jupiters at high-resolution using cross-correlation studies.