Influence of molecular collisions on the atmospheric modelling: new data and updates in spectroscopic databases

The advent of new-generation space missions (such as WFIRST and ARIEL) as well as high-performance observatories both space-born and ground-based (such as JWST and ELTs) should start to answer fundamental questions about the formation, composition and properties of exoplanets. A reliable interpretation of the observations will rely on a huge amount of spectroscopic data, with a particular emphasis on high-resolution spectroscopic data.

Because of the extreme conditions characteristic of exoplanetary atmospheres (high temperatures and high levels of insolation), much information on molecular processes and their spectroscopic signatures is not available. In particular, lack of adequate procedures to measure or calculate collisional line-shape parameters for billions of transitions required limits severely the quality of the atmospheric modelling and retrievals. Line-shape profiles and pressure-broadening coefficients for “exotic” molecular species constitute a basis for cross-sections calculations which are further used to calculate atmospheric opacities. It was shown recently [1] that account of pressure-broadening effect on the spectral line shapes is essential to enhance our capacity for studying exoplanets. Several spectroscopic databases, such as HITRAN [2], GEISA [3], ExoMol [4], TheoReTS [5] and MoLList [6]) provide extensive line lists of positions and intensities of isolated spectral transitions, some of which contain many billions of lines.  However, the associated pressure-broadening and shift parameters, as well as their temperature dependences, remain poorly determined or completely missing. Therefore, there is a huge demand for robust theoretical approaches and estimates that could provide line-shape parameters for a large range of excitations covering wide ranges of temperatures and pressures and large variety of perturbers.

First steps in this direction have been made recently with developing at least approximate theoretical approaches [7] to getting pressure-broadening parameters for most infrared-active molecules detected or expected in exoplanetary atmospheres. Rotationally independent estimates of pressure-broadening coefficients have been generated for more than 50 active species (see Table) and 12 perturbers (Ar, CH₄, CO, CO₂, H₂, H₂O, He, N₂, NH₃, NO, O₂ and self). Full data sets have been put in a specifically designed for this purpose prototype database COLLINE [8] for testing, and one-value data have been selected for each molecular pair for including in the 2024 release [4] of ExoMol.

Table: Exomolecules with new line-broadening coefficients.

AlCl	AlH	AlO	AsH3	BeH	C3	CaF	CaH
CaO	CaOH	CH2	CH3	CP	CrH	CS	FeO
KCl	KF	KOH	LiCl	LiF	LiH	LiH+	MgF
MgH	MgO	NaCl	NaF	NaH	NaOH	NH	NS
PF3	PH	PN	PO	PO2	PS	ScH	SH
SH3	SiC	SiH	SiH2	SiH3	SiO	SiO2	SiS
TiF	TiH	TiO	VO

In this work, we describe the main features of newly produced line-broadening data, their organization in the COLLINE database and selection of some of them for including in ExoMol. We provide also some examples showing the influence of different pertubers on the cross-sections calculated for active molecules. Ways to further populating of databases for exoplanetary molecules will be outlined.

 

References

[1] J.J. Fortney, T.D. Robinson, S. Domagal-Goldman et al., arXiv:1905.07064 (2019)

[2] I.E. Gordon, L.S. Rothman, R.J. Hargreaves et al., J. Quant. Spectrosc. Radiat. Transfer 277, 107949 (2022)

[3] N. Jacquinet-Husson et al., J. Mol. Spectrosc. 327, 31 (2016)

[4] J. Tennyson, S.N. Yurchenko, J. Zhang et al., J. Quant. Spectrosc. Radiat. Transfer, under revision (2024)

[5] A.V. Nikitin, Y.L. Babikov, V. G. Tyuterev, J. Mol. Spectrosc. 327, 138 (2016)

[6] P.F. Bernath, J. Quant. Spectrosc. Radiat. Transfer 240, 106687 (2020)

[7] J. Buldyreva, S.N. Yurchenko, J. Tennyson, RASTI 1, 43 (2022)

[8] COLLINE database: https://colline.u-bourgogne.fr