NON-LTE SPECTROSCOPY OF ETHYLENE (C2H4) IN THE 1.6 μm SPECTRAL RANGE AT 3 DIFFERENT ROTATIONAL TEMPERATURES TO UNDERSTAND THE ATMOSPHERES OF THE HOT JUPITER EXOPLANETS
Résumé
Accurate knowledge of the absorption fingerprint of the species expected in the atmospheres of the hot Jupiter
exoplanets is required to allow their detection. Methane [1] and acetylene [2] were already detected in such environments,
and the presence of other small hydrocarbons is expected. Therefore, it is necessary to provide unambiguous spectral
assignment for ethylene (C2H4), which is not available beyond 3500 cm−1, partly due to its highly congested absorption
spectrum at higher wavenumbers.
We first focus on providing accurate assignments of the ethylene cold transitions. To do so, the jet-cooled gas is
probed in the 1.65-1.68 nm spectral range using a cavity ring-down spectrometer. The low temperature is obtained by
expanding the gas contained in a high pressure reservoir into a low-pressure chamber through a slit. This process leads
to a simplification of the absorption spectrum by reducing drastically the rotational temperature and by narrowing the
absorption line widths, which causes the sample to be in non-local thermodynamic equilibrium (non-LTE). Three spectra
were recorded with a vibrational temperature of about 220K but different rotational temperatures: 5K, 35K and 75K.
Unambiguous assignment of the observed lines was performed using the TheoReTS ab initio line list [3].
[1] M. R. Swain, G. Vasisht, G. Tinetti, Nature, 463 (7281), 637-639 (2008)
[2] P. Giacobbe, et al., Nature, 592 (7853), 205-208 (2021)
[3] M. Rey, A. V. Nikitin, Y. L. Babikov, and V. G. Tyuterev, J. Mol. Spectrosc., 327, 138–158 (2016)