Astronomical CH$_3^+$ rovibrational assignments. A combined theoretical and experimental study validating observational findings in the d203-506 UV-irradiated protoplanetary disky

Abstract

Context: The methyl cation (CH₃⁺) has recently been discovered in the interstellar medium through the detection of 7 µm (1400 cm^-1) features toward the d203-506 protoplanetary disk by the JWST. Line-by-line spectroscopic assignments of these features, however, were unsuccessful due to complex intramolecular perturbations preventing a determination of the excitation and abundance of the species in that source. Aims: Comprehensive rovibrational assignments guided by theoretical and experimental laboratory techniques provide insight into the excitation mechanisms and chemistry of CH₃⁺ in d203-506. Methods: The rovibrational structure of CH₃⁺ was studied theoretically by a combination of coupled-cluster electronic structure theory and (quasi-)variational nuclear motion calculations. Two experimental techniques were used to confirm the rovibrational structure of CH₃⁺: 1. Infrared leak-out spectroscopy of the methyl cation; 2. Rotationally resolved photoelectron spectroscopy of the methyl radical (CH₃). Results: The quantum chemical calculations performed in this study have enabled a comprehensive spectroscopic assignment of the ν⁺₂ and ν⁺₄ bands of CH₃⁺ detected by the JWST. The resulting spectroscopic constants and derived Einstein A coefficients fully reproduce both the infrared and photoelectron spectra and permit the rotational temperature of CH₃⁺ (T = 660 ± 80 K) in d203-506 to be derived. A beam-averaged column density of CH₃⁺ in this protoplanetary disk is also estimated.

Marie-Aline Martin-Drumel

Researcher

My research interests focus on molecular spectroscopy of stable molecules and reactive species, and its applications for astrophysics and physical-chemistry.