Astronomical CH₃⁺ rovibrational assignments

A combined theoretical and experimental study validating observational findings in the d203-506 UV-irradiated protoplanetary disk^★

¹ Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA 02138, USA
e-mail: bryan.changala@cfa.harvard.edu
² Institut des Sciences Moléculaires d’Orsay, Université Paris-Saclay, CNRS, 91405 Orsay, France
e-mail: berenger.gans@universite-paris-saclay.fr
³ Radboud University, FELIX Laboratory, Institute for Molecules and Materials, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands
⁴ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁵ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, CNES, UPS, 9 Av. du colonel Roche, 31028 Toulouse Cedex 04, France
⁶ Institut de Chimie Physique, Université Paris-Saclay, CNRS, UMR8000, 91405 Orsay, France
⁷ Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
⁸ Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany
⁹ Department of Physics and Astronomy, The University of Western Ontario, London ON N6A 3K7, Canada
¹⁰ Institute for Earth and Space Exploration, The University of Western Ontario, London ON N6A 3K7, Canada
¹¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹² Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
¹³ Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA
¹⁴ Carl Sagan Center, SETI Institute, 339 Bernardo Avenue, Suite 200, Mountain View, CA 94043, USA

Received: 19 August 2023
Accepted: 11 October 2023

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, and (2) rotationally resolved photoelectron spectroscopy of the methyl radical (CH₃). In (1), CH₃⁺ ions, produced by the electron impact dissociative ionization of methane, were injected into a 22-pole ion trap where they were probed by the pulses of infrared radiation from the FELIX free electron laser. In (2), neutral CH₃, produced by CH₃NO₂ pyrolysis in a molecular beam, was probed by pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy.

Results. The quantum chemical calculations performed in this study have enabled a comprehensive spectroscopic assignment of the v₂⁺ and v₄⁺ 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.