PDRs4All XI. Detection of infrared CH+ and CH3+ rovibrational emission in the Orion Bar and disk d203-506: evidence of chemical pumping
Abstract
The methylidyne cation (CH+) and the methyl cation (CH3+) are building blocks of organic molecules, yet their coupled formation and excitation mechanisms remain mainly unprobed. The James Webb Space Telescope (JWST), with its high spatial resolution and good spectral resolution, provides unique access to the detection of these molecules. Our goal is to use the first detection of CH+ and CH3+ rovibrational emission in the Orion Bar and in the protoplanetary disk d203-506, irradiated by the Trapezium cluster, to probe their formation and excitation mechanisms and constrain the physico-chemical conditions. We use spectro-imaging acquired using both the NIRSpec and MIRI-MRS instruments to study the CH+ and CH3+ spatial distribution at very small scales, and compare it to excited H2 emission. CH+ and CH3+ emissions originate from the same region as highly excited H2. Our comparison between the Bar and d203-506 reveals that both CH+ and CH3+ excitation and/or formation are highly dependent on gas density. The excitation temperature of the observed CH+ and CH3+ rovibrational lines is around T ~ 1500 K in the Bar and T ~ 800 K in d203-506. Moreover, the column densities derived from the rovibrational emission are less than 0.1 % of the total known (CH+) and expected (CH3+) column densities. These results show that CH+ and CH3+ level populations strongly deviate from ETL. CH+ rovibrational emission can be explained by chemical formation pumping with excited H2 via C+ + H2* = CH+ + H. These results support a gas phase formation pathway of CH+ and CH3+ via successive hydrogen abstraction reactions. However, we do not find any evidence of CH2+ emission in the JWST spectrum. Finally, observed CH+ intensities coupled with chemical formation pumping model provide a diagnostic tool to trace the local density.
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