First detection of the methylidyne cation (CH+) fundamental rotational line with the Herschel/SPIRE FTS

摘要

Aims. To follow the species chemistry arising in diverse sources of the Galaxy with Herschel. Methods. SPIRE FTS sparse sampled maps of the Orion bar & compact HIIregions G29.96-0.02 and G32.80+0.19 have been analyzed. Results. Beyond the wealth of atomic and molecular linesdetected in the high-resolution spectra obtained with the FTS of SPIREin the Orion Bar, one emission line is found to lie at the position ofthe fundamental rotational transition of CH+ as measuredprecisely in the laboratory by Pearson and Drouion. This coincidencesuggests that it is the first detection of the fundamental rotationaltransition of CH+. This claim is strengthened by theobservation of the lambda doublet transitions arising from itsrelative, CH, which are also observed in the same spectrum. The broadspectral coverage of the SPIRE FTS allows for the simultaneousmeasurement of these closely related chemically species, under the sameobserving conditions. The importance of these lines are discussed and acomparison with results obtained from models of the photon dominatedregion (PDR) of Orion are presented. The CH+ line alsoappears in absorption in the spectra of the two galactic compact HIIregions G29.96-0.02 and G32.80+0.19, which is likely due to thepresence of CH+ in the the cold neutral medium of thegalactic plane. These detections will shed light on the formationprocesses and on the existence of CH+, which are still outstanding questions in astrophysics. Key words: ISM: molecules - evolution - ISM: general - space vehicles: instruments - techniques: spectroscopic