THE DISTRIBUTION, EXCITATION, AND ABUNDANCE OF C+, CH+, AND CH IN ORION KL

Abstract

The CH$^+$ ion was one of the first molecules identified in the interstellar gas over 75 years ago, and is postulated to be a key species in the initial steps of interstellar carbon chemistry. The high observed abundances of CH$^+$ in the interstellar gas remain a puzzle, because the main production pathway of CH$^+$, {it viz.}, $mathrm{C^{+} + H_{2} rightarrow CH^{+} + H}$, is so endothermic (4640~K), that it is unlikely to proceed at the typical temperatures of molecular clouds. One way in which the high endothermicity may be overcome, is if a significant fraction of the H$_2$ is vibrationally excited, as is the case in molecular gas exposed to intense far-ultraviolet radiation fields. Elucidating the formation of CH$^+$ in molecular clouds requires characterization of its spatial distribution, as well as that of the key participants in the chemical pathways yielding CH$^+$. Here we present high-resolution spectral maps of the two lowest rotational transitions of CH$^+$, the fine structure transition of C$^+$, and the hyperfine-split fine structure transitions of CH in a $sim 3^{prime} times 3^{prime}$ region around the Orion Kleinmann-Low (KL) nebula, obtained with the {em Herschel Space Observatory's} Heterodyne Instrument for the Far-Infrared (HIFI).footnote{These observations were done as part of the Herschel observations of EXtraordinary sources: the Orion and Sagittarius star-forming regions (HEXOS) Key Programme, led by E. A. Bergin at the University of Michigan, Ann Arbor, MI.} We compare these maps to those of CH$^+$ and C$^+$ in the Orion Bar photodissociation region (PDR), and discuss the excitation and abundance of CH$^+$ toward Orion KL in the context of chemical and radiative transfer models, which have recently been successfully applied to the Orion Bar PDR.footnote{Nagy, Z. et al. 2013, A&A 550, A96