We report the first detection of the ground-state rotational transition of the methylidyne cation CH+ towards the massive star-forming region DR21 with the HIFI instrument onboard the Herschel satellite. The line profile exhibits a broad emission line, in addition to two deep and broad absorption features associated with the DR21 molecular ridge and foreground gas. These observations allow us to determine a CH+ J=1-0 line frequency of 835137 +/- 3 MHz, in good agreement with a recent experimental determination. We estimate the CH+ column density to be a few 1e13 cm^-2 in the gas seen in emission, and > 1e14 cm^-2 in the components responsible for the absorption, which is indicative of a high line of sight average abundance [CH+]/[H] > 1.2x10^-8. We show that the CH+ column densities agree well with the predictions of state-of-the-art C-shock models in dense UV-illuminated gas for the emission line, and with those of turbulent dissipation models in diffuse gas for the absorption lines.
@article{arxiv.1007.1420,
title = {Strong CH+ J=1-0 emission and absorption in DR21},
author = {E. Falgarone and V. Ossenkopf and M. Gerin and P. Lesaffre and B. Godard and J. Pearson and S. Cabrit and Ch. Joblin and A. O. ~Benz and F. Boulanger and A. Fuente and R. Güsten and A. Harris and T. Klein and C. Kramer and S. Lord and P. Martin and J. Martin-Pintado and D. Neufeld and T. G. ~Phillips and M. Röllig and R. Simon and J. Stutzki and F. van der Tak and D. Teyssier and H. Yorke and N. Erickson and M. Fich and W. Jellema and A. Marston and C. Risacher and M. Salez and F. Schmülling},
journal= {arXiv preprint arXiv:1007.1420},
year = {2015}
}