Herschel observations of EXtra-Ordinary Sources (HEXOS): detecting spiral arm clouds by CH absorption lines
Qin, S.-L. and Schilke, P. and Comito, C. and Moller, T. and Rolff, R. and Muller, H.S.P. and Belloche, A. and Menten, K.M. and Lis, D.C. and Phillips , T.G. and Bergin, E.A. and Bell, T.A. and Crockett, N.R. and Blake, G.A. and Cabrit, S. and Caux, E. and Ceccarelli, C. and Cernicharo, J. and Daniel, F. and Dubernet, M.-L. and Emprechtinger, M. and Encrenaz, P. and Falgarone, E. and Gerin, M. and Giesen, T.F. and Goicoechea, J.R. and Goldsmith, P.F. and Gupta, H. and Herbst, E. and Joblin, C. and Johnstone, D. and Langer, W.D. and Lord, S.D. and Maret, S. and Martin, P.G. and Melnick, G.J. and Morris, P. and Murphy, J.A. and Neufeld, D.A. and Ossenkopf, V. and Pagani, L. and Pearson, J.C. and Perault, M. and Plume, R. and Salez, M. and Schlemmer, S. and Stutzki, J. and Trappe, N. and van der Tak, F.F.S. and Vastel, C. and Wang, S. and Yorke, H.W. and Yu, S. and Zmuidzinas, J. and Boogert, A. and Gusten, R. and Hartogh, P. and Honingh, E. and Karpov, A. and Kooi, J. and Krieg, J.-M. and Schieder, R. and Diez-Gonzalez, M.C. and Bachiller, R. and Martin-Pintado, J. and Baechtold, W. and Olberg, M. and Nordh, L.H. and Gill, J.L. and Chattopadhyay, G. (2010) Herschel observations of EXtra-Ordinary Sources (HEXOS): detecting spiral arm clouds by CH absorption lines. Astronomy & Astrophysics, 521 (L14). pp. 1-5. ISSN 0004-6361
We have observed CH absorption lines (J = 3/2, N = 1 ← J = 1/2, N = 1) against the continuum source Sgr B2(M) using the Herschel/HIFI instrument. With the high spectral resolution and wide velocity coverage provided by HIFI, 31 CH absorption features with different radial velocities and line widths are detected and identified. The narrower line width and lower column density clouds show “spiral arm” cloud characteristics, while the absorption component with the broadest line width and highest column density corresponds to the gas from the Sgr B2 envelope. The observations show that each “spiral arm” harbors multiple velocity components, indicating that the clouds are not uniform and that they have internal structure. This line-of-sight through almost the entire Galaxy offers unique possibilities to study the basic chemistry of simple molecules in diffuse clouds, as a variety of different cloud classes are sampled simultaneously. We find that the linear relationship between CH and H2 column densities found at lower AV by UV observations does not continue into the range of higher visual extinction. There, the curve flattens, which probably means that CH is depleted in the denser cores of these clouds.
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