Ossenkopf, V. and Muller, H.S.P. and Lis, D.C. and Schilke, P. and Bell, T.A. and Bruderer, S. and Bergin, E. and Ceccarelli, C. and Comito, C. and Stutzki, J. and Bacman, A. and Baudry, A. and Benz, A.O. and Benedettini, M. and Berne, O. and Blake, G. and Boogert, A. and Bottinelli, S. and Boulanger, F. and Cabrit, S. and Caselli, P. and Caux, E. and Cernicharo, J. and Codella, C. and Coutens, A. and Crimier, N. and Crockett, N.R. and Daniel, F. and Demyk, K. and Dieleman, P. and Dominik, C. and Dubernet, M.-L. and Emprechtinger, M. and Encrenaz, P. and Falgarone, E. and France, K. and Fuente, A. and Gerin, M. and Giesen, T.F. and Di Giorgio, A.M. and Goicoechea, J.R. and Goldsmith, P.F. and Gusten, R. and Harris, A. and Helmich, F. and Herbst, E. and Hily-Blant, P. and Jacobs, K. and Jacq, T. and Joblin, Ch. and Johnstone, D. and Kahane, C. and Kama, M. and Klein, T. and Klotz, A. and Kramer, C. and Langer, W. and Lefloch, B. and Leinz, C. and Lorenzani, A. and Lord, S.D. and Maret, S. and Martin, P.G. and Martin-Pintado, J. and McCoey, C. and Melchior, M. and Melnick, G.J. and Menten, K.M. and Mookerjea, B. and Morris, P. and Murphy, J.A. and Neufeld, D.A. and Nisini, B. and Pacheco, S. and Pagini, L. and Parise, B. and Pearson, J.C. and Perault, M. and Phillips , T.G. and Plume, R. and Quin, S.-L. and Rizzo, R. and Rollig, M. and Salez, M. and Saraceno, P. and Schlemmer, S. and Simon, R. and Schuster, K. and van der Tak, F.F.S. and Tielens, A.G.G.M. and Teyssier, D. and Trappe, N. and Vastel, C. and Viti, S. and Wakelam, V. and Walters, A. and Wang, S. and Whyborn, N. and van der Wiel, M. and Yorke, H.W. and Yu, S. and Zmuidzinas, J.
Detection of interstellar oxidaniumyl: Abundant H2O+ towards
the star-forming regions DR21, Sgr B2, and NGC6334.
Astronomy & Astrophysics, 518 (L.111).
Aims. We identify a prominent absorption feature at 1115 GHz, detected in first HIFI spectra towards high-mass star-forming regions, and interpret
its astrophysical origin.
Methods. The characteristic hyperfine pattern of the H2O+ ground-state rotational transition, and the lack of other known low-energy transitions in
this frequency range, identifies the feature as H2O+ absorption against the dust continuum background and allows us to derive the velocity profile
of the absorbing gas. By comparing this velocity profile with velocity profiles of other tracers in the DR21 star-forming region, we constrain the
frequency of the transition and the conditions for its formation.
Results. In DR21, the velocity distribution of H2O+ matches that of the [Cii] line at 158 μm and of OH cm-wave absorption, both stemming
from the hot and dense clump surfaces facing the Hii-region and dynamically affected by the blister outflow. Diffuse foreground gas dominates
the absorption towards Sgr B2. The integrated intensity of the absorption line allows us to derive lower limits to the H2O+ column density of
7.2 × 1012 cm−2 in NGC 6334, 2.3 × 1013 cm−2 in DR21, and 1.1 × 1015 cm−2 in Sgr B2.
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