Detection of interstellar oxidaniumyl: Abundant H₂O⁺towards the star-forming regions DR21, Sgr B2, and NGC6334

Abstract

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 H 2 O + ground-state rotational transition, and the lack of other known low-energy transitions in this frequency range, identifies the feature as H 2 O + 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.