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.Results. In DR21, the velocity distribution of H 2 O + matches that of the [C ii] line at 158 μm and of OH cm-wave absorption, both stemming from the hot and dense clump surfaces facing the H ii-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 H 2 O + column density of 7.2 × 10 12 cm −2 in NGC 6334, 2.3 × 10 13 cm −2 in DR21, and 1.1 × 10 15 cm −2 in Sgr B2.

2018 Census of Interstellar, Circumstellar, Extragalactic, Protoplanetary Disk, and Exoplanetary Molecules

“…H 2 O + was identified in Herschel /HIFI spectra toward DR21, Sgr B2, and NGC 6334 by Ossenkopf et al (2010). Six hyperfine components of the N Ka,Kc = 1 1,1 → 0 0,0 , J = 3/2 → 1/2 transition in the 2 B 1 electronic ground state at 1.115 THz were observed.…”

HerschelCHESS discovery of the fossil cloud that gave birth to the Trapezium and Orion KL

“…This is most likely due to the uncertainties in the rest frequency measurements reported in the literature for H 2 O + (see, e.g. Table 1 in Ossenkopf et al 2010). Here we have taken the rest frequencies from Mürtz et al 1998. As mentioned above, OH + is not detected towards OMC-3 MM6 down to the noise level of the observations.…”

Low temperature rates for key steps of interstellar gas-phase water formation

“…OH + was first detected using the Atacama Pathfinder Experiment 12m telescope toward the massive star-forming region Sagittarius B2(M) ( 6 ). H 2 O + was found using the Herschel satellite toward the star-forming regions DR21, Sagittarius B2(M), and NGC 6334 ( 7 ). The protonated water molecule H 3 O + was first detected much earlier, using the National Radio Astronomy Observatory 12m telescope toward Orion KL and OMC-1 ( 8 , 9 ).…”