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

TL;DR

This study detects H2O+ absorption in high-mass star-forming regions using Herschel/HIFI spectra, revealing its velocity profile and abundance, and linking it to the physical conditions of the regions.

Contribution

First detection of H2O+ absorption in star-forming regions with detailed velocity and abundance analysis, providing insights into interstellar chemistry and conditions.

Findings

H2O+ absorption detected at 1115 GHz in star-forming regions.

Velocity profiles of H2O+ match those of other tracers like [CII] and OH.

Column densities of H2O+ derived for NGC 6334, DR21, and Sgr B2.

Abstract

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. 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. In DR21, the velocity distribution of H2O+ matches that of the [CII] line at 158\mu\m 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 7.2e12 cm^-2 in NGC 6334, 2.3e13 cm^-2 in DR21, and 1.1e15 cm^-2 in Sgr B2.