First interstellar detection of OH⁺

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: [wyrowski;kmenten;rguesten;belloche]@mpifr-bonn.mpg.de

Received: 5 March 2010
Accepted: 24 May 2010

Abstract

The Atacama Pathfinder Experiment (APEX) 12 m telescope was used to observe the N = 1–0, J = 0–1 ground-state transitions of OH⁺ at 909.1588 GHz with the CHAMP+ heterodyne array receiver. Two blended hyperfine structure transitions were detected in absorption against the strong continuum source Sagittarius B2(M) and in several pixels offset by 18´´. Both absorption from Galactic center gas and absorption from diffuse clouds in intervening spiral arms in a velocity range from –116 to 38.5 km s^-1 is observed. The total OH⁺ column density of absorbing gas is 2.4 × 10¹⁵ cm^-2. A column density local to Sgr B2(M) of 2.6 × 10¹⁴ cm^-2 is found. On the intervening line-of-sight, the column density per unit velocity interval is in the range of 1 to 40 × 10¹² cm^-2/(km s^-1). OH⁺ is found to be on average more abundant than other hydrides, such as SH⁺ and CH⁺. Abundance ratios of OH and atomic oxygen to OH⁺ are found in the range of 10^1-2 and 10^3-4, respectively. The detected absorption of a continuous velocity range on the line-of-sight shows OH⁺ to be an abundant component of diffuse clouds.