First interstellar detection of OH+

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 kms(-1) is observed. The total OH+ column density of absorbing gas is 2.4 x 10(15) cm(-2). A column density local to Sgr B2(M) of 2.6 x 10(14) cm(-2) is found. On the intervening line-of-sight, the column density per unit velocity interval is in the range of 1 to 40x10(12) cm(-2)/(kms(-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.