First interstellar detection of OH+
  The Atacama Pathfinder Experiment (APEX) 12m 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 as well as absorption from diffuse clouds in intervening spiral arms
in a velocity range from -116 to 38.5 km/s is observed. The total OH+ column
density of absorbing gas is 2.4 \times 10^15 cm-2. A column density local to
Sgr B2(M) of 2.6 \times 10^14 cm-2 is found. On the intervening line-of-sight
the column density per unit velocity interval are in the range from 1 to 40
\times 10^12 cm-2/(km/s). 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 to be 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.
