Submillimeter absorption from SH+, a new widespread interstellar
  radical, 13CH+ and HCl
  We have used the Atacama Pathfinder Experiment 12 m telescope (APEX) to carry
out an absorption study of submillimeter wavelength rotational ground-state
lines of H35Cl, H37Cl, 13CH+, and, for the first time, of the SH+ radical
(sulfoniumylidene or sulfanylium). We detected the quartet of ground-state
hyperfine structure lines of SH+ near 683 GHz with the CHAMP+ array receiver
against the strong continuum source Sagittarius B2, which is located close to
the center of our Galaxy. In addition to absorption from various kinematic
components of Galactic center gas, we also see absorption at the radial
velocities belonging to intervening spiral arms. This demonstrates that SH+ is
a ubiquitous component of the diffuse interstellar medium. We do not find clear
evidence for other SH+ lines we searched for, which is partially due to
blending with lines from other molecules. In addition to SH+, we observed
absorption from H35Cl, H37Cl, and 13CH+. The observed submillimeter absorption
is compared in detail with absorption in 3 mm transitions of H13CO+ and c-C3H2
and the CO J = 1-0 and 3-2 transitions.
