N₂H⁺ in Orion: Chemical clues to the dynamics of the quiescent gas

Measurements of the J = 1 → 0 transition of N₂H⁺ have been obtained toward OMC-1 with high spectral resolution, using the NRAO 12 m telescope. In contrast with HCO⁺, N₂H⁺ spectra exclusively exhibit the quiescent extended ridge gas toward Orion-KL. In fact, N₂H⁺ appears to diminish in column density near the KL/IRc2 region, where many other molecules are quite abundant. Toward KL, the spectrum of N₂H⁺ exhibits two narrow velocity components. One component lies to the northeast of Orion-KL with V_LSR ∼9.5 km s^-1, and the other lies southwest of KL with V_LSR ∼7.2 km s^-1. These data suggest that two clouds may be present in the region. Thus, N₂H⁺ appears to be an excellent tracer of ambient, quiescent material. The velocity structure observed in this molecule toward KL may be related to the star formation observed in the Orion region.