Interstellar H₃⁺ line absorption toward LkHα 101

We present a detection of three lines of the H₃⁺ ion in the near-infrared spectrum of the Herbig Be star LkHα 101. H ₃⁺ is the principal initiator of gas-phase chemistry in interstellar clouds and can be used to constrain the ionization rate or the path length through interstellar material along the line of sight. Essentially all of the observed H₃⁺ column of (2.2 ± 0.3) × 10¹⁴ cm^-2 toward LkHα] 101 originates in the same dense, dark cloud; less than 1 mag of the ∼11 total magnitudes of visual extinction is attributable to diffuse material. Constraints on the density [1 × 10⁴ cm^-3 < n(H₂) < 4 × 10⁴ cm^-3], along with estimates of distances to the obscuring cloud, imply H₂ ionization rates of 6.7 × 10 ^-17 s^-1 < ζ < 2.7 × 10^-16 s^-1. The nondetection of fluorescent H₂ emission at the cloud surface implies that the obscuring dark cloud is located at least 20 pc from LkHα 101 and is therefore decoupled from its evolution. The confirmation of significant H₃⁺ in dense material addresses earlier concerns that H₃⁺ may originate in diffuse material surrounding dense clouds.