Excitation of Molecular Material near the Young Stellar Object LkHα 234 in NGC 7129

With the Spitzer Infrared Spectrograph (IRS) we have obtained the first mid-IR spectroscopy of NGC 7129, in the unusually strong outflow and in a ridge of H₂ emission near the Herbig Be star LkHα 234. The UV radiation field strength is estimated from polycyclic aromatic hydrocarbon (PAH) band intensities in the H₂ ridge and found to be comparable to that of NGC 7023. From the rotational H₂ emission lines we have deduced aperture average excitation temperatures and column densities in the two regions, finding the H₂ ridge values to be consistent with pumping by UV fluorescence, but also comparable to warm-gas regions of Cep A that form H₂ in nondissociative C-shocks. The H₂ emission in the outflow is consistent with formation by collisional excitation in J-shocks, with shock velocities of 10-30 km s^-1. A photodissociating component may be present in the outflow, by similarity of S(0) line intensities in both regions. There is no indication of warm dust in the outflow. We also present the first 16 μm imaging of a Galactic nebula using the unique imaging capabilities of the IRS, combined with ground-based 2.12 μm [H₂ 1-0 S(1)] imaging. Candidate pre-main-sequence objects are clearly evident in these data. We also find extended emission not previously observed around the young B star BD +65°1638, ~22" across, showing that the region is not free of material, as otherwise inferred by recent high angular resolution mapping at submillimeter wavelengths. The presence of this material complicates interpretation of the surrounding CO cavity and origin(s) of the photodissociated region, and further spectroscopic observations are needed to characterize its nature.