Excitation of Molecular Material Near the Young Stellar Object LkHalpha-234 in NGC 7129

Abstract

With the Spitzer-IRS we have obtained the first mid-IR spectroscopy of NGC 7129, in the unusually strong outflow and in a ridge of H_2 emission near the Herbig Be star LkHalpha-234. The UV radiation field strength is estimated from PAH band intensities in the H_2 ridge, and from the rotational H_2 emission lines we have deduced aperture average excitation temperatures and column densities in the two regions, finding the H_2 ridge values to be consistent with pumping by UV fluorescence, but also comparable to regions that form H_2 in non-dissociative C-shocks. The H_2 emission in the outflow is consistent with formation by collisional excitation in J-shocks, with shock velocities of 10 - 30 km/sec. 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 16um imaging of a Galactic nebula using the unique imaging capabilities of the IRS, and combine with ground-based 2.12um (H_2 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.