Near‐Infrared Spectroscopy of Molecular Hydrogen Emission in Four Reflection Nebulae: NGC 1333, NGC 2023, NGC 2068, and NGC 7023

We present near-infrared spectroscopy of fluorescent molecular hydrogen (H_2)emission from NGC 1333, NGC 2023, NGC 2068, and NGC 7023 and derive thephysical properties of the molecular material in these reflection nebulae. Ourobservations of NGC 2023 and NGC 7023 and the physical parameters we derive forthese nebulae are in good agreement with previous studies. Both NGC 1333 andNGC 2068 have no previously-published analysis of near-infrared spectra. Ourstudy reveals that the rotational-vibrational states of molecular hydrogen inNGC 1333 are populated quite differently from NGC 2023 and NGC 7023. Wedetermine that the relatively weak UV field illuminating NGC 1333 is theprimary cause of the difference. Further, we find that the density of theemitting material in NGC 1333 is of much lower density, with n ~ 10^2 - 10^4cm^-3. NGC 2068 has molecular hydrogen line ratios more similar to those of NGC7023 and NGC 2023. Our model fits to this nebula show that the bright,H_2-emitting material may have a density as high as n ~ 10^5 cm^-3, similar towhat we find for NGC 2023 and NGC 7023. Our spectra of NGC 2023 and NGC 7023show significant changes in both the near-infrared continuum and H_2 intensityalong the slit and offsets between the peaks of the H_2 and continuum emission.We find that these brightness changes may correspond to real changes in thedensity and temperatures of the emitting region, although uncertainties in thetotal column of emitting material along a given line of sight complicates theinterpretation. The spatial difference in the peak of the H_2 and near-infraredcontinuum peaks in NGC 2023 and NGC 7023 shows that the near-infrared continuumis due to a material which can survive closer to the star than H_2 can.