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

Martini, P., Sellgren, K., and DePoy, D.L. 1999, ApJ, 526, 772

This paper presents near-infrared spectroscopy of fluorescent molecular hydrogen (H₂) emission from NGC 1333, NGC 2023, NGC 2068, and NGC 7023 and derives the physical properties of the molecular material in these reflection nebulae. These observations of NGC 2023 and NGC 7023 and the physical parameters derived for these nebulae are in good agreement with previous studies. Both NGC 1333 and NGC 2068 have no previously-published analysis of near-infrared spectra. This study reveals that the rotational-vibrational states of molecular hydrogen in NGC 1333 are populated quite differently from NGC 2023 and NGC 7023. We determine that the relatively weak UV field illuminating NGC 1333 is the primary cause of the difference. Further, the density of the emitting material in NGC 1333 is of much lower density, with n ~ 10² - 10⁴ cm^-3. NGC 2068 has molecular hydrogen line ratios more similar to those of NGC 7023 and NGC 2023. Model fits to this nebula show that the bright, H₂-emitting material may have a density as high as n ~ 10⁵ cm^-3, similar to what we find for NGC 2023 and NGC 7023.

Our spectra of NGC 2023 and NGC 7023 show significant changes in both the near-infrared continuum and H₂intensity along the slit and offsets between the peaks of the H₂ and continuum emission. These these brightness changes may correspond to real changes in the density and temperatures of the emitting region, although uncertainties in the total column of emitting material along a given line of sight complicates the interpretation. The spatial difference in the peak of the H₂ and near-infrared continuum peaks in NGC 2023 and NGC 7023 shows that the near-infrared continuum is due to a material which can survive closer to the star than H₂ can.