Near-infrared spectroscopy of molecular hydrogen emission in four reflection nebulae: NGC 1333, NGC 2023, NGC 2068, and NGC 7023

This paper presents near-infrared spectroscopy of fluorescent molecular hyd rogen (H-2) emission from NGC 1333, NGC 2023, NGC 2068, and NGC 7023 and de rives the physical properties of the molecular material in these reflection nebulae. These observations of NGC 2023 and NGC 7023 and the physical para meters derived for these nebulae are in good agreement with previous studie s. Both NGC 1333 and NGC 2068 have no previously published analysis of near -infrared spectra. This study reveals that the rotational-vibrational state s of molecular hydrogen in NGC 1333 are populated quite differently from NG C 2023 and NGC 7023. We determine that the relatively weak UV field illumin ating 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 simi lar to 10(2)-10(4) 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, H-2-emitting material may have a density as high as n simi lar to 10(5) cm(-3), similar to NGC 2023 and NGC 7023. Our spectra of NGC 2 023 and NGC 7023 show significant changes in both the near-infrared continu um and H-2 intensity along the slit and offsets between the peaks of the H- 2 and continuum emission. These brightness changes may correspond to real c hanges in the density and temperatures of the emitting region, although unc ertainties in the total column of emitting material along a given line of s ight complicates the interpretation. The spatial difference in the peak of the H-2 and near-infrared continuum peaks in NGC 2023 and NGC 7023 shows th at the near-infrared continuum is due to a material which can survive close r to the star than H-2 can.