PDR Model Mapping of Physical Conditions via Spitzer/IRS Spectroscopy of H₂: Theoretical Success toward NGC 2023-South

Abstract

We use the Infrared Spectrograph on Spitzer to observe the southern part of the reflection nebula NGC 2023, including the Southern Ridge, which is a photodissociation region (PDR) par excellence excited by HD 37903. Five pure-rotational H₂ emission lines are detected and mapped over and around the Southern Ridge in order to compare with predicted level column densities from theoretical PDR models. We find very good agreement between PDR model predictions and emission line intensities and ratios measured with Spitzer, leading us to conclude that grain photoelectric heating sufficiently warms the gas to produce the observed H₂ line emission via collisional excitation. On the Southern Ridge, we infer a hydrogen nucleus density n _H ≈ 2 × 10⁵ cm^-3 and radiation field strength χ ≈ 10⁴ relative to the local Galactic interstellar radiation field. This high value for χ independently predicts a distance toward HD 37903 of 300 pc and is consistent with the most recent Hipparcos results. Over the map we find that both n _H and χ vary by a factor of ~3. Such two-dimensional variations provide clues about the underlying three-dimensional structure of the Southern Ridge field, which appears to be the tip of a molecular cloud. We also map variations in excitation temperature and the ortho-to-para ratio, the latter attaining values of ~1.5-2.0 on the Southern Ridge, and find that PDR modeling can readily reproduce observed ortho-to-para ratios that are <3 for rotational excitation dominated by collisional processes. Last, the stars Sellgren C and G are discovered to be resolved on archival Hubble Space Telescope images into two point sources each, with separations of lsim0farcs5.