Warm dense gas in the reflection nebula NGC 2023

A photodissociation region (PDR) with a known UV excitation source was studied by examining emission from atomic and molecular material. In agreement with models of externally excited PDRs, the photodissociation region southwest of HD 37903 contains both atomic and excited molecular material and arises close to the exciting star than does the bulk of the molecular cloud. The C(+) emission arises closest to the star but is 1.5 x 10 to the 17th cm or less from the peaks in the excited CO and fluorescent H2 emission regions. The (C-18)O J = 2 - 1 line, which traces molecular column density, has its emission peak about 4 x 10 to the 17th cm beyond the C(+) peak. Even in a source with an incident UV flux as low as about 1000 times the mean interstellar radiation field, there is a significant amount of CO 7 - 6 emission arising from regions with T(gas) greater than 85 K which is substantially greater than T(dust).