CONCLUSIONS

We have measured over 100 lines of H from 1-2.5m at two positions in the reflection nebula NGC 2023, selecting 35 for which reliable line fluxes and column densities can be inferred. These cover an energy range from 6000-45,000K above the ground state.

The emission is clearly fluorescent, with the gas in the northern location exhibiting a pure fluorescent spectrum, and that in the southern a collisional fluorescent spectrum. This latter is evident by an excess in the v=1 population over that for fluorescence, but with pure fluorescent populations for higher levels.

The molecular gas in NGC 2023 forms a clumpy photodissociation region, with the clumps at least as dense as the critical density (). Those in the southern location experience a sufficiently high far-UV radiation field that the dense gas exists in a hot self-shielded layer close to the cloud surface (A). Their excited high (v ) levels can be collisionally depopulated, while collisions can thermalise the populations in v=0 and v=1 levels.

The spectrum from 8000Å to 2.5m can be readily observed in NGC 2023, and the strengths of several hundred lines of H determined. Such a data set could be used for detailed investigations of the nebula and the excitation process. These could include (i) the determination of the ortho-para ratio and its variation with vibrational level, (ii) the derivation of the extinction to the emitting regions through minimising the scatter in a column density-energy level diagram, and (iii) the testing of fluorescent excitation models. Furthermore, the physical parameters it would yield will help to constrain the geometry of NGC 2023. Such an analysis forms the subject of the PhD dissertation of M. McCartney (1998), which makes use of the data presented here.

© Copyright Astronomical Society of Australia 1997