Photodissociation regions (PDRs) are described in the context of the interstellar medium and star-forming regions. Observations of PDRs and molecular hydrogen are reviewed and the reflection nebula NGC 2023 is discussed in detail. NGC 2023 is a bright and well-studied reflection nebula at a distance of 450 parsecs in the Orion region. Illuminated primarily by a B-type star, it offers an ideal opportunity to study UV-excited molecular hydrogen.
The theory of the hydrogen molecule is described: the energy states and their relationship with the quantum numbers which represent the vibrational and rotational states of the molecule, the radiative processes which determine the optical and infrared emission spectrum of H2, the effect collisions have on the excited states of the molecule and the processes which govern the formation and destruction of H2. Particular attention is given to the process of formation on the surface of dust grains and the resulting energy states of the ejected H2 molecule.
nfrared and optical far-red observations of fluorescent H2 line emission from NGC 2023 are presented. The resulting datasets contain flux measurements of over ninety lines. These are combined with published data to produce column densities for 81 energy states of the H2 molecule, the most extensive dataset yet compiled for a PDR. The processes of observing in the infrared and optical wavelength regimes are outlined. The emission lines of H2 are intrinsically very faint and thus measurements require careful data reduction to minimise sources of noise wherever possible. The data reduction steps which were applied to the observations are described in detail. An optical extinction of Av = 5.7 ± 1.0 to the H2 emission region and ortho/para abundance ratio of 2.0 ± 0.2 are derived from flux ratios of emission lines and by minimising the scatter on a diagram which plots the logarithm of the column density against the energy level of each state.
The theory of photodissociation regions is summarised and the effect of the incident ultraviolet radiation field and hydrogen nucleus number density on the H2 emission spectrum is discussed. Steady-state and time-dependent PDR models are reviewed and their predictions are compared to the observational data. Deviations from the pure fluorescent case are apparent; the v = 1 vibrational level displays a column density distribution indicative of collisional processes. There is also strong evidence for a trend within each vibrational level, for v > 1, whereby excess populations increase with rotational level, J, with a possible ‘turnover’ and peak excess at high J. The overall intensity of the H2 emission lines is an order of magnitude greater than that predicted by face- on, steady-state PDR models. The emission can be explained by intense ultraviolet radiation (Go _ 5 x 103) incident on dense gas (n ~ 105cm~3) at a high temperature (T ~ 1000K). The ortho/para abundance ratio is discussed and it is proposed that the observed PDR in NGC 2023 has not yet reached equilibrium and the photodissociation front, which is advancing into the molecular cloud, is more accurately described by time-dependent PDR models.