Princeton University Observatory, Peyton Hall, Princeton, NJ 08544-1001 USA
Photodissociation regions (PDRs), or photodissociation fronts, are the boundary layers between cool molecular gas and diffuse regions with ultraviolet radiation which can photodissociate molecular hydrogen.
The properties of a PDR are determined primarily by
the gas density,
the flux of
Å
photons; and
the flux of
Å
photons.
The gas density itself is influenced by the flux of
Å
photons, since the pressure in the ionization front
can drive a shock wave into the
molecular cloud, compressing the gas prior to arrival of the
photodissociation front (Bertoldi & Draine 1996, ApJ 458, 222).
The structure of a PDR is determined by a number of physical processes, including: advection of gas through the PDR; radiative transfer; chemical processes, including formation of H2 on grain surfaces; heating of the gas by a variety of processes; and cooling of the gas by radiation from vibrationally-excited levels of H2, and fine-structure levels of OI, CII, SiII, and other species.
Ground-based observations of NGC 2023 already provided evidence of
high gas temperatures in PDRs (Draine & Bertoldi 1996, ApJ 468, 269 ),
but the definitive evidence has come from
ISO SWS observations of emission from rotationally-excited levels of H2.
Observations of S140 (Timmerman et al. 1996; A&A 315, L81)
and other PDRs revealed that these PDRs contain
zones where H2 is present and the kinetic temperature of the gas
is in the range 500-1000K.
This result was unexpected, as theoretical models had suggested that
the gas temperatures in regions where H2 was abundant should be
K.
There are a number of theoretical challenges confronting us. Perhaps foremost among them is the need to account for the hot gas required to account for the observed rotational excitation of the H2. Because of the potent cooling by rotionally-excited H2, a substantial increase in the heating rate (or a substantial decrease in the cooling rate) is required to raise temperatures from, say, 300K to 600K. Evidently we have not yet got the physics right for one or more heating or cooling processes!
The structure of PDRs will be reviewed, concentrating on the H/H2transition and the processes determining the temperature in this region. Dominant heating processes include: (1) kinetic energy of newly-formed H2; (2) collisional deexcitation of vibrationally-excited H2 produced by UV pumping; (3) photoelectric emission from dust grains. All of these processes depend directly or indirectly on the abundance of dust grains. Weingartner & Draine (this volume) have suggested a mechanism for altering the abundances of dust grains in PDRs; the thermal effects of such concentration will be discussed. Uncertainties in the dust grain physics will also be discussed.