  Gas has been detected in a number of debris disk systems. This gas may have
arisen from grain sublimation or grain photodesorption. It interacts with the
surrounding dust grains through a number of charge and heat exchanges. Studying
the chemical composition and physical state of this gas can therefore reveal
much about the dust component in these debris disks. We have produced a new
code, ontario, to address gas emission from dusty gas-poor disks around A--F
stars. This code computes the gas ionization and thermal balance
self-consistently, with particular care taken of heating/cooling mechanisms.
Line emission spectra are then produced for each species (up to zinc) by
statistical equilibrium calculations of the atomic/ionic energy levels. For
parameters that resemble the observed beta Pictoris gas disk, we find that the
gas is primarily heated by photoelectric emission from dust grains, and
primarily cooled through the CII 157.7 micron line emission. The gas can be
heated to a temperature that is warmer than that of the dust and may in some
cases reach temperature for thermal escape. The dominant cooling line, CII
157.7 micron, should be detectable by Herschel in these disks, while the OI
63.2 micron line will be too faint. We also study the dependence of the cooling
line fluxes on a variety of disk parameters, in light of the much improved
sensitivity to thermal line emission in the mid/far infrared and at
sub-millimeter wavelengths provided by, in particular, Herschel, SOFIA and
ALMA. These new instruments will yield much new information about dusty debris
disks.
