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ISO: a Novel Look at the Photodissociated Surfaces of Molecular Clouds
Frank Bertoldi
Max-Planck-Institut für Extraterrestrische Physik, Garching
Photodissociation regions (PDRs) form at molecular cloud boundaries wherever
the far-UV radiation strikes the cloud surface. They are transition
layers between the dense, cold molecular gas and the tenuous, warm,
ionized gas. The UV radiation here governs the
chemical structure by dissociating H2, CO, and other molecules, by ionizing
carbon, silicon, or iron, e.g., and by heating
the gas, mostly via photoelectric emission, to temperatures of
order 102 - 103 K.
PDRs are responsible for reprocessing much of the energy output from
massive stars, reemitting this energy at other wavelengths, including
a rich mixture of infrared lines. Especially through many
lines not observable from the ground, ISO provides a window into the
star formation environment, revealing the properties of the
molecular gas near newly-formed massive stars, and clarifying
important features in the overall spectrum of galaxies
with significant star formation, particularly starburst galaxies.
ISO now yields beautiful spectra from various PDRs
(S140, M17, NGC 7023, IC 1396, RCW 103, Orion ... ) of
H2 rotational and rotation-vibrational
transition lines and of fine structure
lines such as the major
coolants [OI]63,146
m, and [CII]158m.
These observations, in conjunction
with ground-based imaging and theoretical modeling, now enable
us to address outstanding problems such as the
- density structure and degree of clumpiness of gas in molecular clouds,
- temperature structure in PDRs, now indicating that the temperature in the H/H2 transition region is higher (>500K) than previously thought,
- abundances of elements such as oxygen, carbon, silicon, iron,
- effects of advection of gas through the PDR,
- H2 formation rate coefficient at T> 100 K,
- level distribution of newly formed H2,
- heating yield of photoelectron emission off dust grains,
- ortho-H2 to para-H2 abundance ratio,
- H-H2 collisional excitation rate coefficients,
- nature of the extended warm molecular gas component.
Observations of a PDR near the supernova remnant RCW 103 may illuminate how molecular gas is excited and dissociated by X-rays, which is of great importance in starbursts and AGN.
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