The first ISO workshop on analytical spectroscopy with SWS, LWS, PHT-S and CAM-CVF

(Oct 6-8, 1997, Madrid, Spain)

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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$\mu$m, and [CII]158$\mu$m. These observations, in conjunction with ground-based imaging and theoretical modeling, now enable us to address outstanding problems such as the

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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Postscript version