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Pumping of interstellar molecules by near infrared PAH photons
Giard, M. 1 (giard@cesr.cnes.fr), Crété, E. 1 - Puget, J.L. 2
1 CESR/CNRS, BP4346, F21029 Toulouse cedex, France -
2 IAS, Bat. 121, 91405 Orsay cedex, France
We have developed an excitation model for small molecules
which includes radiative pumping by
dust photons from near infrared to submillimeter wavelengths. This model
applies to molecules
within bright photodissociation regions in galactic
star-forming regions
and starburst galaxies.
In such environments, the near infrared photons
emitted by polycyclic aromatic hydrocarbon molecules (PAHs)
or small carbon grains,
, are able to penetrate molecular regions
and pump the molecules
in an excited vibration state. The far infrared and submillimeter
transitions
involved in the de-excitation cascade are strongly affected by this process.
Their intensities can be enhanced by several orders of magnitude.
We will show for H2O and
NH3 how the behaviour of the FIR rotation lines
with respect to the gas density and the molecule column
density is strongly modified compared to pure collisional models.
At moderate densities, nH2 < 105 cm-3,
radiative pumping dominates the excitation of the rotation ladder, and
the FIR lines are good probes of the molecule column density.
Concerning orthoH2O, a quasi resonant
pumping of the 
transition by photons of the 6.2 
PAH
band occurs.
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Postscript version