Excitation of Interstellar Molecules by Near Infrared PAH Photons

Abstract

We have developed an excitation model for small molecules including 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, 5 micron < lambda < 20 micron, 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 have applied this model to H2O and NH3, ortho and para species. 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 < 10^5 cm-3, radiative pumping dominates the excitation of the rotation ladder, and the FIR lines are good probes of the molecule column density. The near infrared absorption and emission lines predicted to appear between 6 and 7 micron for H2O and from 10 to 12 micron for NH3 can also be used for this purpose. Concerning ortho-H2O, a quasi resonant pumping of the nu2 6.18 micron transition by photons of the 6.2 micron PAH band occurs. A strong de-excitation emission line is expected at 6.64 micron.