The Importance of Nonlinear H2 Photoexcitation in Strongly Irradiated PDRs

Details The Importance of Nonlinear H2 Photoexcitation in Strongly Irradiated PDRs The Importance of Nonlinear H2 Photoexcitation in Strongly Irradiated PDRs

Dec 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000aas...197.0710s&link_type=abstract

American Astronomical Society, 197th AAS Meeting, #07.10; Bulletin of the American Astronomical Society, Vol. 32, p.1403

Astronomy and Astrophysics

Astronomy

Scientific paper

It is shown that, under sufficiently intense OB-star illumination of a stationary photoexcitation front (PDR), the transition rates of nonlinear H 2 photoexcitation processes can far exceed the rates of H 2 linear photoexcitation. A one-dimensional PDR, irradiated by light from a 0.1-pc-distant B0 III star (T ~= 31,500oK, R ~= 16R sun), is considered. Ionizing radiation from the star creates a thin H II region on the PDR surface. Within this H II region, roughly two-thirds of the incident ionizing photons are converted into Ly-α photons, with frequencies spread out to an estimated 20-cm-1 width via elastic scattering by H atoms occurring in the ionized region. It is assumed that half of these photons enter the neutral region and are thus able to drive two Ly-α -resonant, Inverse Raman Scattering (IRS) processes which result in light being nonlinearly absorbed around the transitions B9-0P1 and B3-0R1. The total rate of nonlinear photoexcitation of an H2 molecule in (X0, J''=1) via the above two IRS processes is calculated to be about 2400 times greater than the total rate of linear photoexcitation of an H2 molecule in the same (X0, J'' =1) quantum level. No additional photonic mechanisms (e.g.} trapping of photons via the combined effects of elastic scattering and diffusion) are invoked to enhance the Ly-α photon density in the model.

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