**AAS 197, January 2001**

*Session 7. Gas in the Galactic ISM*

Display, Monday, January 8, 2001, 9:30am-7:00pm, Exhibit Hall
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## [7.10] The Importance of Nonlinear H_{2} Photoexcitation in Strongly Irradiated PDRs

*P. P. Sorokin (IBM Research Division), J. H. Glownia (IBM Research Division)*

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 ~q
31,500^{\circ}K, R ~q 16R _{\odot}), 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-\alpha 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-\alpha-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
H_{2} molecule in (X0, J^{\prime\prime}=1) via the above
two IRS processes is calculated to be about 2400 times
greater than the total rate of linear photoexcitation of an
H_{2} molecule in the same (X0, J^{\prime\prime} =1)
quantum level. No additional photonic mechanisms
(\textit{e.g.} trapping of photons via the combined effects
of elastic scattering and diffusion) are invoked to enhance
the Ly-\alpha photon density in the model.

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