H2 DIBs Model Revisited

A theory advanced by the authors several years ago that H2 molecules located in photon dominated regions (PDRs) surrounding bright stars are the carriers of the diffuse interstellar absorption bands (DIBs) is reexamined in the light of a recently proposed hypothesis that line-driven acceleration of select species ions in the stellar winds of OB-type stars can occur via the nonlinear process of stimulated Rayleigh scattering. In the model describing the latter process, radially-outwards-directed acceleration of an ion species possessing a resonance transition at occurs simultaneously with generation of intense, relatively monochromatic, radially-inwards-propagating, coherent light at this same frequency, with continuum light from the star over a wide spectral range (blueshifted with respect to ) providing the necessary pump power to drive the stimulated scattering process. To explain how DIBs could originate, this model is now modified as follows. Replacing the stellar wind ions of the original model are H2 molecules and H atoms located in a PDR close to an illuminating star. Both neutral species become nonlinearly accelerated radially outwards from the star via stimulated Rayleigh scattering, and concomitant generation of intense, radially-inwards-propagating, monochromatic coherent light occurs at Ly- and at select H2 transition frequencies. However, unlike what happens to ions in a strong OB-type stellar wind, the accelerations received by both the H2 molecules and H atoms are not sufficient to allow their escape from the star’s gravitational field, and they eventually fall back to the base of the PDR. As the neutral species particles approach the latter, they become decelerated by the same stimulated Rayleigh scattering processes thataccelerated them radially outwards - this time, however, with redshifted continuum light from the star being absorbed. Again, intense monochromatic laser light aimed at the star at Ly- and at select H2 frequencies is concomitantly produced. This dynamic process continually repeats itself. With a rational basis thus provided for postulating the presence within a PDR of intense, monochromatic, coherent VUV radiation at Ly- and at specified H2 frequencies, it becomes much simpler both to identify and to justify those nonlinear photonic mechanisms which should lead to selective absorption occurring at DIB frequencies on transitions between H2 excited states. Furthermore, on the basis of the dynamic PDR model here postulated, one can (via the Doppler effect) plausibly account for the random juxtaposition of broad and narrow DIBs that occurs throughout the entire DIB spectrum. In essence, this arises because H2 molecules in levels with different rotational quantum numbers become nonlinearly accelerated/decelerated by different amounts.

By: P. P. Sorokin; J. H. Glownia

Published in: RC23641 in 2005


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