Abstract

The near UV spectra of OB stars are often dominated by a broad extinction band peaking at 2175A. Forty years after its discovery, the origin of this band remains unknown, although it is usually attributed to linear scattering or linear absorption by interstellar dust particles. Here we report that two-photon absorption (TPA) by H2 molecules in gaseous clouds enveloping OB stars should give rise to a strong extinction band peaking near 2175A. We first show that if the product of the H2 density in the gaseous cloud times the emitted stellar VUV flux is sufficiently great, the threshold for stimulated Rayleigh scattering (SRayS) will be exceeded, resulting in the generation of intense, monochromatic VUV light at the rest frame frequencies of H2 B- and C-state resonance lines originating from levels J”=0 and J”=1 of X0. This coherently generated light must necessarily propagate radially inwards towards the photosphere of the illuminating star, and therefore cannot be detected externally. However, this same light effectively constitutes intense “first step” monochromatic radiation that should induce continuum photons emitted by the OB star near 2175A to be strongly absorbed as “second steps” in resonantly-enhanced H2 two-photon transitions to two well known doubly-excited dissociative states of H2. With use of the MAST Scrapbook web resource, we now have found a very strong correlation to exist (in each of 215 separate OB stars we examined) between (i) the strength of the X.2175 band in a given star, and (ii) the extent to which H2 molecules in any gaseous cloud possibly enveloping the star are nonlinearly photoexcited via SRayS. This finding lends strong support to our suggestion that the λ2175 astronomical extinction band results from the combined action of the above mentioned H2 SRayS and TPA nonlinear photonic mechanisms.

© 2007 Optical Society of America