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Session 51 - Interstellar Medium II.
Display session, Thursday, January 08
Exhibit Hall,
The VUV spectra of many B-type giant and supergiant stars whose lines-of-sight are not intersected by dark interstellar clouds were recorded in the range 1000-1200 Åby the Copernicus satellite in the mid-70's and in the range 900-1160 Åwith the ORFEUS-1 spectrometer during the September '93 mission of the Discovery space shuttle. From inspection of these spectra, which vary little from star to star, one concludes that the observed relative absorption intensities of individual vibronic components of the H_2 Lyman-band (B\leftarrowX) and Werner-band (C\leftarrowX) systems cannot at all be comprehended on the basis of linear spectroscopy. Furthermore, the /sim100 cm^-1- wide linewidths of the strong emission bands seen in these spectra cannot be explained in terms of normal H_2 fluorescence. We explain both of these apparent anomalies by proposing that the H_2 molecules are located in a cloud that is close enough to the star to allow the latter's VUV blackbody continuum to induce broadband stimulated Raman scattering (SERS). A natural consequence of SERS is the presence of lightwave coherence. Under such conditions, further coherent coupling or wave mixing is expected. In particular, four-wave parametric oscillation (FWPO) occurring on many of the H_2 levels can be invoked to explain both the regions of absorption and the dominant emission peaks appearing in these spectra. We show that the frequencies, intensities, and linewidths of these spectral features are consistent with coherent optical processes in H_2.
