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D.M. Meyer, J.T. Lauroesch (Northwestern Univ.)
Using the DensePak fiber optic array on the KPNO WIYN telescope, we have obtained high S/N echelle spectra of the Na~I D wavelength region toward the central 27\arcsec~x~43\arcsec of the globular cluster M15 at a spatial resolution of 4\arcsec. The spectra exhibit significant interstellar Na~I absorption at LSR velocities of +3~km~s-1 (LISM~component) and +68~km~s-1 (IVC~component) which varies appreciably in strength on these scales. The derived Na~I column densities differ by a factor of 4 across the LISM~absorption map and a factor of 16 across the IVC~map. Assuming distances of 500~pc and 1500~pc for the LISM and IVC clouds, these maps show evidence of significant ISM structure down to the minimum scales of 2000~AU and 6000~AU probed in these absorbers. The smallest-scale N(Na~I) variations observed in the M15 LISM and IVC maps are typically comparable to or higher than the values found at similar scales in previous studies of interstellar Na~I structure toward binary stars. The physical implications of the small and larger-scale Na~I features observed in the M15 maps will be discussed in terms of variations in the H~I column density as well as the Na ionization equilibrium.
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