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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[7.23] Small-Scale Interstellar Na I Structure Toward M92

S.M. Andrews, D.M. Meyer, J.T. Lauroesch (Northwestern University)

We have used integral field echelle spectroscopy with the DensePak fiber-optic array on the KPNO WIYN telescope to observe the central 27\arcsec \times 43\arcsec of the globular cluster M92 in the Na I D wavelength region at a spatial resolution of 4\arcsec. Two interstellar Na I absorption components are evident in the spectra at LSR velocities of 0 km s-1 (Cloud 1) and -19 km s-1 (Cloud 2). Substantial strength variations in both components are apparent down to scales limited by the fiber-to-fiber separations. The DensePak coverage has been converted into a two-dimensional map of Na I absorption for each component. The derived Na I column densities differ by a factor of 4 across the Cloud 1 absorption map and by a factor of 7 across the Cloud 2 absorption map. Using distance upper limits of 400 and 800 pc for Cloud 1 and Cloud 2, respectively, the absorption maps indicate structure in the ISM down to scales of 1600 and 3200 AU. The smallest scale N(Na I) variations observed in the M92 maps are consistent with the values presented in a nearly identical study of the globular cluster M15 and those found at similar scales in previous observations of interstellar Na I structure toward binary systems. The physical interpretations of these variations are investigated, compared to previous studies, and discussed in terms of possible variations in the H I column density and the Na ionization equilibrium.

S.A. would like to thank the Northwestern University NASA - Illinois Space Grant Consortium Summer Program and the Weinberg College of Arts and Sciences for funding support.

The author(s) of this abstract have provided an email address for comments about the abstract: s-andrews2@northwestern.edu

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