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Recent high spatial and spectral resolution investigations of the diffuseinterstellar medium (ISM) have found significant evidence for small-scalevariations in the interstellar gas on scales less than or equal to 1 pc. Tobetter understand the nature of small-scale variations in the ISM, we have usedthe KPNO WIYN Hydra multi-object spectrograph, which has a mapping advantageover the single-axis, single-scale limitations of studies using high propermotion stars and binary stars, to obtain moderate resolution (~12 km/s)interstellar Na I D absorption spectra of 172 stars toward the double opencluster h and Chi Persei. All of the sightlines toward the 150 stars withspectra that reveal absorption from the Perseus spiral arm show differentinterstellar Na I D absorption profiles in the Perseus arm gas. Additionally,we have utilized the KPNO Coude Feed spectrograph to obtain high-resolution (~3km/s) interstellar Na I D absorption spectra of 24 of the brighter stars towardh and Chi Per. These spectra reveal an even greater complexity in theinterstellar Na I D absorption in the Perseus arm gas and show individualcomponents changing in number, velocity, and strength from sightline tosightline. If each of these individual velocity components represents anisolated cloud, then it would appear that the ISM of the Perseus arm gasconsists of many small clouds. Although the absorption profiles vary even onthe smallest scales probed by these high-resolution data (~30";~0.35pc), ouranalysis reveals that some interstellar Na I D absorption components fromsightline to sightline are related, implying that the ISM toward h and Chi Peris probably comprised of sheets of gas in which we detect variations due todifferences in the local physical conditions of the gas.Comment: 27 pages text; 8 figure

The Complex Interstellar NaiAbsorption toward h and χ Persei