AAS 201st Meeting, January, 2003
Session 77. The Interstellar Medium II
Poster, Wednesday, January 8, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[77.05] The Complex Interstellar Structure Toward h and \chi Persei

S.D. Points, D.M. Meyer, J.T. Lauroesch, D.T. Nguyen (Northwestern U.), S.D. Friedman (STScI)

We have obtained high-resolution (15 km~s-1) spectra of interstellar Na\,{\sc i} absorption toward 183 stars in the double open cluster h and \chi Persei using the Hydra multi-object spectrograph on the KPNO WIYN 3.5m telescope. These sightlines sample both high-velocity gas associated with the Perseus spiral arm as well as local gas associated with the Orion arm. Toward stars in h and \chi Per, the velocity of the Na\,{\sc i} gas in the Perseus arm is blueshifted by about 20 km~s-1 relative to the stars. This has been interpreted in terms of a density-wave shock interaction. Our interstellar Na\,{\sc i} absorption spectra reveal a chaotic velocity structure in the Perseus arm gas from point-to-point, especially when compared to H\,{\sc i} 21-cm emission line profiles which show smoother, continuous variations.

Because Na\,{\sc i} is not a dominant ion in H\,{\sc i} clouds, it is unclear if the variations in the interstellar Na\,{\sc i} profiles are due to differences in the total H column or the Na ionization equilibrium. In the case of the former, Na\,{\sc i} is tracing physical structures in the gas. In the latter case, Na\,{\sc i} is reflecting environmental variations in parameters such as cloud temperature, pressure, electron density, and/or radiation field. The key to understanding this Na\,{\sc i} structure lies in the ultraviolet where many diagnostic lines of important species such as H2 can be observed. We have obtained FUSE\/ observations of six closely-spaced stars in the core of \chi Per. These FUSE\/ spectra show remarkable variations in the H2 line profiles that are reminiscent of the profile variations seen in the individual interstellar Na\,{\sc i} absorption spectra toward these six stars.

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