2004PASP..116..801P

Recent high spatial and spectral resolution investigations of the diffuse interstellar medium (ISM) have found significant evidence for small-scale variations in the interstellar gas on scales ≤1 pc. To better understand the nature of small-scale variations in the ISM, we have used the Kitt Peak National Observatory (KPNO) WIYN Hydra multiobject spectrograph, which has a mapping advantage over the single-axis, single-scale limitations of studies using high proper motion stars and binary stars, to obtain moderate-resolution (∼12 km/s) interstellar Na I D absorption spectra of 172 stars toward the double open cluster h and χ Persei. All of the sight lines toward the 150 stars with spectra that reveal absorption from the Perseus spiral arm show different interstellar Na I D absorption profiles in the Perseus arm gas. Additionally, we have utilized the KPNO coudé feed spectrograph to obtain high-resolution (∼3 km/s) interstellar Na I D absorption spectra of 24 of the brighter stars toward h and χ Per. These spectra reveal an even greater complexity in the interstellar Na I D absorption in the Perseus arm gas and show individual components changing in number, velocity, and strength from sight line to sight line. If each of these individual velocity components represents an isolated cloud, then it would appear that the ISM of the Perseus arm gas consists of many small clouds. Although the absorption profiles vary even on the smallest scales probed by these high-resolution data (∼30", ∼0.35 pc), our analysis reveals that some interstellar Na I D absorption components from sight line to sight line are related, implying that the ISM toward h and χ Per is probably composed of sheets of gas in which we detect variations due to differences in the local physical conditions of the gas.