A Survey of AU-Scale Na I Structure in the Diffuse ISM

We present multi-epoch, high-resolution (R~230,000), high S/N observations of the interstellar Na I absorption toward a sample of 20 stars taken over the past 10 years with the KPNO Coude Feed telescope. These spectra reveal line profile variations (since 2004) in 12 of the 20 stars indicative of Na I structure on scales of ~10 to ~50 AU in the intervening diffuse ISM. Half of these 12 sightlines are located in Orion with the rest in Auriga, Eridanus, Leo, Monoceros, and Ophiuchus. The most dramatic Na I variations have been found in high-velocity gas associated with the Monoceros Loop (a ~100,000 year-old supernova remnant) toward the star HD 47240. Over the past 8 years, a single, narrow, high-velocity Na I component toward this star has oscillated significantly in strength and steadily widened into two narrow components. Given the 1.3 milli-arcsec/yr proper motion of HD 47240 and the ~50 km/s expansion velocity of the ~1.6 kpc distant Monoceros Loop, these absorption variations are sampling the structure of Na I gas in the Loop over a transverse distance scale of ~10 AU/yr. Almost all of the 12 sightlines exhibiting AU-scale Na I structure are associated with known H I shells, supernova remnants, or stellar bow shocks. Since Na I is not a dominant ion in diffuse clouds, temporal variations in its absorption along a particular sightline can be due to AU-scale fluctuations in either the gas column (N(H I)) or environmental conditions such as the cloud density (n(H)). Previous multi-epoch studies of UV interstellar lines have found that temporal dominant-ion absorption variations are exceedingly rare (Lauroesch 2007). Thus, it is most likely that the Na I variations reported here are due to AU-scale density fluctuations stimulated by turbulence or converging gas flows. Our new results suggest that such fluctuations may be common in some diffuse ISM regions (especially Orion).