VARIATIONS IN COLD INTERSTELLAR CLOUDS ON SUBPARSEC SCALES

We have used high-resolution spectra in the region of the Na D lines to explore the structure of two cold interstellar clouds: one between the Sun and the globular cluster M92 (ten target stars), the other between the Sun and the globular cluster M15 (seven target stars). We find that the neutral sodium column density varies by a factor of ∼4 over distances ≲0.2 pc in the M92 cloud. The column density variations are larger in the higher velocity cloud in front of M15 but the distances over which they occur are more uncertain. There is no obvious "pattern" to the density variations in either cloud. We find evidence for velocity variations of about 6 km s-1 within the M15 cloud.