Mid-Infrared Imaging of the Post-Asymptotic Giant Branch Star AC Herculis with the Multiple Mirror Telescope Adaptive Optics System

We report on preliminary results from the recent multiepoch neutral hydrogen absorption measurements toward three pulsars, B0823+26, B1133+16, and B2016+28, using the Arecibo telescope. We do not find significant variations in optical depth profiles over periods of 0.3 and 9-10 yr or on spatial scales of 10-20 and 70-85 AU. The large number of nondetections of the tiny-scale atomic structure suggests that the AU-sized structure is not ubiquitous in the interstellar medium and could be quite a rare phenomenon.