The Expansion of G11.2-0.3, a Radio Composite Supernova Remnant

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.