The Origin and Distance of the High-velocity Cloud MI

The high-velocity, neutral hydrogen feature known as MI may be the result of a supernova that took place about 100,000 yr ago at a distance of 163 pc. Low-velocity H i data show a clear cavity, a structure indicative of regions evacuated by old exploding stars, centered on the spatial coordinates of MI, (l, b) = (165°, 65.°5). The cavity is also visible in 100 μm dust data from IRAS. The invisible companion of the yellow giant star, 56 Ursae Majoris, may be the remains of the supernova that evacuated the cavity and blasted MI itself outward at 120 km s−1. The mass and energy of MI are easily in line with what is expected from a supernova. The diffuse X-rays seen by ROSAT provide evidence of a hot cavity, and the enhanced X-rays may result from the subsequent bow shock.