The Origin and Distance of the High-Velocity Cloud MI

Abstract

The high-velocity, neutral hydrogen feature known as MI may be the result of a supernova that took place about 100,000 years ago at a distance of 163 pc. Low-velocity HI data show a clear cavity, a structure indicative of regions evacuated by old exploding stars, centered on the spatial coordinates of MI, (l,b) = (165o, 65.o5). 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. The mass and energy of MI are easily in line with what is expected from a supernova. The X-rays seen by ROSAT are consistent with an origin in the resulting bow shock. Ironically, this scenario for MI only came together because we were exploring low-velocity gas in the direction of high-velocity clouds.