An HiRing in the Southeast Cygnus Loop

Abstract

A large ring feature is identified in 21 cm neutral hydrogen (H I) line observations of the Cygnus Loop supernova remnant. The observations were made with the Dominion Radio Astrophysical Observatory's Synthesis Telescope and 26 m telescope. The eastern and western parts of the ring are bright, but the central regions are faint. On the sky, the H I ring extends from the southeast rim toward the center of the Cygnus Loop but is argued to lie on the front side, outside the shock front, except at its eastern end. The ring is found in the IRAS 60 and 100 μm maps of the region, allowing H I and dust column densities to be determined. The association of the eastern end of the ring with the optically and X-ray–identified southeast knot supports its physical association with the Cygnus Loop. The ring's mean velocity of -4 km s-1 is offset from the Cygnus Loop galactocentric velocity of ~4 km s-1. This indicates that the ring has been accelerated by ~8 km s-1 by the rocket effect, in a similar manner to that for the H I cloud at the northeast limb of the Cygnus Loop. The Cygnus Loop cavity is proposed to result from the acceleration of several similar clouds.