Radio observations of CTB80: detection of the snowplough in an old supernova remnant

New radio continuum and H i line observations of the old supernova remnant CTB80 are analysed. The radio continuum emission is more extended than previously known. We analyse archival ROSAT Position Sensitive Proportional Counter images of CTB80 and find extended X-ray emission associated with CTB80 over a 1 degrees.2 diameter region. A new H i absorption spectrum towards the core of CTB80 yields a kinematic distance of similar or equal to 1.5 kpc. An analysis of the H i channel maps confirms the inner shell discovered by Koo et al. In addition, we discover an outer slowly moving shell centred on CTB80s centre, with radius 76 arcmin and velocity 40 km s-1. The shells size and velocity are inconsistent with a stellar wind origin, but are consistent with what is expected for a cool dense shell in the cooling (snowplough) phase of a supernova remnant. Taking into account deceleration of the shell, its age also agrees with that of CTB80 (similar to 60 000 yr). The resulting picture of CTB80 is that of a large old supernova remnant, with an H i shell moving out slowly behind the slow outer shock and a hot interior which is still emitting X-rays and visible in the ROSAT images.