THE SUPERNOVA REMNANT CTA1 AND THE SURROUNDING INTERSTELLAR-MEDIUM

Dominion Radio Astrophysical Observatory aperture synthesis observations, in the continuum at 1420 MHz and in the 21 cm line of H I, are presented of the supernova remnant (SNR) CTA1. The angular resolution is 1 arcmin. Full sensitivity to structures of all angular sizes down to this scale has been achieved by combining the aperture synthesis observations with single-antenna data. In the continuum, in addition to the well-known bright arcs giving the SNR the appearance of an incomplete shell structure, diffuse emission can be traced to the north and northeast of the remnant and also in the southwest. Some of these faint features coincide with diffuse [O III] emission. A comparison with the 100 mum coadded IRAS image shows the presence of two cavities of low infrared emissivity adjacent to these regions. The diffuse radio emission within the outline of the SNR is more prominent in the eastern part of the object where most of the observed x-ray emission is also found. There is evidence in the H I data for part of a shell of neutral gas in the northwest where the continuum shell is noticeably absent. The systemic velocity of this shell is -16 +/- 3 km s-1 yielding a kinematic distance of 1.4 +/- 0.3 kpc for CTA1. H I gas, most likely associated with the SNR, is also observed in the range -40 to -80 km s-1 in projection against the geometrical center of the remnant. The centroid of this emission, particularly at -70 km s-1, is near the peak of the fast-expanding H i detected by Koo and Heiles [ApJ, 382, 204 (1991)] in the velocity range -150 to -180 km s-1. A small cloud, containing 10 +/- 4M. of H I, lies within the H I shell at -16 km s-1, and coincides with it in velocity extent. it is coincident with an IP.AS point source. This H I-IR object may be a protostar whose formation has been triggered by the SNR shock. A conspicuous nearly perfectly circular shell of H I is evident at velocities, near -8. 5 km s-1 but its association with the SNR remains uncertain. The morphological evidence of the infrared, optical, and radio observations suggests that, in the north and west, the SNR has broken out into low-density regions of the interstellar medium. A semiquantitative model is developed along these lines.