Interstellar NaI and CaII absorption observed towards the Cygnus Loop SNR

Abstract

We present high resolution spectra ( km s-1) of the interstellar NaI and CaII absorption lines observed towards 9 early-type stars with distances ranging from 250 to 2300 pc in the line-of-sight towards the Cygnus Loop Supernova Remnant (SNR). All but one of these absorption profiles can be fit using a combination of one or more of three absorption components with average best-fit (lsr) velocities of km s-1, km s-1 and km s-1. An additional velocity component at km s-1 is required in order to fit the NaI profile recorded towards the star HD 198946, whose distance of 794 pc places it well in excess of the nominal 440 pc distance to the SNR. The NaI/CaII column density ratios for the three higher velocity components are typically <1.0, which are similar to values found for high-velocity gas components detected towards other evolved SNRs. Even though we have detected the three higher velocity components solely along the sight-lines towards stars with distance estimates greater than that of the Cygnus Loop, we are unable to definitely associate these components with an interaction between the expansion of the SN shock wave and the ambient interstellar medium. We suggest a more likely origin for these absorption components is that of an old pre-cursor SN neutral gas shell, within whose interstellar cavity the Cygnus Loop supernova explosion occured some 20 000 years ago.