Deep optical observations of the supernova remnant G 78.2+2.1

The wide-field covered by the supernova remnant G 78.2+2.1 was observed in the optical emission lines of H (alpha)+ [N II], [S II] and [O III]. The flux calibrated images reveal several H II regions in the field which dominate the optical emission but we were able to identify possible areas of shock-heated emission through the H (alpha)+[N II] and [S II] images. These are mainly found to the north-east of (gamma) Cygni as well as in the south and the morphology of the detected emission is patchy and diffuse. A few patchy structures are also detected in the medium ionization line image of [O III]. Long-slit spectra taken at one of the candidate positions verify that we have detected radiation from shockheated gas ( [S II]/H (alpha)  (asymptotically equal) 0.6). The estimated shock velocity lies below 100 km s^-1, while the measured electron density of ~700 cm^-3 implies preshock cloud densities of ~20 cm^-3. High resolution maps in the infrared show that the optical emission, which may be associated with G 78.2+2.1, lies in areas relatively free of infrared emission. The interstellar extinction measured through the optical spectra is compatible with current estimates of the distance to the remnant. The optical data are in agreement with the explosion energy and interstellar medium density estimated from the X-ray data suggesting that the remnant is still in the adiabatic phase of its evolution. A second set of spectra taken in the north-west suggests that we are probably dealing with a foreground H II region.