SIMBAD references

Flux calibrated CCD images, in the Hα+[NII], [SII], and [OIII] emission lines, of a wide field around the supernova remnant G 6.4-0.1 are presented. The low ionization images identify a front of enhanced [SII]/Hα+[NII] ratio along the east-west direction. This front is very well correlated with the filamentary radio emission of the remnant as well as with molecular CO emission and may indicate the interaction of the primary blast wave with molecular clouds present in the vicinity of the remnant. We estimate a total Hα flux, corrected for interstellar extinction, of 2x10^–8erg/s/cm², and a total [SII] flux of 1.1x10^–8erg/s/cm². The Hα+[NII] and [SII] images provide evidence for the presence of emission from shock heated gas to the south-west and to the east of the bulk of the known optical emission, implying that the primary shock wave is able to drive radiative shocks into the interstellar clouds. The image in the medium ionization line of [OIII]5007Å does not reveal any filamentary structures. On the contrary, the emission is diffuse and very weak, close to our detection limit of 5x10^–17erg/s/cm²/arcsec² (3σ), and appears to be mainly present in the south-east to north-west areas of the remnant. The long-slit spectra indicate significant extinction in all positions observed, while the measured variations are within the 3σ error. The [OIII] emission in the spectra, whenever present, is weaker than the Hβ flux suggesting shock velocities around 70 km/s or less all around the remnant in accordance with the [OIII] imagery. Thus, the low shock velocities are a common characteristic of G 6.4-0.1 and not just of the areas where the spectra were acquired. The average sulfur line ratio suggests postshock electron densities below 120cm^–3 at the 3σ limit.