2007A&A...461..991M

Flux-calibrated CCD images of HB 21 are presented, along with flux calibrated spectra of moderate resolution. The low ionization images reveal filamentary structures in the east, while the emission in the central to west areas may be either filamentary or patchy. The filaments in the east are correlated very well with the radio emission, but a smaller degree of correlation exists for the patchy emission. An area in the west exhibits strong sulfur emission and is probably associated to the remnant. Uncatalogued HII regions are present in the area of the remnant, mainly in the south. We find that an ellipse extending for 116'x122' incorporates most of the radio and optical emission. The actual physical extent of the remnant depends on the distance, which is not well determined since it ranges from 0.8 to 1.7 kpc. Deep long-slit spectra were also acquired at a number of locations. The Hα emission is generally weak, typically below 15x10^–17erg/s/cm2/arcsec2. The Hβ emission was not always detected, suggesting substantial attenuation of the light through the interstellar medium. The medium ionization line of [OIII]5007Å was not detected indicating low shock velocities into the interstellar clouds whose density is estimated to be a few atoms per cm³.