MULTIFREQUENCY STUDIES OF BRIGHT RADIO SUPERNOVA-REMNANTS .1. 3C-391

We report radio observations of the bright, compact supernova remnant 3C 391 using the Very Large Array of the NRAO(3) at 330, 1468, and 484 8 MHz. We present and discuss high-resolution images of total intensit y, polarization, and spectral index. The large-scale morphology consis ts of a bright partial shell with a considerably larger plateau of fai nter emission extending past the open end of the shell, suggesting tha t the remnant is directly interacting with a dense region of the inter stellar medium, possibly a molecular cloud. The partial shell may resu lt from gradients in the external magnetic field that would be expecte d in the presence of strong density gradients. Small-scale extensions beyond the shell edge can be interpreted as due to relativistic electr ons diffusing upstream of the shock along external magnetic field line s with a mean free path about an order of magnitude smaller than chara cteristic of the mean interstellar medium for particles of energy a fe w GeV. If this interpretation is correct, shock-accelerated electrons are dominantly produced where the shock normal is perpendicular to the upstream magnetic field. We find no polarization at 330 or 1468 MHz, with 3 sigma upper limits of 6 mJy beam(-1) (10'' beam) at 330 MHz and 0.3 mJy beam(-1) (6'' beam) at 1468 MHz. We do observe polarized flux at 4848 MHz, but a mean polarized fraction of 0.77% +/- 0.06%, far lo wer than typical for bright supernova remnants. Tangled or disordered magnetic fields in the emitting region of the radio shell may be respo nsible for depolarizing the radio synchrotron radiation, but some inte rnal Faraday depolarization may also occur. We crudely estimate the fo reground Faraday rotation to be about -500 rad m(-2), consistent with previous estimates. Spectral index images created from the total inten sity images show no variation beyond Delta alpha = 0.1. We do see vari ations at lower levels which are formally marginally significant but w hich are not consistent among the three pairs of frequencies. Small er rors in the total flux density at each frequency or in the deconvoluti ons are probably responsible.