Multi-wavelength observations and modelling of shock-cloud interaction regions in the Vela Supernova Remnant

We present optical photometric and spectrophotometric observations of two s elected filaments near the North rim of the Vela supernova remnant also obs erved in UV with the International Ultraviolet Explorer, and in X-rays with the ROSAT PSPC detector. From the optical data, we have derived spatially- resolved (similar to 3 ") profiles of several emission lines across the fil aments, which allowed us to resolve the post-shock region. Starting from mo del-independent line ratio diagnostics, we have estimated densities ((n) ov er bar = 3.2 - 8.2 cm(-3)), temperatures (T = 3 - 10 x 10(4) K), pressures (P = 2.8 x 10(-10) dyne cm(-2)), and ambient magnetic fields (B = 50 - 85 m u G) in the post-shock flow. We have also found clear evidence of a relativ e displacement between the X-ray emission, the emission in the H alpha line and the emission in the [O III] line. We have compared the measured displa cement, the width of the filaments in H alpha and [O III], and the peak val ues of selected line ratios with the predictions of a radiative shock model which describes in detail the structure of the post-shock region. Shock mo dels with velocities around v(s) = 110 km sec(-1) are shown to be consisten t with the optical and UV observations if we assume that the direction of p ropagation is tilted with respect to the line of sight. The layout of the X -ray and optical emission of the filament and their thermodynamical propert ies strongly suggest that the shock is travelling along increasing density gradient.