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We extend the study of the nuclei of 3CR objects as seen with the HST tohigher redshift FR II radio sources (0.4<z<0.6). Our results reflect what hasbeen found for FR II of lower redshift. The position of the nuclei on the planeformed by optical and radio luminosities is related to their optical spectralclassification: the nuclei of both high and low excitation galaxies lie on thecorrelation found for FRI sources, while broad-lined objects have a significantoptical excess. The nuclear properties of these sources are better understoodwhen considering the equivalent width of the [OIII] emission line with respectto their optical luminosities. Even in the range of redshift considered here,low excitation galaxies show peculiar nuclear properties, more similar to thoseobserved in FR I. This confirms that not all FRII are unified with quasars. Ourfindings have important implications for the FRII-quasar unification scheme: byreconsidering the classification of all 3CR radio galaxies with z<1 in thelight of their nuclear properties, we derive the opening angle of the obscuringtorus for different redshift bins. We find that the covering factor of theobscuring structure decreases as redshift increases (theta ~ 44 deg for z<0.3to theta ~ 56 deg for 0.5<z<1). We argue that this behavior may be interpretedin the framework of the receding torus model, in which the opening angle of thetorus increases as the luminosity of the accretion disk around the centralblack hole increases.Comment: 9 pages, 5 figures, accepted for publication in Astronomy &  Astrophysic

The nuclear radio-optical properties of intermediate-redshift FR II radio galaxies and quasars