An optical spectroscopic survey of the 3CR sample of radio galaxies with
  z<0.3. II. Spectroscopic classes and accretion modes in radio-loud AGN
  We use the emission line measurements of 3CR radio sources with redshift <
0.3, to explore their spectroscopic properties. The 3CR sources show a bimodal
distribution of Excitation Index, a new spectroscopic indicator that measures
the relative intensity of low and high excitation lines. This unveils the
presence of two main sub-populations of radio-loud AGN, High and Low Excitation
Galaxies (HEG and LEG, respectively). All broad-line objects are HEG from the
point of view of their narrow emission line ratios and all HEG are FRII
radio-galaxies with log L(178) [erg/s] > 32.8. Conversely LEG cover the whole
range of radio power encompassed by this 3CR subsample (30.7 < log L(178) <
35.4) and they are of both FRI and FRII type. The brightest LEG are all FRII.
HEG and LEG obey to two (quasi) linear correlations between the optical line
and extended radio luminosities, with HEG being brighter than LEG in the [OIII]
line by a factor of ~10. HEG and LEG are offset also in a plane that compares
the black hole mass and the ionizing nuclear luminosity. However, although HEG
are associated with higher nuclear luminosities, we find LEG among the
brightest radio sources of the sample and with a clear FRII morphology,
indistinguishable from those seen in HEG. This suggests that LEG are not simply
objects with a lower level of accretion. We speculate that the differences
between LEG and HEG are related to a different mode of accretion: LEG are
powered by hot gas, while HEG require the presence of cold accreting material.
The high temperature of the accreting gas in LEG accounts for the lack of
"cold" structures (i.e. molecular torus and Broad Line Region), for the reduced
radiative output of the accretion disk, and for the lower gas excitation.
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