The peculiar emission line spectra of core-Extremely Red BOSS Quasars at
  $z\sim$2-3: orientation and/or evolution?
  Core-extremely red quasars (core-ERQ) have been proposed to represent an
intermediate evolutionary phase in which a heavily obscured quasar is blowing
out the circumnuclear interstellar medium with very energetic outflows prior to
becoming an optical quasar. We revise the general UV and optical emission line
properties of core-ERQ in the context of the AGN orientation-based unification
scenario. We use diagnostic diagrams based on ultraviolet (UV) emission line
ratios and UV-optical line kinematic information to compare the physical and
kinematic gas properties of core-ERQ with those of other luminous narrow and
broad line AGN. In particular, we provide a revised comparison of the [OIII]
kinematics in 21 core-ERQ with other samples of quasars matched in luminosity
with the aim of evaluating whether core-ERQ host the most extreme [OIII]
outflows.
  The UV line ratios suggest that the physical properties (for instance,
density, metallicity) of the ionised gas in core-ERQ are similar to those
observed in the BLR of blue Nitrogen-loud QSOs. The [OIII] outflow velocities
of core-ERQ are, on average, consistent with those of very luminous blue QSO1,
although extreme outflows are much more frequent in core-ERQ. These
similarities can be explained in the context of the AGN unification model,
assuming that core-ERQ are viewed with an intermediate orientation between type
2 (edge-on) and type 1 (face-on) QSOs. We propose that core-ERQ are very
luminous but otherwise normal quasars viewed at an intermediate orientation.
Such orientation allows a direct view of the outer part of the large BLR, from
which core-ERQ UV emission originates; the extreme [OIII] outflow velocities
are instead a consequence of the very high luminosity of core-ERQ.
