CLOUDY view of the warm corona
  Bright active galaxies show a range of properties but many of these
properties are correlated which has led to the concept of the Quasar Main
Sequence. We test whether our current understanding of the quasar structure
allows to reproduce the pattern observed in the optical plane formed by the
kinematic line width of H$\beta$ and the relative importance of the Fe II
optical emission. We performed simulations of the H$\beta$ and Fe II production
using the code CLOUDY and well justified assumptions about the broad band
spectra, distance of the emission line region, and the cloud properties. We
show that the presence of the warm corona is an important element of the broad
band spectrum which decreases the dependence of the relative Fe II emissivity
on the Eddington ratio, and allows to reproduce the rare cases of the
particularly strong Fe II emitters. Results are sensitive to the adopted cloud
distance, and strong Fe II emission can be obtain either by adopting strongly
super-solar metallicity, or much shorter distance than traditionally obtained
from reverberation mapping. We modeled in a similar way the UV plane defined by
the Mg II line and Fe II UV pseudo-continuum, but here our approach is less
successful, in general overproducing the Fe II strength. We found that the Fe
II optical and UV emissivity depend in a different way on the turbulent
velocity and metallicity, and the best extension of the model in order to cover
both planes is to allow very large turbulent velocities in the Broad Line
Region clouds.
