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 the pattern observed in the optical plane formed by the kinematic line width of Hβ and the relative importance of the Fe ii optical emission to be reproduced. We performed simulations of the Hβ and Fe ii production using the code CLOUDY and well-justified assumptions about the broadband spectra, distance to the emission line region, and the cloud properties. We show that the presence of the warm corona is an important element of the broadband spectrum, which decreases the dependence of the relative Fe ii emissivity on the Eddington ratio and allows the rare cases of particularly strong Fe ii emitters to be reproduced. Results are sensitive to the adopted cloud distance, and strong Fe ii emission can be obtained either by adopting strongly supersolar metallicity or a 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.
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