High metal content of highly accreting quasars
  We present an analysis of UV spectra of 13 quasars believed to belong to
extreme Population A (xA) quasars, aimed at the estimation of the chemical
abundances of the broad line emitting gas. Metallicity estimates for the broad
line emitting gas of quasars are subject to a number of caveats, although
present data suggest the possibility of an increase along the quasar main
sequence along with prominence of optical Fe II emission. Extreme Population A
sources with the strongest Fe II emission offer several advantages with respect
to the quasar general population, as their optical and UV emission lines can be
interpreted as the sum of a low-ionization component roughly at quasar rest
frame (from virialized gas), plus a blueshifted excess (a disk wind), in
different physical conditions. Specifically, in terms of ionization parameter,
cloud density, metallicity and column density. Capitalizing on these results,
we analyze the component at rest frame and the blueshifted one, exploiting the
dependence (of several intensity line ratios on metallicity $Z$). We find that
the validity of intensity line ratios as metallicity indicators depends on the
physical conditions. We apply the measured diagnostic ratios to estimate the
physical properties of sources such as density, ionization, and metallicity of
the gas. Our results confirm that the two regions (the low-ionization component
and the blue-shifted excess) of different dynamical conditions also show
different physical conditions and suggest metallicity values that are high, and
probably the highest along the quasar main sequence, with $Z \gtrsim 10
Z_{\odot}$. We found some evidence of an overabundance of Aluminium with
respect to Carbon, possibly due to selective enrichment of the broad line
emitting gas by supernova ejecta.
