\ion Ne8 \lambda 774 is an important tracer of the high-ionization gas in QSOs .
We examine the \ion Ne8 emission-line properties using new HST -FOS spectra of four sources , mean spectra derived from two QSO samples in the HST archives , and new photoionization calculations .
The results support our previous claim that broad \ion Ne8 lines are common in QSOs , with an average flux of \sim 42 % of \ion O6 \lambda 1034 and velocity widths that are \sim 2 to 5 times larger than \ion O6 , \ion C4 \lambda 1549 and other broad lines in the same spectra .
The strongest and most reliably measured \ion Ne8 \lambda 774 lines ( in two sources ) have FWHM \sim 14,500 km s ^ { -1 } .
Line profile fits in these cases show that the unusually large widths might be caused by blending with emission from \ion N4 \lambda 765 and \ion O4 \lambda 789 .
However , standard photoionization calculations indicate that \ion N4 , \ion O4 and all other lines near this wavelength should be too weak , leaving ( very broad ) \ion Ne8 as the only viable identification for the \sim 774 Å feature .
( This conclusion might be avoided if there are large radial velocity dispersions [ \ga 1000 km s ^ { -1 } ] in the emitting region and the resonant absorption of continuum photons enhances the flux in weaker lines . )
The calculations also indicate that the \ion Ne8 emitting regions have ionization parameters in the range 5 \la U \la 30 , total hydrogen column densities of 10 ^ { 22 } \la N _ { H } \la 3 \times 10 ^ { 23 } cm ^ { -2 } , and an average covering factor of \ga 30 % ( for solar abundances and a nominal QSO continuum shape ) .
The \ion Ne8 emitting region is therefore more extensive , more highly ionized , and has much higher velocities than the rest of the broad emission line region ( BELR ) .
This highly-ionized BELR component would be a strong X-ray “ warm ” absorber if it lies along our line-of-sight to the X-ray continuum source .