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We describe a technique for determining the HI column density $N$ and the Doppler width $b$ of a Lyman limit system in quasar absorption spectrum. We concentrate our attention on the wavelengths near the HI Lyman limit and calculate the absorptions produced by both the high Lyman series lines and the Lyman continuum. The calculated results are then convolved with a finite instrumental profile to mimic the real observations. We found that ``the apparent Lyman limit'' is shifted redward relative to the expected Lyman limit wavelength. By fitting this offset, as well as the shape of the Lyman limit drop, we can get useful information about $N$ and $b$. Our method works even when the Lyman limit optical depth is very large and the traditional method, which measures the ratio of the flux levels redward of the Lyman limit to that of blueward, fails. Combined with fitting the available lines, this technique gives accurate HI column density $N$ and the Doppler width $b$. We demonstrate this by applying our method to the HST/FOS spectrum of the BAL quasar Q1246-057 ($z_{em}=2.22$). This quasar shines through at least two Lyman limit systems along the line-of-sight, and we show that our technique really helped us in understanding the observed features.
This research has been supported by NASA NAG 5-1630.
