Abundances in the hot DZ star CBS 127: How efficient is hydrogen screening?

We present the results of an effective temperature and abundance analysis of the DZ white dwarf CBS 127, utilizing a revised and updated grid of cool helium-rich model atmospheres including the effect of varying hydrogen abundance. Our model fits to the Ca II H and K lines and the continuum yields T_eff = 10,500 ± 200 K and abundance ratios H/He < 10^-5, Ca/He = 6 × 10^-9, and [Ca/H] > 2.4 (relative to the Sun), assuming log g = 8. We compare CBS 127 to several other DZ white dwarfs with similar effective temperatures and show that CBS 127 is the hottest DZ star and lies near the boundary temperature above which it has been proposed that accretion of hydrogen may be prevented. Based upon our preliminary temperature/abundance analyses of G111 - 54 and G245 - 58 and the effective temperature/abundance parameters of PG 1225 - 081, we reject entirely a specifically defined single lower boundary temperature for screening and demonstrate that the propeller must operate over a very broad range of T_eff, down to T_eff, as low as 5800 K, with highly variable efficiency. We present a number of counterarguments which question the efficiency of hydrogen screening. We briefly speculate on the possibility of an alternative scenario involving bimodal accretion by helium-rich degenerates, cloud accretion producing hydrogen and metals (the DBA and DZA stars), and accretion of volatile-depleted planetary material/cometary matter to explain objects such as GD 40 with no trace of hydrogen.