We present radial velocities , Fe , and Al abundances for 180 red giant branch ( RGB ) stars in the Galactic globular cluster Omega Centauri ( \omega Cen ) .
The majority of our data lie in the range 11.0 < V < 13.5 , which covers the RGB from about 1 mag .
above the horizontal branch to the RGB tip .
The selection procedures are biased towards preferentially observing the more metal–poor and luminous stars of \omega Cen .
Abundances were determined using equivalent width measurements and spectrum synthesis analyses of moderate resolution spectra ( R \approx 13,000 ) obtained with the Blanco 4m telescope and Hydra multifiber spectrograph .
Our results are in agreement with previous studies as we find at least four different metallicity populations with [ Fe/H ] =–1.75 , –1.45 , –1.05 , and –0.75 , with a full range of –2.20 \lesssim [ Fe/H ] \lesssim –0.70 .
[ Al/Fe ] ratios exhibit large star–to–star scatter for all populations , with the more than 1.0 dex range of [ Al/Fe ] decreasing for stars more metal–rich than [ Fe/H ] \sim –1.4 .
The minimum [ Al/Fe ] abundance observed for all metallicity populations is [ Al/Fe ] \sim +0.15 .
The maximum abundance of log \epsilon ( Al ) is reached for stars with [ Fe/H ] \sim –1.4 and does not increase further with stellar metallicity .
We interpret these results as evidence for type II SNe providing the minimum [ Al/Fe ] ratio and a mass spectrum of intermediate mass asymptotic giant branch stars causing the majority of the [ Al/Fe ] scatter .
These results seem to fit in the adopted scheme that star formation occurred in \omega Cen over > 1 Gyr .