Fe and Al abundances for 180 red giants in the globular cluster Omega Centauri (NGC 5139)

We present radial velocities and 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 toward 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 approximate to 13; 000) obtained with the Blanco 4 m 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 <= [Fe/H] less than or similar to -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] similar to -1.4. The minimum [Al/Fe] abundance observed for all metallicity populations is [Al/Fe] similar to +0.15. The maximum abundance of log is an element of(Al) is reached for stars with [Fe/H] similar to -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.