We analyze spectra of 18 stars belonging to the faintest subgiant branch in \omega Centauri ( the SGB-a ) , obtained with GIRAFFE @ VLT at a resolution of R \simeq 17 000 and a S/N ratio between 25 and 50 .
We measure abundances of Al , Ba , Ca , Fe , Ni , Si , and Ti and we find that these stars have < [ Fe/H ] > =–0.73 \pm 0.14 dex , similarly to the corresponding red giant branch population ( the RGB-a ) .
We also measure < [ \alpha /Fe ] > =+0.40 \pm 0.16 dex , and < [ Ba/Fe ] > =+0.87 \pm 0.23 dex , in general agreement with past studies .
It is very interesting to note that we found a uniform Al abundance , < [ Al/Fe ] > =+0.32 \pm 0.14 dex , for all the 18 SGB-a stars analysed here , thus supporting past evidence that the usual ( anti- ) correlations are not present in this population , and suggesting a non globular cluster-like origin of this particular population .
In the dwarf galaxy hypothesis for the formation of \omega Cen , this population might be the best candidate for the field population of its putative parent galaxy , although some of its properties appear contradictory .
It has also been suggested that the most metal-rich population in \omega Cen is significantly enriched in helium .
If this is true , the traditional abundance analysis techniques , based on model atmospheres with normal helium content , might lead to errors .
We have computed helium enhanced atmospheres for three stars in our sample and verified that the abundance errors due to the use of non-enhanced atmospheres are negligible .
Additional , indirect support to the enhanced helium content of the SGB-a population comes from our Li upper limits .