It has been found that globular clusters host multiple stellar populations .
In particular , in NGCÂ 1851 the subgiant branch ( SGB ) can be divided into two components and the distribution of stars along the horizontal branch ( HB ) is multimodal .
Various authors have found that NGCÂ 1851 possibly has a spread in [ Fe/H ] , but the relation between this spread and the division in the SGB is unknown .
We obtained blue ( 3950-4600Â Ã ) intermediate resolution ( R \sim 8 , 000 ) spectra for 47 stars on the bright and 30 on the faint SGB of NGCÂ 1851 ( b-SGB and f-SGB , respectively ) .
The spectra were analysed by comparing with synthetic spectra .
The determination of the atmospheric parameters to extremely high internal accuracy allows small errors to be recovered when comparing different stars in the cluster , in spite of their faintness ( V \sim 19 ) .
Abundances were obtained for Fe , C , Ca , Cr , Sr , and Ba .
We found that the b-SGB is slightly more metal-poor than the f-SGB , with [ Fe/H ] = -1.227 \pm 0.009 Â and [ Fe/H ] = -1.162 \pm 0.012 , respectively .
This implies that the f-SGB is only slightly older by \sim 0.6 Â Gyr than the b-SGB if the total CNO abundance is constant .
There are more C-normal stars in the b-SGB than in the f-SGB .
This is consistent with what is found for HB stars , if b-SGB are the progenitors of red HB stars , and f-SGB those of blue HB ones .
As previously found , the abundances of the n-capture elements Sr and Ba have a bimodal distribution , reflecting the separation between f-SGB ( Sr and Ba-rich ) and b-SGB stars ( Sr and Ba-poor ) .
In both groups , there is a clear correlation between [ Sr/Fe ] and [ Ba/Fe ] , suggesting that there is a real spread in the abundances of n-capture elements .
By looking at the distribution of SGB stars in the [ C/H ] vs. T _ { eff } diagram and in the [ Ba/H ] vs. [ Sr/H ] diagram , not a one-to-one relation is found among these quantities .
There is some correlation between C and Ba abundances , while the same correlation for Sr is much more dubious .
We identified six C-rich stars , which have a moderate overabundance of Sr and Ba and rather low N abundances .
This group of stars might be the progenitors of these on the anomalous RGB in the ( v,v - y ) Â diagram .
These results are discussed within different scenarios for the formation of NGCÂ 1851 .
It is possible that the two populations originated in different regions of a inhomogeneous parent object .
However , the striking similarity with MÂ 22 calls for a similar evolution for these two clusters .
Deriving reliable CNO abundances for the two sequences would be crucial .