We have studied the chemical composition of NGC 1806 , a massive , intermediate-age globular cluster that shows a double main sequence turnoff .
We analyzed a sample of high-resolution spectra ( secured with FLAMES at the Very Large Telescope ) for 8 giant stars , members of the cluster , finding an average iron content of [ Fe/H ] =–0.60 \pm 0.01 dex and no evidence of intrinsic star-to-star variations in the abundances of light elements ( Na , O , Mg , Al ) .
Also , the ( m _ { F 814 W } ; m _ { F 336 W } - m _ { F 814 W } ) color-magnitude diagram obtained by combining optical and near-UV Hubble Space Telescope photometry exhibits a narrow red giant branch , thus ruling out intrinsic variations of C and N abundances in the cluster .
These findings demonstrate that NGC 1806 does not harbor chemically distinct sub-populations , at variance with what was found in old globular clusters .
In turn , this indicates that the double main sequence turnoff phenomenon can not be explained in the context of the self-enrichment processes usually invoked to explain the chemical anomalies observed in old globulars .
Other solutions ( i.e. , stellar rotation , merging between clusters or collisions with giant molecular clouds ) should be envisaged to explain this class of globulars .