We report on a deep spectroscopic survey for star-to-star CN variations along the main sequence ( MS ) of the globular cluster 47 Tuc with ESO ’ s VLT .
We find a significant bimodal distribution in the S ( 3839 ) index for main-sequence stars in the mass range of \sim 0.85 to 0.65 M _ { \odot } , or from the main-sequence turn-off down to \sim 2.5 mag below the main sequence turn-off .
An anti-correlation of CN and CH is evident on the MS .
The result is discussed in the context of the ability of faint MS stars to alter their surface composition through internal evolutionary effects .
We argue against internal stellar evolution as the only origin for the abundance spread in 47 Tuc ; an external origin such as pollution seems to be more likely .