We present a detailed chemical analysis of 22 stars along the colour-magnitude sequence of the intermediate-age ( 1.7 Gyr ) open cluster IC 4651 , based on high-resolution , high S/N ratio spectra from UVES/VLT .
IC 4651 thus becomes one of the few open clusters for which a detailed composition analysis exists for stars spanning 3.5 magnitudes , from solar-type main-sequence stars to giants above the RGB clump .
In a strict comparison with the Sun , we find for the cluster a well-defined Fe abundance of [ Fe/H ] = 0.10 \pm 0.03 ( internal errors ) , with a reddening E ( b-y ) = 0.091 .
We also derive abundances for the \alpha elements Mg , Si , Ti , and Ca and find a moderate enhancement of the three former elements , in excellent agreement with the results for field stars of similar Fe abundance .
Among the Fe group elements , Cr and Ni are slightly overabundant ( [ X/Fe ] \sim 0.05 ) .
As also observed in M67 , the Na abundance among the giants is more than 0.2 dex higher than in the dwarfs .
We interpret this effect as real , and due to dredge-up of ^ { 23 } Na in the giants . Four turnoff stars , all fairly rapid rotators ( v \sin i \leq 25 km s ^ { -1 } ) , appear to have higher [ Fe/H ] than the other stars ; our tests show that , while a spurious enhancement of 0.1 dex can be produced by the effect of high rotation on our spectral analysis , this can not explain the whole difference .
These stars appear overabundant because we tend to overestimate their effective temperatures by forcing excitation equilibrium . Li abundances have been computed for all the stars and show a well-defined pattern : the Solar-type stars have an almost constant Li abundance , just below that of the Hyades , and the Li-dip is pronounced and well determined .
Turnoff stars just above the dip have a ‘ cosmic ’ Li abundance , but within a very small range of magnitudes ( 0.25 mag ) higher on the turnoff , the Li level drops by more than a factor 10 .
This can not be due just to dilution ; rather some extra mixing is required .
Among the giants , two probable clump stars show detectable Li , while all the other ( likely ) RGB stars do not - as is also observed in the similar cluster NGC3680 .
None of these patterns can be explained by classical stellar evolution models .
Again , some extra mixing is required .
We show that rotating stellar models including the most recent developments for meridional circulation and turbulence by shear instabilities explain very well the behaviour of the lithium abundance along the colour-magnitude diagram of IC4651 , including subgiant and giant phases .
The possibility remains open that the giants exhibiting the highest lithium abundances are actual RGB bump stars which have just been through the so-called “ lithium flash ” .