Simon Schuler
National Optical Astronomy Observatory


Nucleosynthesis in the Hyades Open Cluster-
Evidence for the Enhanced Depletion of 12C

The CNO bi-cycle is the dominant set of H burning reactions powering the 
cores of stars more massive than the Sun.  The cycle results in no net 
loss of the CNO nuclei, but due to different lifetimes to proton 
capture, the relative numbers of each do change: 12C is depleted, 14N is 
enhanced, and 16O, if the ON cycle is active, is depleted.  At the end 
of core H burning, stars experience the first dredge-up, and material 
processed by the CN and possibly ON cycles
are mixed to the surface layers where the products of these core nuclear 
processes can be observed.  Here I present the results of our 
observational program designed to test standard stellar evolution models 
and our understanding of core nuclear processes on the main sequence 
(MS).  We have derived CNO abundances of three giants and three 
solar-type MS dwarfs in the Hyades open cluster and used the abundances 
of the dwarfs as a proxy for the initial composition of the giants.  The 
observed N and O abundances are found to match well a stellar evolution 
model tailored to the Hyades giants, but the
observed C abundance is a factor of 1.5 lower than the model prediction.

Empirically, the observed C+N+O abundance of the dwarfs does not equal 
the C+N+O abundance of the giants, as would be expected from the CNO 
bi-cycle.  I describe our efforts to account for the missing 12C, 
including the possibility that a heretofore unknown nucleosynthetic 
process may be active in the cores of near-solar metallicity 2.5 M_solar 
stars.