Using a Hipparcos derived HR diagram to limit the metallicity scatter of   stars in the Hyades -- Are Stars Polluted?
  Hipparcos parallaxes and proper motions have made it possible to construct Hertzsprung-Russell (HR) diagrams of nearby clusters with unprecedented accuracy. The standard deviation of high fidelity non-binary non-variable stars about a model stellar evolution isochrone in the Hyades cluster is about 0.04 magnitudes. We use this deviation to estimate an upper limit on the scatter in metallicities in stars in this cluster. From the gradient of the isochrones evolution in the HR diagram we estimate an upper limit for the scatter of metallicities Delta [Fe/H] <~ 0.03 dex, a smaller limit than has been measured previously spectroscopically. This suggests that stars in open clusters are formed from gas that is nearly homogeneous in its metallicity. We consider the hypothesis that processes associated with planet formation can pollute the convection zone of stars. If the position on the HR diagram is insensitive to the metallicity of the convection zone and atmosphere, then stars which have very polluted convection zones can be identified from a comparison between their metallicity and position on the HR diagram. Alternatively if the pollution of the star by metals results in a large change in the position of the star on the HR diagram in a direction perpendicular to the isochrone, then the low scatter of stars in the Hyades can be used to place constraints on quantity of high-Z material that could have polluted the stars. 
A.C. Quillen