Using a Hipparcos-derived Hertzsprung-russell 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 (H-R) diagrams of nearby clusters with unprecedented accuracy. The standard deviation of high-fidelity, nonbinary, nonvariable stars about a model stellar evolution isochrone in the Hyades cluster is about 0.04 mag. We use this deviation to estimate an upper limit on the scatter in metallicities in stars in this cluster. From the gradient of the isochrone’s evolution in the H-R diagram, we estimate an upper limit for the scatter of metallicities D[Fe/H] d 0.03 dex, a smaller limit than has previously been measured 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. The low observed scatter about the isochrone in the Hyades suggests that pollution effects are not common and strong. If the position on the H-R diagram is insensitive to the metallicity of the convection zone and atmosphere, then stars that have very polluted convection zones can be identified from a comparison between their metallicity and position on the H-R diagram. Alternatively, if the pollution of the star by metals results in a large change in the position of the star on the H-R 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.