The Impact of Pollution on Stellar Evolution Models

An approach is introduced for incorporating the concept of stellar pollutioninto stellar evolution models. The approach involves enhancing the metalcontent of the surface layers of stellar models. In addition, the surfacelayers of stars in the mass range of 0.5-2.0 Solar masses are mixed to anartificial depth motivated by observations of lithium abundance. The behaviorof polluted stellar evolution models is explored assuming the pollution occursafter the star has left the fully convective pre main sequence phase. Stellarmodels polluted with a few Earth masses of iron are significantly hotter thanstars of the same mass with an equivalent bulk metallicity. Polluted stellarevolution models can successfully reproduce the metal-rich, parent star tauBootis and suggest a slightly lower mass than standard evolution models.Finally, the possibility that stars in the Hyades open cluster have accreted anaverage of 0.5 Earth masses of iron is explored. The results indicate that itis not possible to rule out stellar pollution on this scale from the scatter ofHyades stars on a color-magnitude diagram. The small amount of scatter in theobservational data set does rule out pollution on the order of 1.5 Earth massesof iron. Pollution effects at the low level of 0.5 Earth masses of iron do notproduce substantial changes in a star's evolution.Comment: 14 pages, 3 figures, AASTeX, to appear in the 10/10/03 issue of Ap