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I. Lithium in F-G Giants at the Rotational Break
As a star crosses the Hertzprung Gap, its surface rotational velocity declines abruptly between between spectral types G0 and G3 III. This rotational break must in large part be due to the growth of of the convective envelope and, thus, the star must exhibit a decline of the surface Li abundance. I have determined Li abundances and rotational velocities for the 174 program stars and have shown that 1) theoretical dilution curves do not match the observed decline in Li abundances; 2) supergiants deplete Li faster as a function of effective temperature than normal giants; and 3) Li abundances are correlated with rotational velocites.
II. A Survey of Lithium in 400 G0-K2 Subgiants
G0-K2 subgiants, similar in mass to our sun, represent a critical and dynamical stage of stellar evolution. Li, located in only a thin outer layer, is spread over the growing convective envelope and, hence, should be accompanied by a depletion(=dilution) of the surface abundance of Li as the stars evolve. In addition, the G0-K2 subgiants encompass a group of stars, ``Li-dip'' stars, known to be depleted by factor of 100 in Li as seen in their younger counterparts. Because Li is used to constrain theoretical models of stellar evolution and continues to represent a challenge to such models, there remains a lot of unanswered questions. How does the Li abundance change in this critical region of transition? Is the internal evolution affected by the different environments in which these stars where born? What is the physical process behind the Li-dip and which stars are affected? What is the relative contribution of dilution, diffusion, and convective mixing? Accurate, detailed knowledge of the physical properties of subgiant stars is crucial for us to address the above questions. Thus, our spectra will very accurately provide the temperature, spectral class, gravity, metallicity, and atmospheric abundances of carefully selected elements, for example, Al, Ca, Li, and Fe, for these stars.
Wednesday program listing