Rotation and Lithium Surface Abundances, Revisited

For giants in the Hertzsprung gap, the relations between T_eff, v sin i, and lithium surface abundances are reinvestigated and compared with the relations found for Hyades main-sequence F stars. For the Hyades main-sequence F stars, the v sin i decrease steeply around T_eff ∼ 6450 K. At the same temperature the lithium surface abundances show a narrow, deep dip. For most giants there is also a steep decrease of v sin i for T_eff around 6450 K. At this temperature the lithium surface abundances of the giants also decrease steeply and remain low for T_eff < 6400 K. The changes in rotation and Li surface abundances occur over a temperature interval of less than 300 K, which for a 2 M_⊙ giant corresponds to an age interval of about 10⁶ yr. The simultaneous steep decreases of rotation velocities and Li surface abundances indicate that for the giants these changes are due to the same cause, which we suggest to be deep mixing. It then seems rather likely that for the Hyades main-sequence F5 V stars the decrease of rotation and Li surface abundance is also caused by deep mixing. We suggest that in both cases the changes are related to the merging of the hydrogen and helium convection zones.