The evolution of chromospheric activity and the spin-down of solar-type stars.

Abstract

The authors report on observations made with the IUE satellite of nearly two dozen F7 - G2 dwarfs, dated on the basis of their surface Li content in order to examine the age dependence of ultraviolet chromospheric and transition region emission lines. The IUE data for solar-type stars are compared with observations of T Tauri stars (both published and newly reported here) in order to determine whether the pattern of main-sequence chromospheric decay shown by stars older than ≡10⁸yr extends back to ages of 10⁶yr appropriate for T Tauri stars. The analysis of the time decay of stellar ultraviolet and X-ray emission establishes a relationship between emission level and axial rotation, which is expressed in terms of the Rossby parameter (here taken to be the stellar rotation period divided by a characteristic turnover time for subphotospheric convection). It is suggested that the evolution of main- sequence activity reflects a diminishing coverage of stellar active regions with age.