2005MNRAS.363L..51V

Stellar parameters of 11 G-type stars with ages ranging from 0.1 to 8.5 Gyr, from the Sun in Time programme, were used to compute the Rossby number, ℜ, for each star. The Rossby number for each star was calculated from the rotation period and the convective overturn time derived from spectral type (B-V). It was found to vary essentially as t^0.5, where t is the stellar age in Gyr. The Rossby number is used as an index of X-ray-ultraviolet (XUV) (1-1200 Å) and Lyα activity, defined as the ratio of the total emission flux in these spectral regions to the total bolometric emission. Expressions for the ratio of the stellar surface XUV and Lyα emission flux relative to present mean solar surface flux values are given in terms of ℜ. It is shown that the observed activity in these stars varies as ℜ^–β, where β takes values of 2.5 and 1.5 for XUV and Lyα, respectively. Expressions for deriving the Rossby number from B-V and age are also given. Thus, one can use the stellar B-V and effective temperature variation with age to calculate the XUV and Lyα emission flux relative to present solar values. As an example, the evolution of the solar XUV and Lyα with age from 0.1 to 8.5 Gyr is given. The variation of the stellar ultraviolet flux with age can be used in photochemical models to study the evolution of planetary atmospheres orbiting such stars.