the emission at wavelengths shorter than $\sim$200 nm and this radiation is absorbed high in the atmospheres of planets, driving heating and photochemistry. This emission depends sensitively on the rotation rate of the star (@Reiners14). Since rotation rates decline with age due to angular momentum removal by stellar winds, there is also a corresponding decline in XUV emission (@Guedel97). This means that when the solar system was young the upper atmospheres of solar system planets were likely significantly hotter and more expanded than currently (@Kulikov07; @Tian08; @Johnstone18). However, the situation is complicated by the fact that stars can be born with different rotation rates, and stars born as slow rotators will evolve very differently to stars born as fast rotators (@Johnstone15; @Tu15). A solar mass star born as a rapid rotator will remain highly active for much longer than a star born as a slow rotator, which has important consequences for the subsequent evolution of planetary atmospheric escape (@Johnstone15letter).

In this paper, I study the reaction of a water vapor atmosphere to the high XUV spectrum of a very active star. The aim is to use a newly developed state-of-the-art physical upper atmosphere model to study hydrodynamically outflowing water vapor 