The starpatch on the G8 dwarf XI Bootis A

The G8 dwarf ξ Boo A has been spectroscopically monitored for four observing seasons. The authors find systematic variations in spectral line asymmetries and equivalent widths with a 6.43±0.01 day period. There is no evidence of a change in period or phase shift over the four seasons. The observations are understandable in terms of a surface feature carried across the apparent disk of the star by rotation. The authors have developed a numerical simulation that reproduces both the asymmetry variations and the line strength variations. This leads to the gross characteristics of the feature: temperature 3.7% (≡200K) cooler than the rest of the star, areal coverage 10%±5% of the visible disk, latitude 55°±8°, and velocity fields that are enhanced over those for the rest of the surface.