In astronomy, the Kraft break refers to the abrupt decrease in stars' average rotation rates at surface temperatures below about 6,500 Kelvin. This temperature corresponds to mid-F type stars. The so-called break bears the name of astronomer Robert Kraft, though its existence was recognized prior to his publications on the topic. The break is understood to separate stars with deep convective envelopes and efficient magnetic dynamos from those without. The dynamos are thought to maintain magnetic fields that transfer angular momentum to the stellar wind, thus slowing down the star's surface through magnetic braking. In hot stars the process is less efficient (because the convective envelopes are shallow) so the stars continue to rotate quickly. The Kraft break coincides with a transition in hot Jupiters' spin-orbit alignment. Hot Jupiters orbiting cooler stars tend to orbit in the same plane as stellar rotation, while those orbiting hotter stars are more likely to be misaligned.
References