Subgiants in NGC 188 reveal that rotationally induced mixing creates the main sequence Li-Dip

The Lithium-Dip is a severe lithium depletion observed in mid-F (6200-6650 K) dwarfs, which has puzzled astronomers since it was discovered in 1986. Proposed mechanisms include effects related to rotation, magnetic fields, diffusion, gravity waves, and mass loss. Which, if any, of these is realistic remains unclear. Here we show that mixing due to shear induced by stellar angular momentum loss is the unique mechanism driving the lithium depletion. Each mechanism leaves a different signature in the subsurface lithium distribution. The deepening surface convection zones of subgiants of NGC 188 evolving out of the Lithium-Dip dredge up the subsurface material and thus reveal the signature of the responsible mechanism, rotation. Subgiants can also be used more generally, thereby improving fundamental understanding of stellar evolution. Rotational mixing may be the dominant lithium-depleting mechanism in a wide range of solar-type stars, including in the Sun. Our results may further reconcile the cosmological lithium discrepancy.