MPS Seminar: Weakened Magnetic Braking and the Evolution of Stellar Dynamos (Travis Metcalfe)

Weakened magnetic braking (WMB) was originally proposed in 2016 to explain anomalously rapid rotation in old field stars observed by the Kepler mission. Since then, indirect evidence of this phenomenon has accumulated in the form of reliable rotation periods and ages for larger samples of stars. More recently, direct evidence has started to emerge from spectropolarimetric inferences of magnetic field strength and morphology for a sample of field stars that span the transition to WMB. These measurements demonstrate unambiguously that solar-type stars enter the WMB regime at a Rossby number slightly below the solar value, suggesting that our own solar system is already in this regime. Time series observations and numerical simulations of the Sun and other cool stars provide important constraints on the underlying transition in stellar dynamos that ultimately leads to WMB, through the disruption of magnetic organization on the largest spatial scales. I will provide an overview of the direct evidence of WMB from spectropolarimetry, and I will summarize what these constraints tell us about the evolution of stellar dynamos beyond the middle of main-sequence lifetimes.