Dynamos are still active in the weakened magnetic braking regime of sun-like stars

Contributed talk in plenary session. Abstract: The rotational spin-down of cool main-sequence stars governs their long-term magnetic evolution. In recent years, substructure in the age-rotation relationship of cool stars has been identified, showing deviations from the historical Skumanich law. A weakened magnetic braking regime and potentially a break-down of the magnetic dynamo have been proposed as reasons for this deviation. We have investigated a sample of mature main-sequence stars with solar-like masses which display faster rotation than expected. We collected deep observations of those stars in soft X-rays, yielding information on their coronal magnetic activity, and in particular on their X-ray surface fluxes and average coronal temperatures. We detected the majority of the sample in X-rays, at flux levels that are at the low end of the empirical rotation-activity relationship, but not indicative of a full break-down of the magnetic dynamo itself. However, we do find a trend that the more over-rotating a star is for its age, the more under-active it is for its rotation period. We also find that the strongest outliers from the empirical rotation-activity relationship are characterized by very low X-ray fluxes through the stellar surface and very low coronal temperatures. Therefore, angular momentum loss through the stellar wind may be impeded for such stars, while overall magnetic activity can still be observed. An open question is why there are no rotational outliers detected so far in middle-aged open clusters such as M67, challenging an interpretation that relies purely on a reduction of the angular momentum loss rate for middle-aged suns.