Breaking Gyrochronology through the Collapse of Coronal Winds

Abstract

Gyrochronology, a method for dating aged field stars (≳a few Gyr) based on their rotation rate, has recently been shown to fail for many stars older than the Sun. The explanation most often put forth is that a shutdown or mode change in the stellar dynamo leads to a sharp decrease in angular momentum loss in magnetized coronal winds. In this paper, we explore an alternate possibility, namely, a collapse of the wind itself through a reduction of coronal heating. We show that in the low coronal temperature (T0) limit, even at solar-like low rotation rates (Ω) and coronal magnetic field strength (Br0), magnetocentrifugal effects are important and preclude expression of the mass and angular momentum loss rates as power laws of T0 or Ω when T0 drops below ≃1.5 MK. Mass loss is found to scale linearly with power input into the wind at all coronal temperatures. Introducing an ad hoc power-law relationship while retaining the “standard” dynamo relationship Br0 ∝ Ω, we show that reproducing the observed break in gyrochronology requires an exponent σ ≳ 1.5, which is associated with a drop by over 3 orders of magnitude in power input into the quiet corona. This appears physically unrealistic, given current observations of chromospheric and coronal nonthermal emission in aged solar-type stars.