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 ( T 0 ) limit, even at solar-like low rotation rates (Ω) and coronal magnetic field strength ( B r 0 ), magnetocentrifugal effects are important and preclude expression of the mass and angular momentum loss rates as power laws of T 0 or Ω when T 0 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>B</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>r</mml:mi> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mi>σ</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> while retaining the “standard” dynamo relationship B r 0 ∝ 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.