Towards a holistic magnetic braking model – II: explaining several long-term internal- and surface-spin properties of solar-like stars and the Sun
arxiv(2024)
摘要
We extend our model of magnetic braking (MB) from fully convective M-dwarfs
(FCMDs) to explain the surface and internal spin P_spin evolution of
partly convective dwarfs (PCDs) starting from disc-dispersal stage to
main-sequence turnoff. In our model, the spin of the core is governed by shear
at the core-envelope boundary while the spin of the envelope is governed by MB
and shear. We show that (1) the most massive FCMDs experience a stronger
spin-down than PCDs and less massive FCMDs; (2) the stalled spin-down and
enhanced activity of K-dwarfs and the pileup of G-dwarfs older than a few Gyr
are stellar-structure- and MB-dependent, and weakly dependent on core-envelope
coupling effects; (3) our empirical expression of the core-envelope convergence
time-scale τ_converge(M_∗, P_spin) between a few 10
to 100 Myr is strongly dependent on stellar structure and weakly dependent on
MB strength and shear, where fast and massive rotators achieve corotation
earlier; (4) our estimates of the surface magnetic fields are in general
agreement with observations and our wind mass loss evolution explains the weak
winds from the solar analog π^1 UMa; (5) the massive young Sun theory as a
solution to the faint young Sun problem, which states that the early Sun was
sufficiently more massive to maintain liquid water on Earth when the Sun's
luminosity would have been about 30 percent lower, can likely be ruled out
because the maximum mass lost by winds from our Sun with our model is about
0.001M_⊙, an order of magnitude smaller than required to solve the
problem with this theory.
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