Transition of latitudinal differential rotation as a possible cause of weakened magnetic braking of solar-type stars. (arXiv:2211.13522v3 [astro-ph.SR] UPDATED)

Transition of latitudinal differential rotation as a possible cause of weakened magnetic braking of solar-type stars. (arXiv:2211.13522v3 [astro-ph.SR] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Tokuno_T/0/1/0/all/0/1">Takato Tokuno</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Suzuki_T/0/1/0/all/0/1">Takeru K. Suzuki</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shoda_M/0/1/0/all/0/1">Munehito Shoda</a>

We investigate the role of latitudinal differential rotation (DR) in the spin
evolution of solar-type stars. Recent asteroseismic observation detected the
strong equator-fast DR in some solar-type stars. Numerical simulations show
that the strong equator-fast DR is a typical feature of young fast-rotating
stars and that this tendency is gradually reduced with stellar age.
Incorporating these properties, we develop a model for the long-term evolution
of stellar rotation. The magnetic braking is assumed to be regulated dominantly
by the rotation rate in the low-latitude region. Therefore, in our model, stars
with the equator-fast DR spin down more efficiently than those with the
rigid-body rotation. We calculate the evolution of stellar rotation in ranges
of stellar mass, $0.9 , mathrm{M}_{odot} le M le 1.2,
mathrm{M}_{odot}$, and metallicity, $0.5, mathrm{Z}_{odot} le Z le 2,
mathrm{Z}_{odot}$, where $mathrm{M}_{odot}$ and $mathrm{Z}_{odot}$ are
the solar mass and metallicity, respectively. Our model, using the observed
torque in the present solar wind, nicely explains both the current solar
rotation and the average trend of the rotation of solar-type stars, including
the dependence on metallicity. In addition, our model naturally reproduces the
observed trend of the weakened magnetic braking in old slowly rotating
solar-type stars because strong equator-fast DR becomes reduced. Our results
indicate that LDR and its transition are essential factors that control the
stellar spin down.

We investigate the role of latitudinal differential rotation (DR) in the spin
evolution of solar-type stars. Recent asteroseismic observation detected the
strong equator-fast DR in some solar-type stars. Numerical simulations show
that the strong equator-fast DR is a typical feature of young fast-rotating
stars and that this tendency is gradually reduced with stellar age.
Incorporating these properties, we develop a model for the long-term evolution
of stellar rotation. The magnetic braking is assumed to be regulated dominantly
by the rotation rate in the low-latitude region. Therefore, in our model, stars
with the equator-fast DR spin down more efficiently than those with the
rigid-body rotation. We calculate the evolution of stellar rotation in ranges
of stellar mass, $0.9 , mathrm{M}_{odot} le M le 1.2,
mathrm{M}_{odot}$, and metallicity, $0.5, mathrm{Z}_{odot} le Z le 2,
mathrm{Z}_{odot}$, where $mathrm{M}_{odot}$ and $mathrm{Z}_{odot}$ are
the solar mass and metallicity, respectively. Our model, using the observed
torque in the present solar wind, nicely explains both the current solar
rotation and the average trend of the rotation of solar-type stars, including
the dependence on metallicity. In addition, our model naturally reproduces the
observed trend of the weakened magnetic braking in old slowly rotating
solar-type stars because strong equator-fast DR becomes reduced. Our results
indicate that LDR and its transition are essential factors that control the
stellar spin down.

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