7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Transition of latitudinal differential rotation as a possible cause of weakened
magnetic braking of solar-type stars
1

Takato Tokuno1,2, Takeru Suzuki1,2, Munehito Shoda1
School of Science, The University of Tokyo, 2 School of Arts & Sciences, The University of Tokyoe-mail (speaker): tokuno-takato@g.ecc.u-tokyo.ac.jp

The rotation of solar-type stars, including the Sun, is
actively studied because it governs the stellar magnetic
activity through the dynamo mechanism. Becausemagnetized stellar winds emanate from a corona ofsolar-type stars, they carry off not only the mass but also
the angular momentum of a star. Then, solar-type stars
generally spin down through this magnetic braking
mechanism[1]. The formulation of spin-down ofsolar-type stars is investigated through numericalsimulations and observations. Previous formulations ofspin-down explain the basic observed trend of solar-typestars that are younger than the Sun[2]. However, there arestill unsolved problems remained: Stars older than theSun spin down considerably slower than the trend
predicted from the a standard theoretical model[3]. This
breakdown of the spin-down trend, called “weakenedmagnetic breaking”, means that a regime change in themagnetic braking process occurs around the solar
rotation period.
Recent observations using the asteroseismic
method[4] and 3D MHD simulations[5] reveal that thedifferential rotation (DR), especially latitudinaldifferential rotation (LDR), is a typical feature of young
solar-type stars and that this tendency is gradually
reduced with stellar age. Incorporating these properties,
we construct the phenomenological spin-down
formulation[6], which considers an effect of LDR, toresolve the two problems mentioned above. Assuming
the magnetic braking is regulated dominantly by therotation rate in the low-latitude region, stars with theequator-fast DR (pole-fast DR) spin down more
efficiently (inefficiently) than those with the rigid-bodyrotation (see Fig.1).The spin-down of the stars with solar
mass and metallicity under our formulation reproduces
the observed trend of both solar-type stars and the Sun,
including weakened magnetic braking (see Fig.2). Our
results indicate that latitudinal DR and its transition are
essential factors that control the stellar spin down.

Figure 1. The schematic picture of the LDR effect on thestellar spin-down. The top horizontal color-bar indicates thestellar angular velocity, Ω(𝑟, 𝜃), where 𝑟 and 𝜃 denote thedistance from the center and co-latitude, respectively. The
upper and lower quarter sector represents a star withequator-fast DR and pole-fast DR, respectively. Under theassumption that the magnetic braking is regulated by thelow-latitude rotation speed, AM is removed more strongly(weakly) under the equator-fast DR (pole-fast DR).

-References[1] Weber E. J., Davis L.J., ApJ, 147, 217 (1967)[2] Matt S. P. et al., 799, L23 (2015)[3] van Saders J. L. et al. Nature, 529, 181 (2016)[4] Bernomar O. et al., Science, 361, 1231 (2018)[5] Brun A.S. et al., ApJ, 926, 35 (2022)[6] Tokuno T. et al., MNRAS, 520, 418 (2023)
Figure 2. The spin-down of the stars with solar mass andmetallicity considering LDR (orange shaded region). The
abscissa and ordinate represent the stellar age and the stellar
rotation period, respectively. The circles and stars represent theobservational values obtained by open clusters and field stars,respectively. The orange dashed line is the case without LDR.