Photometric magnetic activity proxy (Sph) as a function of the Rossby number (ratio between the rotation period of the star and the convective overturn timescale) for the Kepler stars as a function of the spectral type. For G and K dwarfs, we can see the dip in Sph. The red dash-dot line shows the value of the level of magnetic activity of the Sun between minimum and maximum activity cycles.
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Authors
Savita
Mathur
Ângela R. G. Santos
Zachary R. Claytor
et al.
References
It’s been decades since the need to study other stars to understand the past, present and future of the Sun was realized. One important aspect that has been investigated is the magnetic activity of stars for which we cannot fully grasp the mechanisms involved. Indeed, the origin of stellar magnetic cycles or the dependence of the magnetic activity on the stellar properties are not completely understood. This knowledge improves not only our understanding of the physics involved in stellar evolution but also affects the study of the Sun to better predict high-energy events and the better characterization of the habitability zones of exoplanets.
In this work, we took advantage of the largest sample of solar-like stars observed by the NASA/Kepler mission with measured rotation periods and photometric magnetic activity index (called Sph) to investigate how magnetic activity evolves and depends on properties such as effective temperature, metallicity, luminosity, and rotation period. With such a large sample, we found very interesting distinct behaviors with different spectral types from hot F dwarfs to cool K dwarfs. While K and G dwarfs show a general decrease of magnetic activity as they evolve (or with Rossby number) with a dip around 0.3 solar Rossby, F dwarfs do not present any trend. We could also see that the range of magnetic activity is different between K and G dwarfs, suggesting different positions of the active regions (group of spots) or cycle lengths.
We demonstrated that the magnetic dip in K and G dwarfs is related to the moment when a stalling in the spin down has been discovered previously in Kepler field and cluster stars. One theory to explain the stalling is the coupling between the core and the envelope of a star. Our findings that the photometric magnetic activity increases during that stalling provide hints that the coupling triggers enhanced magnetic activity. This work is very important for dynamo models that intend to reproduce the mechanisms involved in the stellar magnetic cycles.
Finally, our work also showed that the Sun has similar magnetic activity levels to stars with very similar properties, showing that it is not so peculiar in that sense.
