New study reveals enhanced magnetic activity across the stellar atmosphere after the intermediate rotation-period gap
Stars change as they age. Like our Sun, they rotate and generate magnetic fields, producing phenomena such as dark spots on their surfaces and influencing the activity of their atmospheres. As stars age, they generally rotate more slowly and their magnetic activity declines. However, observations have revealed an unexpected feature in this evolution: a shortage of stars with certain intermediate rotation periods. This intermediate rotation-period gap suggests that stellar rotational evolution is more complex than a smooth progression from rapid to slow rotation.
Previous studies have found that the gap is associated with changes in photospheric magnetic activity, measured through variations in a star's brightness caused by features such as dark spots. The photosphere is the visible layer from which most of a star's light emerges, while the chromosphere lies above it and provides a different view of stellar magnetic activity. Until recently, it was unknown whether similar changes in magnetic activity would also appear across these different layers of the stellar atmosphere.
To investigate this question, researchers at the Instituto de Astrofísica de Canarias (IAC) studied stars observed by NASA's Kepler space telescope, combining their rotation periods with spectroscopic observations from the European Space Agency's Gaia mission. They examined the near-infrared spectral lines of ionised calcium, known as the Ca II infrared triplet. These lines provide a way to trace chromospheric activity and offer an independent perspective on how stellar magnetic behaviour changes with rotation.
The results reveal hints of enhanced chromospheric activity after the intermediate rotation-period gap, on its slower-rotating side. Activity reaches a minimum near the gap and increases at longer rotation periods. This overall behaviour resembles the pattern previously identified in photospheric magnetic activity, although the corresponding features occur at different rotation periods. The agreement between these two independent indicators suggests that the enhancement in magnetic activity extends across different layers of the stellar atmosphere, rather than being confined to the visible surface.
The intermediate rotation-period gap is thought to be linked to changes in how angular momentum is exchanged between a star's core and its outer layers, a process known as core–envelope coupling. By finding evidence of enhanced magnetic activity beyond the gap in the chromosphere, this study provides a new observational clue to understanding the connection between stellar rotation and magnetism. These insights can also help refine the use of stellar rotation and activity as indicators of stellar age, improving our understanding of how stars and their planetary systems evolve.
