Kyoto, Japan -- From birth to death, stars generally slow by 100 to 1000 times their initial rotation rates; in other words, they spin down. The Sun's total angular momentum has declined as material is gradually blown off at the surface as solar wind. By observing this, astronomers have theorized the interaction between magnetic fields and plasma flow to be the most efficient way to spin down stars.

Why and how this happens has long interested astronomers, and recently an observational technique called astroseismology, which measures a star's natural oscillation frequencies, has made it possible to measure the internal rotation rates and magnetic fields of other stars in our galaxy. From this huge population, a picture of how stellar rotation decreases with stellar age has emerged, one that suggests that current theory is insufficient to explain the dramatic decrease in rotation.

Fascinated by astroseismology and by other researchers' 3D simulations of the solar convective zone, a team of researchers at Kyoto University was inspired to investigate how magnetic fields affect rotation inside massive stars..

Kyoto, Japan --  The Fe Kα line, or iron Kα line, is often used in astronomical research to understand the physical composition of astronomical objects. This line is produced when a K-shell electron of an iron ion in the photosphere -- the gas on the stellar surface -- is ejected by an external process, and has been detected in X-ray spectra of solar and stellar flares. Yet the dominant mechanism behind this ionization process has remained an open question for many years.

Astronomers have proposed two possible mechanisms: photoionization by X-ray photons emitting from hot flare plasma, or collisional ionization by high-energy electrons accelerating at the onset of the flare. With these two possibilities in mind, a team of researchers at Kyoto University set out to uncover the truth behind the iron Kα line.

The team focused on the triple star system UX Arietis, conducting several days of simultaneous ultraviolet and X-ray observations using NICER, NASA's X-ray telescope aboard the International Space Station, and Hisaki, JAXA's ultraviolet space telescope. While Hisaki was developed primarily for observations of planets in the Solar System, the researchers demonstrated that it can also be used to study distant stars.