Astronomers using NASA’s Chandra X-ray Observatory and ESA’s Gaia satellite have studied a sample of 6,003 stars in 10 different open clusters with ages of 7-25 million years. Their goals was to learn how the magnetic activity levels of stars like our Sun change during the first tens of millions of years after they form.
Penn State University astronomers Konstantin Getman and his colleagues combined the Chandra data of the stars’ activity with data from ESA’s Gaia satellite to determine which stars are in the open clusters and which ones are in the foreground or background.
They combined their results for the open clusters with previously published Chandra studies of stars as young as 500,000 years old.
They found that the X-ray brightness of young, Sun-like stars is roughly constant for the first few million years, and then fades from 7 to 25 million years of age.
To explain this decline in activity, they used astronomers’ understanding of the interior of the Sun and Sun-like stars.
Magnetic fields in such stars are generated by a dynamo, a process involving the rotation of the star as well as convection, the rising and falling of hot gas in the star’s interior.
According to the team, the dynamos of Sun-like stars become much less efficient because their convection zones become smaller as they age.
For stars with masses smaller than that of the Sun, this is a relatively gradual process. For more massive stars, a dynamo dies away because the convection zone of the stars disappears.
How active a star is directly affects the formation processes of planets in the disk of gas and dust that surrounds all nascent stars.
The most boisterous, magnetically active young stars quickly clear away their disks, halting the growth of planets.
This activity, measured in X-rays, also affects the potential habitability of the planets that emerge after the disk has disappeared.
If a star is extremely active, then scientists predict it will blast planets in its system with energetic X-rays and ultraviolet light.
In some cases, this high-energy barrage could cause an Earth-sized rocky planet to lose much of its original, hydrogen-rich atmosphere through evaporation within a few million years.
It might also strip away a carbon dioxide-rich atmosphere that forms later, unless it is protected by a magnetic field.
“Using new Chandra observations of 6,003 stellar members of 10 open clusters with ages 7-25 million years, we’ve improved characterization of magnetic activity for the late pre-main-sequence (l-PMS) phase of stellar evolution,” the astronomers said.
“In addition to quantifying the evolution of X-ray activity in young stars for a range of stellar masses, the results give insight into two astrophysical issues: the response of magnetic dynamo processes to rapid l-PMS changes in interior structure, and the effects of high-energy radiation on protoplanetary disks and primordial planetary atmospheres.”
The findings were published in the Astrophysical Journal.
Konstantin V. Getman et al. 2022. Evolution of X-Ray Activity in <25 Myr Old Pre-main Sequence Stars. ApJ 935, 43; doi: 10.3847/1538-4357/ac7c69
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