The debris cloud in the terrestrial zone of the 10-million-year-old star HD 166191 was created by a large impact involving objects of several hundred kilometers in size, according to a paper published in the Astrophysical Journal.
“There is no substitute for being an eyewitness to an event,” said Dr. George Rieke, an astronomer with the Steward Observatory and the Lunar and Planetary Laboratory at the University of Arizona.
“All the cases reported previously from NASA’s Spitzer Space Telescope have been unresolved, with only theoretical hypotheses about what the actual event and debris cloud might have looked like.”
HD 166191 is a young F- to early G-type star located 329 light-years away in the constellation of Sagittarius.
Also known as HIC 89046, IRAS 18074-2334, SAO 186404, and TIC 114619435, the star is surrounded by a large amount of circumstellar dust.
Beginning in 2015, Dr. Rieke and colleagues started making routine observations of HD 166191.
“Around this early time in a star’s life, dust left over from its formation has clumped together to form rocky bodies called planetesimals — seeds of future planets,” they said.
“Once the gas that previously filled the space between those objects has dispersed, catastrophic collisions between them become common.”
Anticipating they might see evidence of one of these collisions around HD 166191, the astronomers used Spitzer to conduct more than 100 observations of the system between 2015 and 2019.
While the planetesimals are too small and distant to resolve by telescope, their smashups produce large amounts of dust.
Spitzer detected infrared light — or wavelengths slightly longer than what human eyes can see. Infrared is ideal for detecting dust, including the debris created by protoplanet collisions.
In mid-2018, the telescope saw the HD 166191 system become significantly brighter, suggesting an increase in debris production.
During that time, Spitzer also detected a debris cloud blocking the star.
Combining Spitzer’s observation of the transit with observations by telescopes on the ground, the team could deduce the size and shape of the debris cloud.
Their work suggests the cloud was highly elongated, with a minimum estimated area three times that of the star.
However, the amount of infrared brightening Spitzer saw suggests only a small portion of the cloud passed in front of the star and that the debris from this event covered an area hundreds of times larger than that of the star.
To produce a cloud that big, the objects in the main collision must have been the size of dwarf planets, like Vesta in our Solar System.
The initial clash generated enough energy and heat to vaporize some of the material.
It also set off a chain reaction of impacts between fragments from the first collision and other small bodies in the system, which likely created a significant amount of the dust Spitzer saw.
Over the next few months, the large dust cloud grew in size and became more translucent, indicating that the dust and other debris were quickly dispersing throughout the young star system.
By 2019, the cloud that passed in front of the star was no longer visible, but the system contained twice as much dust as it had before Spitzer spotted the cloud.
“By looking at dusty debris disks around young stars, we can essentially look back in time and see the processes that may have shaped our own Solar System,” said Dr. Kate Su, an astronomer with the Steward Observatory at the University of Arizona.
“Learning about the outcome of collisions in these systems, we may also get a better idea of how frequently rocky planets form around other stars.”
Kate Y.L. Su et al. 2022. A Star-sized Impact-produced Dust Clump in the Terrestrial Zone of the HD 166191 System. ApJ 927, 135; doi: 10.3847/1538-4357/ac4bbb
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