Young Star Caught Snacking on Planetary Debris

For decades, astronomers have puzzled over the variability of young stars residing in Taurus-Auriga dark clouds, a group of molecular clouds located in the constellations of Taurus and Auriga, about 450 light-years away. Since 1937, they have recorded noticeable dips in the brightness of RW Aur A — the primary star of a low-mass binary system — every few decades. Each dimming event appeared to last for about a month. In 2011, the star dimmed again, this time for about half a year. RW Aur A eventually brightened, only to fade again in mid-2014. In November 2016, the star returned to its full luminosity. Now Hans Moritz Günther of MIT and co-authors have observed RW Aur A using NASA’s Chandra X-Ray Observatory. They’ve found evidence for what may have caused its most recent dimming event: a collision of two protoplanetary bodies, which produced in its aftermath a dense cloud of gas and dust. As this planetary debris fell into RW Aur A, it generated a thick veil, temporarily obscuring the star’s light.

“Computer simulations have long predicted that planets can fall into a young star, but we have never before observed that,” Dr. Günther said.

“If our interpretation of the data is correct, this would be the first time that we directly observe a young star devouring a planet or planets.”

Dr. Günther and co-authors focus on stars that are young enough to still host protoplanetary disks — rotating disks of debris, including gas, dust, and clumps of material ranging in size from small dust grains to pebbles, and possibly to fledgling planets.

They were particularly interested in RW Aur A, which is at the older end of the age range for young stars, as it is estimated to be 10 million years old.

RW Aur A is part of a binary system, meaning that it circles another young star, RW Aur B. Both these stars are about the same mass as the Sun.

Some astronomers have proposed that RW Aur A’s mysterious dimming is caused by a passing stream of gas at the outer edge of the star’s disk. Still others have theorized that the dimming is due to processes occurring closer to the star’s center.

“We wanted to study the material that covers the star up, which is presumably related to the disk in some way. It’s a rare opportunity,” Dr. Günther said.

In January 2017, RW Aur A dimmed again, and the team used NASA’s Chandra X-Ray Observatory to record X-ray emission from the star.

“The X-rays come from the star, and the spectrum of the X-rays changes as the rays move through the gas in the disk. We’re looking for certain signatures in the X-rays that the gas leaves in the X-ray spectrum,” Dr. Günther noted.

In total, Chandra recorded almost 14 hours of X-ray data from RW Aur A.

After analyzing these data, the astronomers came away with several surprising revelations: the star’s disk hosts a large amount of material; the star is much hotter than expected; and the disk contains much more iron than expected — not as much iron as is found in the Earth, but more than a typical moon in our Solar System. This last point was the most intriguing for the team.

Typically, an X-ray spectrum of a star can show various elements, such as oxygen, iron, silicon, and magnesium, and the amount of each element present depends on the temperature within a star’s disk.

“Here, we see a lot more iron, at least a factor of 10 times more than before, which is very unusual, because typically stars that are active and hot have less iron than others, whereas this one has more. Where does all this iron come from?” Dr. Günther said.

The researchers speculate that this excess iron may have come from one of two possible sources.

The first is a phenomenon known as a dust pressure trap, in which small grains or particles such as iron can become trapped in ‘dead zones’ of a disk. If the disk’s structure changes suddenly, such as when the star’s partner star passes close by, the resulting tidal forces can release the trapped particles, creating an excess of iron that can fall into the star.

In the second scenario, excess iron is created when two planetesimals, or infant planetary bodies, collide, releasing a thick cloud of particles. If one or both planets are made partly of iron, their smash-up could release a large amount of iron into the star’s disk and temporarily obscure its light as the material falls into the star.

“There are many processes that happen in young stars, but these two scenarios could possibly make something that looks like what we observed,” Dr. Günther said.

The research is published in the Astronomical Journal.

Source: Sci News

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