By their nature, stars burn bright – so when they start dimming and fading, astronomers pay attention. It's been speculated that one star that dims on an irregular schedule is home to some sort of "alien megastructure," and although a swarm of comets is the more likely culprit, the star is still a point of interest. Now, astronomers have observed another strangely-twinkling star and found that it might be the result of a collision between proto-planets orbiting it, and the star gobbling up the remains.
The star in question is named RW Aur A, and it resides in the Taurus-Auriga constellation about 450 light-years from Earth. Observations since 1937 have shown that the star's brightness tends to dip every few decades, with that dimmer phase lasting about a month each time. But more recently it's been kicking that activity up a notch. The star dimmed for about six months beginning in 2011, and later it dimmed for more than two years between 2014 and 2016.
The leading theory on why this is happening is that as a young star, RW Aur A is still surrounded by a disk of dust and gas, and the dimming is the result of particularly thick clouds sweeping in front.
When the star dimmed again in January 2017, a team of researchers jumped on the opportunity to find out. Using NASA's Chandra X-Ray Observatory, they recorded the x-ray emissions from RW Aur A to determine what kind of material was in the dust around it.
"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," says Hans Moritz Guenther, lead researcher on the study. "We're looking for certain signatures in the X-rays that the gas leaves in the X-ray spectrum."
After analyzing almost 14 hours worth of data, the team learned a few new things about RW Aur A. Not only is the star hotter than expected, but the disk surrounding it has much more iron in it than assumed.
"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," says Guenther. "Where does all this iron come from?"
The team's best theory explains both the extra iron and the star's latest dimming event. Two planetesimals orbiting the star, the researchers suggest, may have collided, spewing vast quantities of iron into the dusty disk, which then temporarily blocks the light as the star swallows it up. Given the turbulent conditions around the star, this could also conveniently explain the regular dimming sessions.
"Computer simulations have long predicted that planets can fall into a young star, but we have never before observed that," says Guenther. "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."
To get a sense of the size of these proposed protoplanets, the team plans to make more observations of the star, to measure how the iron levels in the cloud change over time. The more that remains, the larger the body it could have come from.
The research was published in the Astronomical Journal.
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