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Being a planet can be a
dangerous business, particularly at the start and end of a star's life. We have
reason to believe stars often devour their own planets, and have even
pinpointed cases where it seems to have happened long ago, but now we’re
getting to watch the effects in real time.
RW Aur A is a young star 450
light-years away. Eighty years ago astronomers noticed a dip in brightness
lasting around a month, something that has been repeated every few decades.
Recently these dips have become more frequent, with one lasting six months.
These weren’t subtle changes, like when a planet passes in front of a star –
the most recent saw light reduced by a factor of six. Meanwhile, the stellar
companion RW Aur B has remained unchanged.
Given RW Aur A's relative
youth, these changes are not as far outside the norms of stellar behavior as
those of Tabby's Star, so we haven't had to resort to theories like alien megastructures,
but something definitely needed explaining.
He thinks two objects
orbiting the star collided, creating a cloud of debris. The dimming is a result
of the dust released by the collision passing between us and RW Aur A, blocking
some of its light. Follow-up collisions between remnants have renewed this
occasionally. Some of this dust has been absorbed into the star's corona,
changing its composition and the spectrum we see.
We can't tell a lot about
the collision’s components, but Guenther says at least one must have been large
enough to be considered a planet. "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,” he said in a statement.
By studying the changes in
brightness at both X-ray and visible light wavelengths, Guenther and co-authors
of a paper published in The Astronomical Journal were able to determine that
there is a lot of iron in the dust cloud, and in the coronal absorption, with
10 times as much present during the dim phase in 2017 as when RW Aur A was at
its brightest in 2013.
We've previously found two
stars, thought to be born from the same cloud, with very different metal contents.
The likeliest explanation is that the more metal-rich star consumed rocky
material equivalent to 15 times the mass of the Earth. However, the
cannibalistic ways of that Sun-aged star were probably long in the past.
We've also seen less-certain
examples where the process seems to have been more recent, but these were still
millions of years ago, rather than occurring as we watch.
RW Aur A is thought to be
less than 10 million years old, exceptionally young by star standards. We
expect most cases of planet consumption to happen at times like this, before
larger objects on collision orbits are weeded out, or towards the end of a
star's life as it puffs up to become a red giant. The gravitational effects of
RW Aur B probably disturb planetary orbits, making such collisions particularly
likely.
Even aside from the dimming,
RW Aur A is an interesting star because it has jets of material visible in
X-ray wavelengths coming from its poles.
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