Boston University astronomers analyzed light data from a piece of supernova shrapnel—a star called LP 40−365—to gain clues about where it came from
From:
Boston University
August
2, 2021 -- About 2,000 light-years away from Earth, there is a star catapulting
toward the edge of the Milky Way. This particular star, known as LP 40−365, is
one of a unique breed of fast-moving stars—remnant pieces of massive white
dwarf stars—that have survived in chunks after a gigantic stellar
explosion.
“This star is moving so fast that it’s
almost certainly leaving the galaxy…[it’s] moving almost two million miles an
hour,” says JJ Hermes, Boston University College of Arts & Sciences
assistant professor of astronomy. But why is this flying object speeding out of
the Milky Way? Because it’s a piece of shrapnel from a past explosion—a cosmic
event known as a supernova—that’s still being propelled forward.
“To have gone through partial detonation
and still survive is very cool and unique, and it’s only in the last few years
that we’ve started to think this kind of star could exist,” says Odelia
Putterman, a former BU student who has worked in Hermes’ lab.
In a new paper published in The
Astrophysical Journal Letters, Hermes and Putterman uncover new
observations about this leftover “star shrapnel” that gives insight to other
stars with similar catastrophic pasts.
Putterman and Hermes analyzed data from
NASA’s Hubble Space Telescope and Transiting Exoplanet Survey Satellite (TESS),
which surveys the sky and collects light information on stars near and far. By
looking at various kinds of light data from both telescopes, the researchers
and their collaborators found that LP 40−365 is not only being hurled out of
the galaxy, but based on the brightness patterns in the data, is also rotating
on its way out.
“The star is basically being
slingshotted from the explosion, and we’re [observing] its rotation on its way
out,” says Putterman, who is second author on the paper.
“We dug a little deeper to figure out
why that star [was repeatedly] getting brighter and fainter, and the simplest
explanation is that we’re seeing something at [its] surface rotate in and out
of view every nine hours,” suggesting its rotation rate, Hermes says. All stars
rotate—even our sun slowly rotates on its axis every 27 days. But for a star
fragment that’s survived a supernova, nine hours is considered relatively
slow.
Supernovas occur when a white dwarf gets
too massive to support itself, eventually triggering a cosmic detonation of
energy. Finding the rotation rate of a star like LP 40−365 after a supernova
can lend clues into the original two-star system it came from. It’s common in
the universe for stars to come in close pairs, including white dwarfs, which
are highly dense stars that form toward the end of a star’s life. If one white
dwarf gives too much mass to the other, the star being dumped on can
self-destruct, resulting in a supernova. Supernovas are commonplace in the
galaxy and can happen in many different ways, according to the researchers, but
they are usually very hard to see. This makes it hard to know which star did the
imploding and which star dumped too much mass onto its star partner.
Based on LP 40−365’s relatively slow
rotation rate, Hermes and Putterman feel more confident that it is shrapnel
from the star that self-destructed after being fed too much mass by its partner,
when they were once orbiting each other at high speed. Because the stars were
orbiting each other so quickly and closely, the explosion slingshotted both
stars, and now we only see LP 40–365.
“This [paper] adds one more layer of
knowledge into what role these stars played when the supernova occurred,” and
what can happen after the explosion, Putterman says. “By understanding what’s
happening with this particular star, we can start to understand what’s
happening with many other similar stars that came from a similar
situation.”
“These are very weird stars,” Hermes
says. Stars like LP 40–365 are not only some of the fastest stars known to
astronomers, but also the most metal-rich stars ever detected. Stars like our
sun are composed of helium and hydrogen, but a star that has survived a
supernova is primarily composed of metal material, because “what we’re seeing
are the by-products of violent nuclear reactions that happen when a star blows
itself up,” Hermes says, making star shrapnel like this especially fascinating
to study.
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