Why is this weird, metallic star hurtling out of the Milky Way?
Astronomers analyzed light data from a piece of supernova shrapnel to
gain clues about where it came from

Date:
August 2, 2021
Source:
Boston University
Summary:
About 2,000 light-years away from Earth, there is a star catapulting
toward the edge of the Milky Way. This particular star 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.



FULL STORY ========================================================================== 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.

========================================================================== Story Source: Materials provided by Boston_University. Original written
by Jessica Colarossi.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. J. J. Hermes, Odelia Putterman, Mark A. Hollands, David J. Wilson,
Andrew
Swan, Roberto Raddi, Ken J. Shen, Boris T. Ga"nsicke. 8.9 hr
Rotation in the Partly Burnt Runaway Stellar Remnant LP 40-365
(GD 492). The Astrophysical Journal Letters, 2021; 914 (1): L3 DOI:
10.3847/2041-8213/ ac00a8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210802160646.htm

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