Hubble reveals surviving companion star in aftermath of supernova
The discovery helps explain the puzzle of hydrogen loss pre-supernova,
and supports the theory that most massive stars are paired.

Date:
May 5, 2022
Source:
NASA/Goddard Space Flight Center
Summary:
It's not unheard of to find a surviving star at the scene of a
titanic supernova explosion, which would be expected to obliterate
everything around it, but new research has provided a long-awaited
clue to a specific type of stellar death. In some supernova cases,
astronomers find no trace of the former star's outermost layer of
hydrogen. What happened to the hydrogen? Suspicions that companion
stars are responsible - - siphoning away their partners' outer shell
before their death -- are supported by the recent identification
of a surviving companion star on the scene of supernova 2013ge.



FULL STORY ==========================================================================
It's not unheard of to find a surviving star at the scene of a titanic supernova explosion, which would be expected to obliterate everything
around it, but the latest research from the Hubble Space Telescope has
provided a long-awaited clue to a specific type of stellar death. In
some supernova cases, astronomers find no trace of the former star's
outermost layer of hydrogen.

What happened to the hydrogen? Suspicions that companion stars are
responsible -- siphoning away their partners' outer shell before their
death -- are supported by Hubble's identification of a surviving companion
star on the scene of supernova 2013ge.


==========================================================================
The discovery also lends support to the theory that the majority of
massive stars form and evolve as binary systems. It could also be
the prequel to another cosmic drama: In time, the surviving, massive
companion star will also undergo a supernova, and if both the stars'
remnant cores are not flung from the system, they will eventually merge
and produce gravitational waves, shaking the fabric of space itself.

NASA's Hubble Space Telescope has uncovered a witness at the scene of a
star's explosive death: a companion star previously hidden in the glare
of its partner's supernova. The discovery is a first for a particular
type of supernova -- one in which the star was stripped of its entire
outer gas envelope before exploding.

The finding provides crucial insight into the binary nature of massive
stars, as well as the potential prequel to the ultimate merger of the
companion stars that would rattle across the universe as gravitational
waves, ripples in the fabric of spacetime itself.

Astronomers detect the signature of various elements in supernova
explosions.

These elements are layered like an onion pre-supernova. Hydrogen is found
in the outermost layer of a star, and if no hydrogen is detected in the aftermath of the supernova, that means it was stripped away before the explosion occurred.

The cause of the hydrogen loss had been a mystery, and astronomers have
been using Hubble to search for clues and test theories to explain these stripped supernovae. The new Hubble observations provide the best evidence
yet to support the theory that an unseen companion star siphons off the
gas envelope from its partner star before it explodes.



========================================================================== "This was the moment we had been waiting for, finally seeing the evidence
for a binary system progenitor of a fully stripped supernova," said
astronomer Ori Fox of the Space Telescope Science Institute in Baltimore, Maryland, lead investigator on the Hubble research program. "The goal is
to move this area of study from theory to working with data and seeing
what these systems really look like." Fox's team used Hubble's Wide
Field Camera 3 to study the region of supernova (SN) 2013ge in ultraviolet light, as well as previous Hubble observations in the Barbara A. Mikulski Archive for Space Telescopes. Astronomers saw the light of the supernova
fading over time from 2016 to 2020 -- but another nearby source of
ultraviolet light at the same position maintained its brightness.

This underlying source of ultraviolet emission is what the team proposes
is the surviving binary companion to SN 2013ge.

Two by two? Previously, scientists theorized that a massive progenitor
star's strong winds could blow away its hydrogen gas envelope, but observational evidence didn't support that. To explain the disconnect, astronomers developed theories and models in which a binary companion
siphons off the hydrogen.

"In recent years many different lines of evidence have told us that
stripped supernovae are likely formed in binaries, but we had yet to
actually see the companion. So much of studying cosmic explosions is
like forensic science - - searching for clues and seeing what theories
match. Thanks to Hubble, we are able to see this directly," said Maria
Drout of the University of Toronto, a member of the Hubble research team.



==========================================================================
In prior observations of SN 2013ge, Hubble saw two peaks in the
ultraviolet light, rather than just the one typically seen in most
supernovae. Fox said that one explanation for this double brightening was
that the second peak shows when the supernova's shock wave hit a companion star, a possibility that now seems much more likely. Hubble's latest observations indicate that while the companion star was significantly
jostled, including the hydrogen gas it had siphoned off its partner,
it was not destroyed. Fox likens the effect to a jiggling bowl of jelly,
which will eventually settle back to its original form.

While additional confirmation and similar supporting discoveries need
to be found, Fox said that the implications of the discovery are still substantial, lending support to theories that the majority of massive
stars form and evolve as binary systems.

One to Watch Unlike supernovae that have a puffy shell of gas to light
up, the progenitors of fully stripped-envelope supernovae have proven
difficult to identify in pre- explosion images. Now that astronomers
have been lucky enough to identify the surviving companion star, they
can use it to work backward and determine characteristics of the star
that exploded, as well as the unprecedented opportunity to watch the
aftermath unfold with the survivor.

As a massive star itself, SN 2013ge's companion is also destined to
undergo a supernova. Its former partner is now likely a compact object,
such as a neutron star or black hole, and the companion will likely go
that route as well.

The closeness of the original companion stars will determine if they
stay together. If the distance is too great, the companion star will be
flung out of the system to wander alone across our galaxy, a fate that
could explain many seemingly solitary supernovae.

However, if the stars were close enough to each other pre-supernova, they
will continue orbiting each other as black holes or neutron stars. In
that case, they would eventually spiral toward each other and merge,
creating gravitational waves in the process.

That is an exciting prospect for astronomers, as gravitational waves
are a branch of astrophysics that has only begun to be explored. They
are waves or ripples in the fabric of spacetime itself, predicted by
Albert Einstein in the early 20th century. Gravitational waves were
first directly observed by the Laser Interferometer Gravitational-Wave Observatory.

"With the surviving companion of SN 2013ge, we could potentially be seeing
the prequel to a gravitational wave event, although such an event would
still be about a billion years in the future," Fox said.

Fox and his collaborators will be working with Hubble to build up a larger sample of surviving companion stars to other supernovae, in effect giving
SN 2013ge some company again.

"There is great potential beyond just understanding the supernova
itself. Since we now know most massive stars in the universe form in
binary pairs, observations of surviving companion stars are necessary to
help understand the details behind binary formation, material-swapping,
and co-evolutionary development. It's an exciting time to be studying
the stars," Fox said.

"Understanding the lifecycle of massive stars is particularly important
to us because all heavy elements are forged in their cores and through
their supernovae. Those elements make up much of the observable universe, including life as we know it," added co-author Alex Filippenko of the University of California at Berkeley.


========================================================================== Story Source: Materials provided by
NASA/Goddard_Space_Flight_Center. Note: Content may be edited for style
and length.


========================================================================== Related Multimedia:
* Supernova_2013ge_with_its_companion_star ========================================================================== Journal Reference:
1. Ori D. Fox, Schuyler D. Van Dyk, Benjamin F. Williams, Maria Drout,
Emmanouil Zapartas, Nathan Smith, Dan Milisavljevic, Jennifer
E. Andrews, K. Azalee Bostroem, Alexei V. Filippenko, Sebastian
Gomez, Patrick L.

Kelly, S. E. de Mink, Justin Pierel, Armin Rest, Stuart
Ryder, Niharika Sravan, Lou Strolger, Qinan Wang, Kathryn
E. Weil. The Candidate Progenitor Companion Star of the Type
Ib/c SN 2013ge. The Astrophysical Journal Letters, 2022; 929 (1):
L15 DOI: 10.3847/2041-8213/ac5890 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/05/220505114702.htm

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