In Aftermath of a Supernova, Hubble reveals a Surviving Companion Star

It’s not uncommon to find a surviving star at the site of a massive supernova explosion, which would normally wipe everything around it, but new Hubble Space Telescope research has revealed a long-awaited clue to a specific sort of stellar death. Astronomers have discovered no trace of the former star’s outermost layer of hydrogen in some supernova events. What became of the hydrogen? Hubble’s detection of a surviving companion star on the scene of supernova 2013ge lends credence to the theory that companion stars are to blame, siphoning away their partners’ outer shells before their deaths.

The finding also gives credence to the hypothesis that the vast majority of large stars begin and evolve as binary systems. It might potentially be a precursor to another cosmic drama: In time, the surviving, massive companion star will likewise supernova, and if the leftover cores of both stars are not flung from the system, they will fuse and emit gravitational waves, disturbing the fabric of space itself.

A companion star previously buried in the brilliance of its partner’s supernova has been discovered by NASA’s Hubble Space Telescope as a witness at the scene of a star’s cataclysmic death. The discovery is the first of its kind for a specific form of supernova, one in which the star was stripped of its entire outer gas envelope before exploding.

The discovery sheds light on the binary nature of huge stars, as well as a possible precursor to the final merging of the companion stars, which would reverberate across the universe as gravitational waves, vibrations in the fabric of spacetime itself.

The signatures of numerous components in supernova explosions are detected by astronomers. These ingredients are piled together like an onion before a supernova. Hydrogen is located in a star’s outermost layer, and if no hydrogen is found in the aftermath of a supernova, it suggests it was stripped away before the explosion.

The origin of the hydrogen loss had previously been unknown, and astronomers have used Hubble to look for clues and test theories to explain these stripped supernovae. The latest Hubble data provide the greatest evidence yet to support the notion that an unseen companion star siphons off its partner star’s gas envelope before it explodes.

“This was the moment we had been waiting for, finally seeing evidence for a binary system progenitor of a fully stripped supernova,” stated principal scientist on the Hubble research program, astronomer Ori Fox of the Space Telescope Science Institute in Baltimore, Maryland. “The idea is to advance this area of study from theory to working with data and seeing what these systems look like in practice.”

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.

With the surviving companion of SN 2013ge, we might potentially be seeing the prelude to a gravitational wave event, although such an event would still be approximately a billion years in the future.

Ori Fox

Two by two?

Previously, scientists hypothesized that the high winds of a giant progenitor star may sweep away its hydrogen gas envelope, but empirical evidence did not support this. Astronomers created ideas and models to explain the disconnect, in which a binary companion siphons off the hydrogen.

“In recent years several different lines of evidence have told us that stripped supernovae are likely created in binaries, but we had yet to actually see the companion. So much of analyzing cosmic explosions is like forensic science — hunting for clues and evaluating what ideas match. Thanks to Hubble, we are able to see this firsthand,” said Maria Drout of the University of Toronto, a member of the Hubble research team.

In prior observations of SN 2013ge, Hubble noticed two peaks in the ultraviolet radiation, rather from simply the one generally seen in most supernovae. Fox stated that one theory 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 feasible. Hubble’s latest observations reveal that while the companion star was considerably jostled, including the hydrogen gas it had drained off its partner, it was not destroyed. Fox likens the impact to a jiggling bowl of jelly, which will eventually settle back to its former 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

In contrast to supernovae that have a puffy shell of gas to illuminate, the progenitors of entirely stripped-envelope supernovae have been difficult to spot in pre-explosion photographs. Astronomers can now utilize the surviving companion star to work backward and discover features of the star that exploded, as well as the unprecedented ability to witness the aftermath unfold with the survivor.

As a big star in its own right, SN 2013ge’s companion will also go supernova. Its former partner is now likely a compact entity, such as a neutron star or black hole, and the companion will likely travel that road as well.

The initial companion stars’ closeness will determine whether they remain together. If the distance is too enormous, the companion star will be thrown out of the system, wandering alone throughout our galaxy, a destiny that could explain many seemingly lonely supernovae.

If the stars were close enough to each other before the explosion, they will continue to orbit each other as black holes or neutron stars. In that situation, they would gradually spiral toward one other and unite, creating gravitational waves in the process.

That is an exciting potential for astronomers, as gravitational waves are an area of astrophysics that has only recently begun to be investigated. 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 seen by the Laser Interferometer Gravitational-Wave Observatory.

“With the surviving companion of SN 2013ge, we might potentially be seeing the prelude to a gravitational wave event, although such an event would still be approximately a billion years in the future,” Fox added. Fox and his colleagues will collaborate with Hubble to assemble a bigger sample of surviving partner stars to other supernovae, effectively giving SN 2013ge some company once more.

“Beyond simply understanding the supernova, there is enormous potential. Because we now know that the majority of big stars in the cosmos form in binary pairs, investigations of surviving partner stars are required to help understand the mechanics of binary formation, material-swapping, and co-evolutionary growth. It’s an exciting time to be studying the stars,” Fox said.

“Understanding the lifespan of large stars is very crucial because all heavy elements are created in their cores and supernovae. These components comprise a large portion of the observable cosmos, including life as we know it “Alex Filippenko of the University of California, Berkeley, is a co-author.

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