Total Annihilation for Supermassive Stars

Artist’s concept of the SN 2016iet pair-instability supernova. (Image: Gemini Observatory / NSF / AURA / illustration by Joy Pollard)

A renegade star exploding in a distant galaxy has forced astronomers to set aside decades of research and focus on a new breed of supernova that can utterly annihilate its parent star — leaving no remnant behind. The signature event, something astronomers had never witnessed before, may represent the way in which the most supermassive stars in the Universe, including the first stars, die.

The European Space Agency’s (ESA) Gaia satellite first noticed the supernova, known as SN 2016iet, on November 14, 2016.

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Three years of intensive follow-up observations with a variety of telescopes, including the Gemini North telescope and its Multi-Object Spectrograph on Maunakea in Hawai’i, the CfA | Harvard & Smithsonian’s MMT Observatory located at the Fred Lawrence Whipple Observatory in Amado, AZ, and the Magellan Telescopes at the Las Campanas Observatory in Chile, provided crucial perspectives on the object’s distance and composition.

Edo Berger, of the Harvard-Smithsonian Center for Astrophysics and a member of the investigation’s team, said:

Chris Davis, program director at the National Science Foundation (NSF), one of Gemini’s sponsoring agencies, added:

In this case, this deep look revealed only weak hydrogen emission at the location of the supernova, evidence that the progenitor star of SN 2016iet lived in an isolated region with very little star formation. This is an unusual environment for such a massive star. Berger added:

SN 2016iet has a multitude of oddities, including its incredibly long duration, large energy, unusual chemical fingerprints, and environment poor in heavier elements — for which no obvious analogs exist in the astronomical literature.

Sebastian Gomez, also of the Center for Astrophysics and lead author of the investigation, which was published in The Astrophysical Journal, said:

The unusual nature of SN 2016iet, as revealed by Gemini and other data, suggest that it began its life as a star with about 200 times the mass of our Sun — making it one of the most massive and powerful single star explosions ever observed. Growing evidence suggests the first stars born in the Universe may have been just as massive.

Astronomers predicted that if such behemoths retain their mass throughout their brief life (a few million years), they will die as pair-instability supernovae, which gets its name from matter-antimatter pairs formed in the explosion.

Most massive stars end their lives in an explosive event that spews matter rich in heavy metals into space, while their core collapses into a neutron star or black hole. But pair-instability supernovae are a different breed.

Image of SN 2016iet and its most likely host galaxy taken with the Low Dispersion Survey Spectrograph on the Magellan Clay 6.5-m telescope at Las Campanas Observatory in i-band on July 9, 2018.
Image of SN 2016iet and its most likely host galaxy taken with the Low Dispersion Survey Spectrograph on the Magellan Clay 6.5-m telescope at Las Campanas Observatory in i-band on July 9, 2018. (Image: GEMINI Observatory)

The collapsing core produces copious gamma-ray radiation, leading to a runaway production of particle and antiparticle pairs that eventually trigger a catastrophic thermonuclear explosion that annihilates the entire star, including the core.

Models of pair-instability supernovae predict they will occur in environments poor in metals (astronomer’s term for elements heavier than hydrogen and helium), such as dwarf galaxies and the early Universe — and the team’s investigation found just that.

The event occurred at a distance of 1 billion light-years in a previously uncatalogued dwarf galaxy poor in metals. Gomez said:

Another surprising feature is SN 2016iet’s stark location. Most massive stars are born in dense clusters of stars, but SN 2016iet formed in isolation some 54,000 light-years away from the center of its dwarf host galaxy. Gomez added:

To explain the event’s long duration and slow brightness evolution, the team advances the idea that the progenitor star ejected matter into its surrounding environment at a rate of about three times the mass of the Sun per year for a decade before the star blew itself into oblivion.

When the star ultimately exploded, the supernova debris collided with this material powering SN 2016iet’s emission. Gomez said:

Berger added:

Not long ago, it was not known if such supermassive stars could actually exist. The discovery and follow-up observations of SN 2016iet have provided clear evidence for their existence and potential for affecting the development of the early Universe. Gomez said:

Provided by: Gemini Observatory [Note: Materials may be edited for content and length.]

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