A shocking supernova discovery challenges the standard theory of stellar evolution

The year before it went supernova, the red giant star now known as SN 2023ixf shed an unexpected amount of mass equivalent to that of the Sun. This artist’s concept shows what the final stages of mass loss looked like before the star exploded. Credit: Melissa Weiss/CfA

Evidence of extreme mass loss before the explosion in a recently discovered supernova suggests that there may be more going on in the last year of a star’s life than previously thought.

A newly discovered nearby supernova whose star ejected an entire solar mass of material in the year before its explosion challenges the standard theory of stellar evolution. The new observations give astronomers insight into what happens in the final year before the star dies and explodes.

Core-collapse supernovae and SN 2023ixf

SN 2023ixf is a new type II supernova discovered by amateur astronomer Koichi Itagaki of Yamagata, Japan, in May 2023 shortly after its predecessor, or parent star, exploded. SN 2023ixf is located about 20 million light-years away in the Pinwheel Galaxy, and its proximity to Earth, extreme supernova brightness, and young age make it a treasure trove of observable data for scientists studying the death of massive stars in supernova explosions.

Type II supernovae, or central collapse supernovae, occur when at least eight red giant stars, with a mass of about 25 times the mass of the Sun, collapse under their own weight and explode. While SN 2023ixf fits the Type II description, multi-wavelength follow-ups are led by astronomers at the Center for Astrophysics | Harvard and Smithsonian (CFA) and using a wide range of CfA telescopes, revealed new and unexpected behavior.

One of the closest Type II supernovae in a decade and among the brightest to date, SN 2023ixf is a young supernova, discovered earlier this year by amateur astronomer Koichi Itagaki of Yamagata, Japan. This artist’s rendering shows the bright explosion of SN 2023ixf, which occurred after an unexpected amount of mass loss unlike anything astronomers have seen before. Credit: Melissa Weiss/CfA

Within hours of going supernova, collapsed supernovas in the core of a star produce a flash of light that occurs when the shock wave from the explosion reaches the outer edge of the star. However, SN 2023ixf produced a slight curve that did not appear to fit this expected behavior. To better understand the sudden breakthrough of SN 2023ixf, a team of scientists led by CfA postdoctoral fellow Daiichi Hiramatsu analyzed data from the Tillinghast 1.5-meter telescope, the 1.2-meter telescope, and the MMT in Fred Lawrence Whipple Observatorya CfA facility located in Arizona, as well as data from the Global Supernova Project – a flagship project of the Las Cumbres Observatory, NASANeil Gehrells Swift Observatory, and many more. This multi-wavelength study, published this week in the Astrophysical Journal LettersIt was revealed that, in sharp contrast to expectations and stellar evolution theory, the breakthrough of the SN 2023ixf shock was delayed by several days.

Implications of delayed shock onset

“The late penetration of the shock is direct evidence of the presence of dense material resulting from recent mass loss,” Hiramatsu said, adding that such extreme mass loss is unusual in Type II supernovae. “Our new observations have revealed a significant and unexpected amount of mass loss — close to that of the Sun — in the last year before the explosion.”

Taken using the 1.2-meter telescope at the CfA’s Fred Lawrence Whipple Observatory on June 27, 2023, just over a month after SN 2023ixf’s predecessor star exploded, the image in this composite combines green, red, near-infrared and infrared light to highlight Light on both. SN 2023ixf and the Pinwheel Galaxy. SN 2023ixf is located in one of the galaxy’s spiral arms, as expected for massive star explosions. Credit: S. Gomez/STScI

SN 2023ixf challenges astronomers’ understanding of the evolution of massive stars and the supernovae they turn into. Although scientists know that collapsed supernovae at their cores are the fundamental origin points for the formation of the universe and the evolution of atoms, neutron stars and black holes, very little is known about the years before stellar explosions. The new observations indicate possible instability in the final years of a star’s life, leading to severe mass loss. This could be related to the final stages of nuclear burning of high-mass elements, such as silicon, in the star’s core.

More feedback and collaboration

Concurrent with the multi-wavelength observations led by Hiramatsu, Edo Berger, a professor of astronomy at Harvard and CfA, and Hiramatsu’s advisor, conducted millimeter-wave observations of the supernova using CfA’s Submillimeter matrix (SMA) on Maunakea Peak, Hawaii. This data published in Astrophysical Journal Letters, directly tracked the collision between supernova debris and dense material lost before the explosion. “SN 2023ixf exploded at just the right time,” Berger said. “Just a few days ago, we began an ambitious new three-year program to study supernova explosions using SMA, and this exciting, nearby supernova was our first target.”

“The only way to understand how massive stars behave in the last years of their lives up to the point of explosion is to detect supernovae when they are very small, preferably nearby, and then study them across multiple wavelengths,” Berger said. “Using both optical and millimeter telescopes, we have effectively been able to turn SN 2023ixf into a time machine to reconstruct what its progenitor star was doing up until the moment of its death.”

The importance of amateur astronomers

The discovery of the supernova itself, and the immediate follow-up, have great meaning for astronomers around the world, including those who do science in their own backyards. Itagaki discovered the supernova on May 19, 2023, from his private observatory in Okayama, Japan. Data collected by Itagaki and other amateur astronomers largely pinpointed the time of the explosion Accuracy Within two hours, giving professional astronomers at the CfA and other observatories a head start in their investigations. CfA astronomers have continued to collaborate with Itagaki on ongoing optical observations.

“The partnership between amateur and professional astronomers has a long-standing tradition of success in the field of supernovae,” Hiramatsu said. “In the case of SN 2023ixf, I received an urgent email from Kōichi Itagaki as soon as he discovered SN 2023ixf. Without this relationship, and Itagaki’s work and dedication, we would have missed the opportunity to gain a critical understanding of the evolution of massive stars and their supernova explosions.”

References:

“From discovery to first month of type II supernova 2023ixf: high and variable mass loss in the last year before explosion” by Daichi Hiramatsu, Daichi Tsuna, Edo Berger, Koichi Itagaki, and Jared A. Goldberg, Sebastian Gomes, Kishalai D., Griffin Hosseinzadeh, K. Azali Bostrom, Peter J. Brown, Iyer Arcavi, Alison Perrella, Peter K. Blanchard, Gilbert A. Esquierdo, Joseph Farah, Dr. Andrew Howell, Tatsuya Matsumoto, Curtis McAuley, Megan Newsom, Estefania Padilla Gonzalez, Craig Pellegrino, Jaehyun Rhee, Giacomo Teran, Joseph Venco, and J. Craig Wheeler, September 19, 2023, Astrophysical Journal Letters.
doi: 10.3847/2041-8213/acf299

“Millimeter observations of type II SN 2023ixf: constraints on the surrounding interstellar medium” by Ido Berger, Garrett K. Keating, Raffaella Margutti, Keiichi Maeda, Kate D. Alexander, Yvette Sindis, Taranah Eftekhari, Mark Jurwell, Daichi Hiramatsu, Anna Y. Q. Ho, Tanmoy Laskar, Ramprasad Rao and Peter K. G. Williams, July 10, 2023, Astrophysical Journal Letters.
doi: 10.3847/2041-8213/ace0c4