A newly discovered star that went supernova ejected up to a solar mass of material in the year before it exploded, challenging standard theories of stellar evolution. The new observations give astronomers insight into what happens in a star's last year before it dies and explodes.
A year before going supernova, the red supergiant star now known as SN2023ixf unexpectedly lost the equivalent of the Sun's mass. This artist's conception shows what might happen in the final stages of mass loss before a star explodes. Image credit: Melissa Weiss/CfA.
Core collapse supernovae and SN2023ixf
SN2023ixf is a new Type II supernova discovered by amateur astronomer Koichi Itagaki in Yamagata Prefecture, Japan, in May 2023, shortly after the explosion of its native or origin star. SN2023ixf is located in the Pine Wheel Galaxy about 20 million light-years away from the Earth. It is very close to the Earth, the supernova is extremely bright, and its age is very young, which makes it a treasure trove of observable data for scientists studying the death of massive stars in supernova explosions.
When the mass of a red supergiant star is at least 8 times that of the Sun, and can be up to 25 times that of the Sun, it will collapse under its own weight and explode. This is a Type II supernova or a core-collapse supernova. While SN2023ixf fits the description of Type II, follow-up multiwavelength observations led by astronomers at Harvard and the Smithsonian Center for Astrophysics (CfA), as well as using various CfA telescopes, discovered new and unexpected behavior.
SN2023ixf is a young supernova discovered earlier this year by amateur astronomer Koichi Itagaki in Yamagata Prefecture, Japan. It is one of the closest Type II supernovae in a decade and one of the brightest supernovae to date. This artist's rendering shows the bright explosion of SN2023ixf, which followed an unexpected mass loss that astronomers had never seen before. Photo credit: Melissa Weiss/CfA
A core-collapse supernova produces a flash of light within hours of going supernova when the shock wave from the explosion reaches the outer edge of the star. However, the light curve produced by SN2023ixf does not appear to conform to this expected behavior. To better understand the impact burst of SN2023ixf, a team of scientists led by CfA postdoctoral researcher Daichi Hiramatsu analyzed data from the 1.5-meter Tillinghast Telescope, the 1.2-meter Telescope, and the CfA in Arizona. Data from the MMT at the Fred Lawrence Whipple Observatory at fA, as well as data from the Global Supernova Project (an important project of the Las Cumbres Observatory), NASA's Neil Gehrels Swift Observatory, and many others. The multi-wavelength study, published this week in The Astrophysical Journal Letters, shows that contrary to expectations and stellar evolution theory, SN2023ixf's impact burst was delayed by several days.
Effects of Delayed Shock Burst
"The delayed shock burst is direct evidence of the presence of dense material produced by recent mass loss," Hiramatsu said, adding that such extreme mass loss is not typical of Type II supernovae. Our new observations show that the mass loss in the last year before the explosion was huge and unexpected - close to the mass of the Sun. "
Taken with the CfA's Fred Lawrence Whipple Observatory's 1.2m telescope on June 27, 2023, just over a month after SN2023ixf's progenitor star exploded, this composite image combines green, red, near-infrared and infrared light to highlight SN2023ixf and the Pinwheel Galaxy. SN2023ixf is located in one of the spiral arms of the Milky Way, which is where massive stars are expected to explode. Source: S.Gomez/STScI
SN2023ixf challenges astronomers' understanding of the evolution of massive stars and their evolution into supernovae. While scientists know that core-collapse supernovae are the primary origin points for the formation and evolution of atoms, neutron stars, and black holes in the universe, less is known about the years before a star explodes. New observations suggest that instability can occur during the final years of a star's life, leading to extreme mass loss. This may be related to the final stages of nuclear burning of high-quality elements such as silicon in the stellar core.
Further observations and cooperation
While conducting multi-wavelength observations led by Daichi Hiramatsu, Edo Berger, an astronomy professor and mentor at Harvard University and CfA, conducted millimeter-wave observations of this supernova using the CfA Submillimeter Array (SMA) on the top of Mauna Kea, Hawaii. The data, published in The Astrophysical Journal Letters, directly track collisions between supernova debris and dense material lost before the explosion.
"The timing of the SN2023ixf explosion was perfect," said Berger. "Just days ago, we began an ambitious new three-year program to study supernova explosions with SMA, and this exciting nearby supernova was our first target." The only way to understand how massive stars behave in the final years of their lives until they explode is to spot supernovae when they are very young, preferably near them, and study them at a variety of wavelengths. Using optical and millimeter-wave telescopes, we effectively turned SN2023ixf into a time machine, reconstructing what it was like before its death. "
What it means to be an amateur astronomer
The discovery of this supernova itself, and the work that followed immediately after it, is of great significance to astronomers around the world, including those conducting science in their own backyards. Itagaki discovered the supernova on May 19, 2023, at a private observatory in Okayama, Japan. The combined data from Itagaki and other amateur astronomers determined the time of the explosion to within two hours, providing a head start for research by professional astronomers at the CfA and other observatories. CfA astronomers continue to collaborate with Itagaki on ongoing optical observations.
Daichi Hiramatsu said: "Collaboration between amateur and professional astronomers has a long tradition of success in the field of supernovae. In the case of SN2023ixf, I received an urgent email from Koichi Itagaki immediately after his discovery of SN2023ixf. Without this relationship, and without Itagaki's work and dedication, we would have missed the opportunity to gain important insights into the evolution of massive stars and their supernova explosions."