Two articles, available here and here, published in “The Astrophysical Journal Letters” report as many studies on the supernova cataloged as SN 2023ixf. Two teams of researchers with members in common examined the evolution of this supernova discovered in the so-called Pinwheel Galaxy. To do this, they used various instruments including some from the Center for Astrophysics (CFA) Harvard & Smithsonian which allowed observations in different electromagnetic bands. The results were different from what was expected from the explosion of a massive star with a delay in the time of the peak of the light pulse just before the explosion. The conclusion is that this was due to the presence of dense materials ejected from the star in the year preceding the supernova.

The top image (Courtesy S. Gomez/STScI. All rights reserved) shows an image of the Pinwheel Galaxy with the location of supernova 2023ixf captured on June 27, 2023, using various optical and infrared frequency filters.

The Pinwheel Galaxy, also known by the catalog acronyms M 101 or NGC 5457, is about 20 million light-years away from Earth. This means it’s relatively close when it comes to studying a supernova and that is why it’s possible to obtain a lot of data on the one cataloged as SN 2023ixf. It was discovered on May 19, 2023, by the Japanese amateur astronomer Kōichi Itagaki and subsequently studied with follow-up observations thanks to his alarm.

SN 2023ixf is the result of the explosion of a massive star, a so-called type II supernova. Knowledge about this type of supernova led to certain expectations about its evolution but the observations revealed something different. In particular, what is called a shock breakout indicates the peak of the light pulse just before the explosion was delayed by several days.

The shock breakout is generated when the shock wave from the explosion reaches the outer edge of the star. A team led by Daichi Hiramatsu, a CFA postdoctoral fellow, examined data collected with various instruments that detected the delay in the shock breakout. The conclusion was that this is direct evidence of the presence of dense materials from a recent mass loss. The observations revealed a loss comparable to the mass of the Sun in the year before the explosion.

This is an unexpected but important discovery to better understand the processes in place before a supernova. Such extreme mass loss indicates a potential instability of the progenitor star in the last years of its life before the supernova. Edo Berger, professor of astronomy at Harvard and CFA, conducted other observations which, combined with those of Daichi Hiramatsu’s team, provided a more complete picture of the situation. In particular, millimeter-wave detections conducted with CFA’s Submillimeter Array (SMA) showed the collision between the supernova debris and the dense materials ejected previously.

The instability of the progenitor star could be linked to the final stages of nuclear fusion of heavy elements such as silicon in the stellar core. Observations will continue to better understand the situation in the interstellar medium around the progenitor star. This kind of information is useful for improving models about supernovae generated by the explosion of massive stars which spread the heavy elements created in their cores into space, contributing to the formation of new planets with the inclusion of the possible ingredients necessary for the emergence of life forms.