A distant supernova exposed elements from a star’s core. The result reshapes ideas of how massive stars evolve.
According to long-standing theory, stars are built in layers like onions, with each layer composed of different elements that grow heavier toward the core. While this model is widely accepted, directly observing a star’s deeper layers has been nearly impossible.
Until now.
Astronomers using the Keck Observatory in Hawaii have collected spectroscopic data from a supernova first identified by the Zwicky Transient Facility in 2021. The event, designated SN 2021yf, occurred 2.2 billion light-years away. The Keck observations revealed ionized silicon, sulfur, and argon, elements never before detected in a supernova because they are normally hidden beneath outer layers.
Confirming and challenging theory
The finding supports some theoretical predictions about the structure of exploding stars but also raises new challenges.
It is well established that massive stars shed material from their outer layers as they near the point of collapse into a supernova. This process has been documented many times, and the new data confirm it again. However, SN 2021yf appears to have lost far more material than any star observed before, leaving astronomers to reconsider how extreme this stripping process can be.
The observations are presented in a new paper titled “A cosmic formation site of silicon and sulfur revealed by a new type of supernova explosion.” The lead author is Steve Schulze, a research associate at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).
“This is the first time we have seen a star that was essentially stripped to the bone,” said lead author Schulze. “It shows us how stars are structured and proves that stars can lose a lot of material before they explode. Not only can they lose their outermost layers, but they can be completely stripped all the way down to the core and still produce a brilliant explosion that we can observe from very, very far distances.”
When astronomers study supernovae, the most prominent signals usually come from light elements such as hydrogen and helium. If these outer layers have already been stripped away, signatures of carbon, oxygen, neon, and magnesium may also become visible. The deeper shells, which contain heavier elements like silicon, sulfur, and argon, generally remain hidden from view.
For decades, stellar theory has described massive stars as being structured in concentric layers, much like the rings of an onion. The new Keck data provide striking confirmation of this model. By revealing the star’s inner composition just before its explosion, these observations offer the clearest evidence yet in direct support of the layered structure predicted by theory.
Unexpected mass loss
However, the observations also pose a challenge. Astrophysicists know that massive stars eject material before exploding as supernova. Shock waves from the expelled matter interact with the surrounding medium, heating it and creating observable light signatures. But SN 2021yf must have ejected much more material than thought possible, because it’s stripped down to its core.
“This event quite literally looks like nothing anyone has ever seen before,” added Adam Miller, an assistant professor of physics and astronomy at Northwestern and senior author on the study. “This star is telling us that our ideas and theories for how stars evolve are too narrow. It’s not that our textbooks are incorrect, but they clearly do not fully capture everything produced in nature. There must be more exotic pathways for a massive star to end its life that we hadn’t considered.”
Massive stars have the power to fuse lighter elements into heavier elements in a process called nucleosynthesis. (Without stellar nucleosynthesis, the only elements in the Universe would be those created during the Big Bang.) Throughout its life of fusion, a massive star burns lighter elements like hydrogen and helium in its outer shells, while in its core it burns successively heavier elements in its deeper layers. Eventually, a star ends up with an iron core. Iron can’t be burned to release more energy, so once the core is dominated by iron, fusion virtually ceases. Without the outward pressure from fusion, the star collapses in on itself and explodes as a supernova.
Seeing the hidden layers
Astrophysicists have observed layers of helium, carbon, and oxygen in exploding stars before, which are visible after the star has ejected its outer layer of hydrogen. By observing the silicon, sulfur, and argon, it means that this star has ejected not only its outer helium layer, but other layers as well. This likely happened in multiple episodes rather than all at once.
“Stars experience very strong instabilities,” Schulze said. “These instabilities are so violent that they can cause the star to contract. Then, it suddenly liberates so much energy that it sheds its outermost layers. It can do this multiple times.”
Alex Filippenko is a professor of astronomy at UC Berkeley and a co-author of the paper. He happened to be working with the Keck when SN 2021yf was discovered, and quickly pivoted to capture its spectrum withe the Keck’s LRIS. “It’s so exciting to discover a new class of exploding star, especially one that provides a confirmation of some of our theories of how massive stars evolve with time yet also reveals interesting new puzzles,” said Filippenko. “It was very fortunate that my team was using the Keck I telescope the night SN 2021yfj was discovered — we were able to obtain a spectrum that directly led to the realization that this was an incredibly special new type of supernova. Opportunities of this kind are rare!”
Toward a new supernova type
The silicon, sulfur, and argon in the star weren’t always present. These elements were created via nucleosynthesis in the star’s interior as it approached the end of its life.
“This star lost most of the material that it produced throughout its lifetime,” Schulze said. “So, we could only see the material formed during the months right before its explosion. Something very violent must have happened to cause that.”
That question is at the heart of this discovery. Is SN 2021yfj a new type of supernova defined by a powerful new process that stripped it of its outer layers? Some of the explanations the team is considering are interactions with a companion star, unusual and extremely powerful stellar winds, and a massive eruption that preceded the supernova explosion.
Rethinking stellar explosions
“Massive stars can lose a substantial amount of their birth mass through stellar winds, eruptions, and interaction with a companion star,” the researchers write in their paper. The presence of helium in this star’s circumstellar material is puzzling, since helium is usually ejected earlier in the SN process. “Since massive stars tend to live in binary systems, it may not be too unlikely to have a helium-star companion with a strong wind,” they write. This could explain the helium.
The researchers think that the most likely explanation is that this massive star simply tore itself apart. Stellar cores of massive stars are under intense gravitational pressure that raises their core temperatures until nuclear fusion is reignited, generating a powerful explosion. The explosion blasts away the star’s outer layers. The process is repetitive, and each time it happens, more material is ejected until the deeper core is visible.
Supernovae are classified according to spectroscopy, and classifications are centered on hydrogen. Type 1 show helium but no hydrogen and Type 2 show hydrogen. Then there are sub-types under each of those classifications based on other spectral lines. The sequence of labelled sub-types reflects the amount of stripping in the progenitor stars.
First of its kind
“Our observations … suggest that SN 2021yfj is indeed the first example of a Type Ien SN,” the researchers write in their paper. This is a new type that lacks lines from hydrogen or helium, and is instead dominated by emission lines from highly ionized silicon, sulfur, and argon.
Since there’s only one example of this type, there are still many questions. As is often the case in astronomy, a larger dataset will likely lead to some answers.
“While we have a theory for how nature created this particular explosion,” Miller said, “I wouldn’t bet my life that it’s correct, because we still only have one discovered example. We still don’t fully understand how nature created this particular explosion. This star underscores the need to uncover more of these rare supernovae, so we can continue to study them.”
Reference: “A cosmic formation site of silicon and sulphur revealed by a new type of supernova explosion” by Steve Schulze, Avishay Gal-Yam, Luc Dessart, Adam A. Miller, Stan E. Woosley, Yi Yang, Mattia Bulla, Ofer Yaron, Jesper Sollerman, Alexei V. Filippenko, K-Ryan Hinds, Daniel A. Perley, Daichi Tsuna, Ragnhild Lunnan, Nikhil Sarin, Sean J. Brennan, Thomas G. Brink, Rachel J. Bruch, Ping Chen, Kaustav K. Das, Suhail Dhawan, Claes Fransson, Christoffer Fremling, Anjasha Gangopadhyay, Ido Irani, Anders Jerkstrand, Nikola Knezevic, Doron Kushnir, Keiichi Maeda, Kate Maguire, Eran Ofek, Conor M. B. Omand, Yu-Jing Qin, Yashvi Sharma, Tawny Sit, Gokul P. Srinivasaragavan, Nora L. Strothjohann, Yuki Takei, Eli Waxman, Lin Yan, Yuhan Yao, WeiKang Zheng, Erez A. Zimmerman, Eric C. Bellm, Michael W. Coughlin, Frank. J. Masci, Josiah Purdum, Mickael Rigault, Avery Wold and Shrinivas R. Kulkarni, 3 September 2024, arXiv.
DOI: 10.48550/arXiv.2409.02054
Adapted from an article originally published on Universe Today.
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