Authors: Tea Temim, J. Martin Laming, P. J. Kavanagh, Nathan Smith, Patrick Slane, William P. Blair, Ilse De Looze, Niccolò Bucciantini, Anders Jerkstrand, Nicole Marcelina Gountanis, Ravi Sankrit, Dan Milisavljevic, Armin Rest, Maxim Lyutikov, Joseph DePasquale, Thomas Martin, Laurent Drissen, John Raymond, Ori D. Fox, Maryam Modjaz, Anatoly Spitkovsky, Lou Strolger
First Author’s Institution: Department of Astrophysical Sciences, Princeton University, NJ, USA
Status: Published in The Astrophysical Journals Letters [open access]
Observing supernova remnants is like piecing together a cosmic mystery. These remnants are the leftover traces of massive star explosions, known as core-collapse supernova (CCSN). By studying them, astronomers can learn a lot about what actually happens when a star goes supernova. One of the most famous of these objects, the Crab Nebula, has fascinated astronomers for centuries. Today’s authors used the powerful James Webb Space Telescope (JWST) to explore the heart of the Crab Nebula in infrared light (Figure 1), uncovering new details about its structure, composition, and the supernova explosion that created it.
Unusually Low Numbers in the Crab Nebula
The Crab Nebula holds a special place in astronomy as the first celestial object identified as the remnant of a supernova, observed by Chinese astronomers in 1054 CE. Despite its brilliance, the Crab Nebula has unusually low kinetic energy—the energy carried by the material ejected as it expands outward from the exploding star. In a typical CCSN, the kinetic energy released is around 1051 ergs, but the Crab Nebula has less than 1050 ergs. However, despite this relatively low kinetic energy, the explosion was remarkably bright—about ten times more luminous than a typical CCSN. This unexpected luminosity has puzzled astronomers, leading to theories that an extended supernova remnant might exist beyond the visible Crab to account for the missing energy. Yet, despite extensive searches, no such component has been found.
Another intriguing aspect of the Crab Nebula is the estimated mass of its ejecta, which is around seven times the mass of the Sun. This suggests that the star that exploded had a mass of only about nine solar masses, placing it at the lower end of the mass range typically associated with CCSN. Despite decades of research, the exact type of star and the precise nature of the supernova explosion that produced the Crab Nebula remain an ongoing mystery.
Exploring the Crab Nebula with JWST
Building on the Crab Nebula’s rich history of scientific study, today’s authors have leveraged the cutting-edge capabilities of the JWST to gain unprecedented insights into this iconic remnant. The study utilized both the Near Infrared Camera (NIRCam) and the Mid-Infrared Instrument (MIRI) aboard JWST to create detailed mosaic images that cover nearly the entire nebula, providing a comprehensive view of its structure in infrared wavelengths. In addition to these imaging data, the authors conducted targeted spectroscopic observations using MIRI’s Medium-Resolution Spectrometer (MRS). These spectroscopic observations focused on two specific areas within the nebula’s ejecta filaments.
Mapping the Dust Distribution
Infrared observations, like those provided by JWST, are key for studying cosmic dust because they can penetrate through dense clouds of gas and dust that often obscure optical observations. Using JWST data, the authors created a high-resolution map of dust emission within the Crab Nebula. This dust map was derived using a combination of NIRCam and MIRI imaging data, which allowed for a detailed separation of the dust emission from other components, such as synchrotron radiation from the Crab’s pulsar wind nebula (PWN, Figure 2 left panel) and sulfur line emission (Figure 2 right panel).
The authors found that dust grains are concentrated in the innermost, high-density filaments of the Crab Nebula. These dense, dusty filaments appear to overlap with multiple synchrotron “bays” around the outskirts of the Crab PWN (white arrows in Figure 2). A PWN is a region of highly energetic particles and magnetic fields created by a rapidly rotating neutron star, or pulsar, left behind after the supernova explosion. The pulsar in the Crab Nebula emits a powerful wind of charged particles that interacts with the surrounding supernova ejecta. The positional correlation between dust and synchrotron features provides new insights into the interaction between dust formation and the energetic environment of the PWN.
Insights into Particle Acceleration
JWST data has also shed light on how particles are accelerated within the Crab’s PWN. The authors detected variation in the synchrotron spectral index—a measure of how radiation is emitted from high-speed electrons spiralling in magnetic fields—within small-scale features of the Crab’s PWN, particularly in the inner pulsar region (Figure 3). These variations are crucial because they provide information about the energy distribution of the emitting particles.
By analyzing these variations, the authors found evidence supporting the theory linking particle acceleration to the termination shock–a region where the pulsar wind, traveling outward at nearly the speed of light, collides with the slower-moving, denser material. This encounter causes the pulsar wind to suddenly slow down, generating a shockwave. At this termination shock, the energy from the fast-moving pulsar wind is transferred to individual particles within the wind, boosting their kinetic energy significantly and accelerating them to extremely high energies.
Decoding the Chemical Composition
The nickel-to-iron (Ni/Fe) ratio serves as a crucial diagnostic tool for understanding supernova explosions, including the type of explosion and the mass of the star that exploded, as well as the nucleosynthesis processes involved. By deriving the Ni/Fe ratios, astronomers can place important constraints on the nature of the explosion that created the Crab Nebula.
Today’s authors detected multiple nickel and iron emission lines within the Crab’s ejecta filaments. By using photoionization models, they calculated the Ni/Fe ratios and found them to be consistent with previously reported values from optical data. These results further support the theory that the Crab Nebula was likely produced by a low-mass iron CCSN, rather than the previously favored electron-capture supernova model.
A low-mass iron CCSN occurs when a star with a relatively low mass—just enough to trigger a supernova—runs out of nuclear fuel. The core, primarily composed of iron, collapses under gravity, leading to an explosive event. This type of supernova is known for producing heavy elements like nickel and iron. In contrast, an electron-capture supernova happens when a star’s core is composed of oxygen, neon, and magnesium, and it reaches a point where it can no longer support itself due to the capture of electrons by atomic nuclei. This process triggers the collapse of the core and results in an explosion.
Refining the Supernova Playbook
Today’s paper demonstrates that as our understanding deepens, the Crab Nebula continues to be a key player in unraveling the mysteries of stellar death and the violent forces that shape our universe. The high-resolution dust emission map from JWST data reveals that dust grains are densely packed within the innermost filaments of the Crab Nebula, offering fresh insights into how dust forms and spreads in the aftermath of a supernova. The observed Ni/Fe ratios in the nebula’s ejecta point to a low-mass iron core-collapse supernova, challenging previous theories and reinforcing the idea that the explosion stemmed from a star with an initial mass around nine times that of our Sun. These findings don’t just add another chapter to the story of the Crab Nebula; they reshape the narrative, imposing new constraints on our models of supernova explosions, especially those involving low-mass progenitors.
Astrobite edited by Skylar Grayson
Featured image credit: NASA, ESA, CSA, STScI, T. Temim, J. DePasquale
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