The details of a supernova explosion are still clouded in mystery and subject to vigorous debate. What exactly happens when they explode? What underlying mechanisms are involved? New observations of a supernova with the European Southern Observatory's Very Large Telescope are removing some of the mystery.
The powerful telescope observed a SN only 26 hours after the explosion began. For the first time, astrophysicists have observational data from an exploding star very soon after it began. The observations caught the explosion as it was breaching the star's outer surface, revealing its true shape.
The supernova in question is a Type II SN named SN 2024ggi, and it exploded in NGC 3621, a disk spiral galaxy about 22 million light-years away. The observations are presented in a new research article titled "An axisymmetric shock breakout indicated by prompt polarized emission from the type II supernova 2024ggi." It's published in Science Advances, and the lead author is Yi Yang from the Department of Physics at Tsinghua University in Beijing.
This image shows the location of the supernova SN 2024ggi in the NGC 3621 galaxy. It was taken on 11 April 2024, just 26 hours after the initial detection of the supernova. The VLT's FORS2 instrument captured it and provided critical spectropolarimetric information. Image Credit: ESO/Y. Yang et al.
An SN explosion has several stages, but the core collapse is where things really get going. This is when the star has formed an iron core, which can't release energy through fusion. Once it reaches the Chandrasekhar limit, the star's outward radiation can't support the star against its own gravity, and it collapses in on the iron core. That's the core collapse stage.
The next stage is the core bounce and shock wave. The star's infalling outer core slams into the dense inner core, bounces off of it, and generates a powerful outward shock wave. There are unanswered questions about this stage.
"The death of massive stars is triggered by an infall-induced bounce shock that disrupts the star," the authors write in their article. "How such a shock is launched and propagates through the star is a decade-long puzzle."
This new data of the shock wave as it breaks out is part of the solution to that puzzle. "The geometry of a supernova explosion provides fundamental information on stellar evolution and the physical processes leading to these cosmic fireworks," said lead author Yang in a press release.
SN 2024ggi was first detected by ATLAS (Asteroid Terrestrial-impact Last Alert System) on April 10th, 2024. Only a few hours later, Yang asked that the VLT observe the supernova. "The first VLT observations captured the phase during which matter accelerated by the explosion near the centre of the star shot through the star’s surface. For a few hours, the geometry of the star and its explosion could be, and were, observed together,” says study co-author Dietrich Baade, an ESO astronomer in Germany.
The supernova's progenitor star was a classic example of a core-collapse supernova. It was a red supergiant star with between 12 and 15 solar masses. The precise mechanisms behind a SN explosion are still unclear, and this was an opportunity to bring some clarity to this fundamental issue in astrophysics.
The shock wave breaching the surface is what most people think of when they think of a supernova. This releases an enormous amount of energy and it's when the supernova brightens. This is when it becomes observable, even from a distant galaxy. Supernovae are known to light up the sky for six months.
For a short period at the beginning of the shock wave breaking out, its shape is visible. Before too long, the shock wave slams into its surroundings and interacts with gas, changing its shape. The VLT observed this brief period for the first time using spectropolarimetry, which can measure the polarization of light across multiple wavelengths. That in turn reveals things about the SN's magnetic fields, temperatures, and especially, its shape, or geometry.
“Spectropolarimetry delivers information about the geometry of the explosion that other types of observation cannot provide because the angular scales are too tiny,” said co-author Lifan Wang, a professor at the Texas A&M University.
The data showed that the initial blast of material was olive-shaped. As the explosion spread outward and struck the matter around the star, the shape flattened. But the ejecta's axis of symmetry remained the same. Here's why that's relevant.
There are two main competing models for how a stalled bounce shock gathers enough energy to make the entire star explode. One is a neutrino-driven mechanism, and one is a jet-driven mechanism, and the explosion's geometry can help determine which mechanism is at work. "The critical link between the shock breakout and the explosion mechanism that drives the expansion of the ejecta may be facilitated by comparing their geometries," the researchers write in their article.
In the neutrino-driven mechanism, the shock is revived by neutrinos from the exploding star that heat up the material behind the stalled shock. According to modelling, this creates uneven heating, which in turn produces an aspherical explosion because of instabilities in its convection. "Such a neutrino-driven explosion would result in a break of spherical symmetry," the authors write.
In the jet-driven mechanism, bi-polar jets are launched along the star's axis of rotation. These jets punch through the stellar surface, and models show they produce explosions with very strong axial symmetry. Since the VLT's observations of SN 2024ggi show strong symmetry, they don't support the neutrino-driven mechanism.
"These findings suggest a common physical mechanism that drives the explosion of many massive stars, which manifests a well-defined axial symmetry and acts on large scales,” according to Yang. This is better explained by a jet-driven mechanism, or perhaps by magneto-rotational mechanisms, which are rarer.
This research helps narrow down the possible explanations for the core bounce and shock wave in SN explosions. It also adds more detail for refining SN models.
"Spectropolarimetry of SN 2024ggi reveals a moderately aspherical explosion that shows a well-defined symmetry axis shared by the prompt shock-breakout emission and the SN ejecta," the authors write in their conclusion. "This variability illustrates that instead of an amorphous/spherical setup resulting from small-scale instabilities, the core-collapse explosion of SN 2024ggi can be driven by a mechanism that shapes the explosion from the earliest shock breakout throughout the entire ejecta expansion.
"This discovery not only reshapes our understanding of stellar explosions, but also demonstrates what can be achieved when science transcends borders,” says co-author and ESO astronomer Ferdinando Patat. “It’s a powerful reminder that curiosity, collaboration, and swift action can unlock profound insights into the physics shaping our Universe."
