An article published in the journal “Science Advances” reports the first reconstruction ever of the shape of a Type II supernova in its early stages. A team of researchers led by Professor Yi Yang of Tsinghua University in Beijing, China, used observations conducted with ESO’s Very Large Telescope (VLT) in Chile to study the supernova cataloged as SN 2024ggi on April 11, 2024, the day after the explosion began.
The top image (ESO/Y. Yang et al.) shows galaxy NGC 3621 with the location of supernova SN 2024ggi in the small circle.
This is the first time astronomers have made such observations of a supernova so close to its start. This result was achieved using the spectropolarimetry technique and revealed that the initial explosion had an elongated shape. For astronomers, the evolution of a supernova’s geometry offers insights into the mechanisms behind the explosion of massive stars.
When supernova SN 2024ggi was first detected on April 10, 2024, Professor Yi Yang knew that obtaining high-quality observations very quickly was essential to gathering new information on the explosion of massive stars. He submitted a proposal to ESO within hours and managed to secure the start of observations with the Very Large Telescope (VLT), specifically the FORS 2 (FOcal Reducer and low dispersion Spectrograph) instrument, for the following day, April 11.
Supernova SN 2024ggi was detected in the galaxy NGC 3621, about 22 million light-years from Earth. This is not very far away in astronomical terms, another reason to strive for high-quality observations. To obtain the desired information on the geometry of the supernova, they used the technique of spectropolarimetry. This provides information on the geometry of a supernova thanks to the polarization of the light emitted. Several instruments observed this supernova, but the FORS 2 spectrograph was the only one in the southern hemisphere that could be used to exploit the spectropolarimetry technique.
The results of the observations were as hoped. The astronomers captured what can technically be called the breakout shape, the initial shape of the supernova before it starts interacting with the surrounding materials, which were ejected during the progenitor star’s death throes. The shape is elongated and was compared to that of an olive. As the explosion spreads outward and collides with the surrounding materials, the shape flattens, but the axis of symmetry of the ejected materials remains the same.
Precisely because of the difficulties in obtaining observations of the explosion of a massive star of such quality and so quickly, astronomers had proposed various models to describe its early stages. The discoveries made thanks to this study have already ruled out some of those models. Other models may be improved using the newly available information. The goal remains to reconstruct the processes at work in supernovae, which lead, among other things, to the generation of heavy elements that are scattered into interstellar space and may become part of new star systems.
Here’s ESO’s video showing the first stages of the evolution of supernova SN 2024ggi’s shape.
