It’s another first for astronomy.

For the first time, a team of astronomers have imaged in real-time as a red supergiant star reached the end of its life. They watched as the star convulsed in its death throes before finally exploding as a supernova.

And their observations contradict previous thinking into how red supergiants behave before they blow up.

A team of astronomers watched the drama unfold through the eyes of two observatories in Hawaii: Pan-STARRS on Haleakala, Maui, and the W. M. Keck Observatory on Maunakea, Hawaii Island. Their observations were part of the Young Supernova Experiment (YSE) transient survey. They watched the supernova explosion, named SN 2020tlf, during the final 130 days leading up to its detonation.

“For the first time, we watched a red supergiant star explode!”

Wynn Jacobson-Galán, UC Berkeley

The title of the paper presenting the discovery is “Final Moments. I. Precursor Emission, Envelope Inflation, and Enhanced Mass Loss Preceding the Luminous Type II Supernova 2020tlf.” The paper is published in The Astrophysical Journal and the lead author is Wynn Jacobson-Galán, an NSF Graduate Research Fellow at UC Berkeley.

“This is a breakthrough in our understanding of what massive stars do moments before they die,” said Jacobson-Galán, in a press release. “Direct detection of pre-supernova activity in a red supergiant star has never been observed before in an ordinary Type II supernova. For the first time, we watched a red supergiant star explode!”

“It’s like watching a ticking time-bomb.”

Raffaella Margutti, UC Berkeley

The discovery dates back to the Summer of 2020. At that time, the progenitor star experienced a dramatic rise in luminosity. Pan-STARRS detected that brightening, and when Fall came around the star exploded as SN 2020tlf. The supernova is a Type II supernova, where a massive star experiences a rapid collapse and then explodes.

This video is an artist’s rendition of the red supergiant star transitioning into a Type II supernova, emitting a violent eruption of radiation and gas on its dying breath before collapsing and exploding. Credit: W. M. Keck Observatory/Adam Makarenko

The team used the Keck Observatory’s Low-Resolution Imaging Spectrometer (LRIS) to capture the supernova’s first spectrum. The LRIS data showed circumstellar material around the star when it exploded. That material is likely what Pan-STARRS saw the star ejecting in the summer before it exploded.

“Keck was instrumental in providing direct evidence of a massive star transitioning into a supernova explosion,” said senior author Raffaella Margutti, an associate professor of astronomy at UC Berkeley. “It’s like watching a ticking time bomb. We’ve never confirmed such violent activity in a dying red supergiant star where we see it produce such a luminous emission, then collapse and combust, until now.”

This figure from the study shows the supernova pre- and post-explosion. The top panel shows the total of all electromagnetic radiation (light) across all wavelengths released by the supernova, in green. The middle panel shows black-body temperatures in red, and the bottom panel shows the radii in blue. Image Credit: Jacobson-Galán et al, 2022.
This figure from the study shows the supernova pre- and post-explosion. The top panel shows the total of all electromagnetic radiation emitted by the event across all wavelengths, in green. The middle panel shows black-body temperatures in red, and the bottom panel shows the radii in blue. Image Credit: Jacobson-Galán et al, 2022.

After the explosion, the team turned to other Keck instruments to continue their observations. Data from the DEep Imaging and Multi-Object Spectrograph (DEIMOS) and Near Infrared Echellette Spectrograph (NIRES) showed that the progenitor star was 10 times more massive than the Sun. The star is in the NGC 5731 galaxy about 120 million light-years away.

The team’s observations led to some new insight into Type II supernovae and their progenitor stars. Prior to these observations, nobody had seen a red supergiant display such a spike in luminosity and undergo such powerful eruptions before exploding. They were much more placid in their final days as if they accepted their fates.

Red supergiant stars eject material prior to core collapse. But that material ejection takes

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