Artist's rendition of a red supergiant exploding

For the first time, astronomers have captured the death of a red supergiant star in real time: revealing a dramatic surge in brightness in the months preceding its final explosion. For researchers of the Young Supernova Experiment, led by Wynn Jacobson-Galán at the University of California, Berkeley, the event was far more violent than would be expected from previous observations. The result could transform astronomers’ conceptions of how massive stars spend the last few months and days of their lives.

To study the evolution of massive stars in their final moments, astronomers can observe the material surrounding them at the instant that they collapse and explode, in dramatic Type II supernovae.

This material is supplied as the star loses mass via wind and violent outbursts, and after the supernova produces an intense flash, it becomes ionized by highly energetic photons. By analysing its resulting emission spectra in the hours and days following this explosion, astronomers can use modelling techniques to reconstruct the evolving environment surrounding the star in its last few months. In turn, this can shed light on how the star’s internal structure is changing.

In the summer of 2020, observations by the Pan-STARRS survey in Hawaii, part of the Young Supernova Experiment, detected excessive amounts of light emanating from a red supergiant roughly 10 times the mass of the Sun, located in the galaxy NGC 5731. At first, this brightness remained remarkably stable and persistent. Yet after 130 days, observations from the W M Keck Observatory, also in Hawaii, recorded the star suddenly collapsing and exploding in real time.

By modelling the photoionization they observed in the dense material surrounding the star, Jacobson-Galán’s team showed that it had shed large amounts of mass prior to going supernova, at a rate of roughly 0.01 solar masses per year. Such violent behaviour was particularly surprising for a red supergiant. Based on previous observations within our own galaxy, these stars were thought to be relatively quiescent in their final moments; typically shedding mass at considerably slower rates.

This suggests that at least some red supergiants must experience turbulent changes to their internal structures prior to going supernova. From further analysis, Jacobson-Galán’s team determined that the power for the star’s bright emission likely originated from the burning of neon, oxygen or silicon. The products of this burning may then trigger buoyancy-balancing gravity waves – which would deposit energy into the star’s outer envelope, intensifying both its brightness and mass loss.

If similar events are discovered in the future, they could have a profound impact on astronomers’ understanding of how stellar evolution unfolds prior to supernovae.

Quelle:physicsworld