The supernova reveals secrets to astronomers

An international group of astronomers led by Benjamin Thomas of the University of Texas at Austin has used observations from the Hobby-Eberly Telescope (HET) at McDonald’s University Observatory to unravel a puzzling mystery about a stellar explosion discovered there. a few years ago and is still evolving today. . The results, published in today’s issue of Journal of Astrophysics, it will help astronomers better understand the life and death process of massive stars.

When an exploding star is first detected, astronomers around the world begin tracking it with telescopes because the light it emits changes rapidly over time. They see that the light of a supernova becomes brighter, that it finally reaches its maximum and then begins to fade. By observing the times of these peaks and falls in the brightness of the light, called the “light curve”, as well as the characteristic wavelengths of the light emitted at different times, the physical characteristics of the system can be deduced.

“I think what’s really cool about this kind of science is that we’re watching the emission of matter that was launched by the parent system before it exploded into a supernova,” Thomas said. “And so it becomes a kind of time machine.”

In the case of the 2014C supernova, the parent was a binary star, a system in which two stars orbit each other. The most massive star evolved faster, expanded, and lost its outer hydrogen blanket to the accompanying star. The inner core of the first star continued to burn lighter chemical elements in heavier ones, until it ran out of fuel. When this happened, the outer pressure of the core that had borne the heavy weight of the star dropped. The star’s core collapsed, causing a giant explosion.

This makes it a type of supernova that astronomers call “Type Ib”. In particular, Ib-type supernovae are characterized by the absence of hydrogen in their expelled matter, at least initially.

Thomas and his team have been following the 2014C SN from the McDonald Observatory telescopes since its discovery that year. Many other computers around the world have also studied it with telescopes on the ground and in space, and in different types of light, including Very Large Array radio waves on the ground, infrared light, and X-rays from space. Chandra Observatory.

But the 2014C SN studies of all the different telescopes failed to get a coherent picture of how astronomers thought an Ib-type supernova should behave.

On the one hand, the optical signature of the Hobby-Eberly Telescope (HET) showed that SN 2014C contained hydrogen, a surprising finding that was also discovered independently by another team using a different telescope.

“It’s very strange for an Ib-type supernova to start showing hydrogen,” Thomas said. “There are only a handful of events that have been found to be similar.”

Second, the optical brightness (light curve) of this hydrogen behaved strangely.

Most of the light curves in the 2014C SN — radio, infrared, and X-rays — followed the expected pattern: they became brighter, peaked, and began to fall. But the optical light of hydrogen remained stable.

“The mystery we struggled with was‘ How do we incorporate our Texas HET observations of hydrogen and its characteristics into that? [Type Ib] photo? ‘ said J. Craig Wheeler, a professor at the University of Austin and a member of the team.

The team realized that the problem was that previous models of this system assumed that the supernova had exploded and sent its shock wave spherically. HET data showed that this hypothesis was impossible: something more must have happened.

“It just wouldn’t fit into a spherically symmetrical image,” Wheeler said.

The team proposes a model where the hydrogen envelopes of the two stars of the parent binary system merged to form a “common envelope configuration”, where the two were contained in a single gas envelope. The couple then ejected this envelope into an expanding disk-shaped structure surrounding the two stars. When one of the stars exploded, its rapid expulsion collided with the slow disk and also slid across the surface of the disk to a “boundary layer” of intermediate velocity.

The team suggests that this boundary layer is the source of the hydrogen they detected and then studied for seven years with HET.

Thus, HET data turned out to be the key to unraveling the mystery of the SN 2014C supernova.

“In a broad sense, the question of how massive stars lose their mass is the big scientific issue we were pursuing,” Wheeler said. “What mass? Where is it? When was he expelled? By what physical process?

“And 2014C turned out to be a very important one-off event that exemplifies the process,” Wheeler said.

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