Saturday, September 24, 2022

Understanding the process of how massive stars live and die

Supernova reveals secrets to a Texas-led team of astronomers.

The light of an exploding star changes rapidly over time. The light from the supernova gets brighter and reaches its peak, and then starts to deam.

It is necessary to note the times of these peaks and valleys in the light’s brightness, called a “light curve,” as well as the characteristic wavelengths of light emitted at different times.

Astronomers around the world follow these exploding stars with telescopes to deduce the physical characteristics of the system.

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 the university’s McDonald Observatory to unlock a puzzling mystery about a stellar explosion discovered several years ago and evolving even now. The results were published in the issue of The Astrophysical Journal.

These results will help astronomers better understand the process of how massive stars live and die.

“I think what’s really cool about this kind of science is that we’re looking at the emission that’s coming from matter that’s been cast off from the progenitor system before it exploded as a supernova,” Thomas said. “And so this makes a sort of time machine.”

Explosion of Stars:

In the case of supernova 2014C, the progenitor was a binary star, a system in which two stars were orbiting each other. The more massive star evolved more quickly, expanded, and lost its outer blanket of hydrogen to the companion star. The first star’s inner core continued burning lighter chemical elements into heavier ones until it ran out of fuel. When that happened, the outward pressure from the core that had held up the star’s great weight dropped. The star’s core collapsed, triggering a gigantic explosion.

This makes it a type of supernova astronomers call a “Type Ib.” In particular, Type Ib supernovae are characterized by not showing any hydrogen in their ejected material, at least at first.

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

But the studies of SN 2014C from all of the various telescopes did not add up into a cohesive picture of how astronomers thought a Type Ib supernova should behave.

For one thing, the optical signature from the Hobby-Eberly Telescope (HET) showed SN 2014C contained hydrogen. Another team also discovered this surprising finding independently using a different telescope.

“For a Type Ib supernova to begin showing hydrogen is completely weird,” Thomas said. “There’s just a handful of events that have been shown to be similar.”

For a second thing, that hydrogen’s optical brightness (light curve) was behaving strangely.

Most of the light curves from SN 2014C radio, infrared, and X-rays seemed to follow the expected pattern: they got brighter, peaked, and started to fall. But the optical light from the hydrogen stayed steady.

“The mystery that we’ve wrestled with has been ‘How do we fit our Texas HET observations of hydrogen and its characteristics into that [Type Ib] picture?’,” said UT Austin professor and team member J. Craig Wheeler.

Are shockwaves not spherical?

The problem, the team realized, was that previous models of this system assumed that the supernova had exploded and sent out its shockwave in a spherical manner.

The data from HET showed that this hypothesis was impossible. It projects that something else must have happened.

“In particular, we find that the assumption of a dense spherically-symmetric shell of hydrogen is not consistent with all the data.” Study mentions.

“It just would not fit into a spherically symmetric picture,” Wheeler said.

What researchers proposed?

“We propose a multi-component, non-spherical configuration of SN 2014C and its immediate circumstellar environment that appears to accommodate the available data.” Study mentions

The team proposes a model where the hydrogen envelopes of the two stars in the progenitor binary system merged to form a “common-envelope configuration,” where both were contained within a single gas envelope. The pair then expelled that envelope in an expanding, disk-like structure surrounding the two stars. When one of the stars exploded, its fast-moving ejecta collided with the slow-moving disk and slid along the disk surface at a “boundary layer” of intermediate velocity.

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

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

“In a broad sense, the question of how massive stars lose their mass is the big scientific question we were pursuing,” Wheeler said. “How much mass? Where is it? When was it ejected? By what physical process? Those were the macro questions we were going after.

“And 2014C just turned out to be a really important single event that’s illustrating the process,” Wheeler said.

Futuristic view study posed:

Future observations of SN 2014C are desirable in order to determine the epoch of disappearance of hydrogen that will constrain the extent of the torus and the future evolution of the radio and X-ray emission.

The X-ray flux is declining, suggesting that the main interaction of the shock with the CSM is over, in analogy with the behavior of SN 1987A.

SN 2014C seems to be a more rapidly-evolving version of SN 1987A and hence may yield clues to the future behavior of SN 1987A.

Further observations are also encouraged to determine whether researchers are observing the effects of a pulsar wind nebula as suggested by their observations of the velocity width and high excitation emission lines.

The toroidal aspect of interpretation of study is an integral concept of this paper and may apply to supernova and stellar evolution science far beyond the scope of SN 2014C.

Journal Reference

  1. Benjamin P. Thomas, J. Craig Wheeler, Vikram V. Dwarkadas, Christopher Stockdale, Jozsef Vinko, David Pooley, Yerong Xu, Greg Zeimann, Phillip MacQueen. Seven Years of SN 2014C: a Multi-Wavelength Synthesis of an Extraordinary Supernova. 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 the university’s McDonald Observatory to unlock a puzzling mystery about a stellar explosion discovered several years ago and evolving even now. The results were published in the issue of The Astrophysical Journal. DOI: 10.48550/arXiv.2203.12747
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