From AAS NOVA: “Exploring an Odd Stellar Death”

AASNOVA

From AAS NOVA

17 July 2019
Susanna Kohler

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Artist’s impression of a supernova explosion. A new study explores whether the merger of two massive stars could lead to a unique kind of supernova. [ESO/M. Kornmesser]

Massive stars can die in a lot of different ways! A new study explores one possible channel in more detail.

Detectives Are on the Case

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Artist’s illustration of a star exploding in a supernova at the end of its lifetime. [NASA/CXC/M. Weiss]

NASA/Chandra X-ray Telescope

Studying supernovae is a little like being a detective in an odd sort of murder mystery. You’ve witnessed the death of a massive star — and from this evidence, you must determine what type of star died, how it died, and even what interactions it had before its death.

As we enter the era of ever more expansive sky surveys, we can expect to amass not just evidence of typical stellar deaths, but also some more unusual ones. In the process, piecing together the evidence to solve each mystery becomes progressively more challenging — but also more intriguing!

In a recent study, a team of scientists led by Alejandro Vigna-Gómez (U. of Birmingham, UK; Monash U., Australia; U. of Copenhagen, Denmark) have explored one particular oddball type of theorized stellar death: pulsational pair-instability supernovae (PISNe).

Gravity (Usually) Wins

According to theory, PISNe occur when a very massive (hundreds of solar masses) star gets hot enough to start producing pairs of electrons and positions. This process saps the star’s internal energy, leading to its sudden collapse as the force of gravity triumphs.

This collapse can end in the dramatic explosion of a PISN, or it may lead to a smaller eruption that only sheds some of the star’s mass. In the latter case, the star may go through multiple rounds of smaller eruptions before eventually running out of nuclear fuel and undergoing a final explosion — as a pulsational PISN.

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Schematic showing three possible ways massive stars can die; click to enlarge. Top and bottom panels describe outcomes of single-star evolution, depending on the star’s mass. The center channel depicts the merger of two evolved, massive stars to form an object with a large envelope of hydrogen. This can lead to a hydrogen-rich pulsational PISN. [Vigna-Gómez et al. 2019]

Starting with a Merger

If this weren’t complicated enough, Vigna-Gómez and collaborators propose one further twist on this stellar death scenario: the object exploding in a pulsational PISN needn’t simply be a massive star. Instead, it might be the product of the merger of two massive stars.

Vigna-Gómez and collaborators argue that this type of merger is expected to be common, and it would produce a very massive object with a large outer hydrogen shell. By running a series of simulations using the Modules for Experiments in Stellar Astrophysics (MESA), the authors demonstrate that such a merger product could undergo a pulsational PISN and still retain a significant portion of its hydrogen shell up to the final explosion, leaving the fingerprint of hydrogen in the supernova spectrum.

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The light curve of iPTF14hls is extremely unusual, featuring multiple apparent explosions. [Adapted from Las Cumbres Observatory/S. Wilkinson]

LCO_map_2017. Map of the Las Cumbres Observatory global network of robotic telescopes

Explanation for a Zombie Star?

Why does this particular theorized death matter? Stellar detectives are currently working to explain the deaths in a number of especially weird observed supernovae, and this model might match some of them. One example is iPTF14hls, the “zombie star” that’s made headlines for apparently erupting multiple times and defying explanation — in part because of the unexpected hydrogen signatures in its spectra.

We can’t yet say for sure whether iPTF14hls is an example of a stellar-merger-turned-pulsational-PISN — that will require more extensive modeling and analysis of observations — but Vigna-Gómez and collaborators think it’s a good candidate! And while we wait on the verdict of that mystery, we can be sure that transient surveys are busy finding many more examples of stellar deaths for us to puzzle over.

Citation

“Massive Stellar Mergers as Precursors of Hydrogen-rich Pulsational Pair Instability Supernovae,” Alejandro Vigna-Gómez et al 2019 ApJL 876 L29.
https://iopscience.iop.org/article/10.3847/2041-8213/ab1bdf

See the full article here .


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