'Zombie' stars return from the dead

October 31, 2018 by Nolan O'brien, Lawrence Livermore National Laboratory
Supercomputer simulations show the evolution of a dormant white dwarf star reigniting as it whizzes around an intermediate-mass black hole. The top series of images show density, the bottom show temperature. Credit: Lawrence Livermore National Laboratory

Black holes are among the most elusive objects in the universe, but research out of Lawrence Livermore National Laboratory (LLNL) suggests the remnant cores of burned-out stars could be the key to making the first observation of the most elusive class of black holes.

The research explored whether a dormant white dwarf star—sometimes referred to as a "zombie" star—could reignite if it had a close encounter with an intermediate-mass black hole. While data exists to corroborate the existence of supermassive black holes, there have been no confirmed observations of black holes in the intermediate class, which range in size from 100 to 100,000 solar masses. This intermediate class, the research team posited, might offer just the right amount of gravitational force to reignite a white dwarf before it's torn apart.

The team ran supercomputer simulations of dozens of different close encounter scenarios to test this theory. Not only did they find that a close encounter would reignite the once-dead star, but they saw evidence that the process could create significant electromagnetic and gravitational wave energies that might be visible from detectors in near-Earth orbit. The research was published in the September issue of the Astrophysical Journal.

"It was exciting to see that the zombie star reignited in each of the close encounter scenarios we looked at," said LLNL physicist Peter Anninos, lead author on the paper. "But what really captured my imagination was the idea that these energetic events could be visible. If the align, so to speak, a zombie star could serve as a homing beacon for a never-before-detected class of ."

The simulations showed that the stellar matter fused into varying amounts of calcium and iron, depending on how close the star passed by the black hole. The closer the pass, the more efficient the nucleosynthesis, and the greater the iron production. All told, the research suggests that an "optimal" close could fuse up to 60 percent of the stellar matter into iron. This peak mass conversion took place with a white dwarf passing at a distance of two or three black hole radii.

"The stretching phenomena can be very complicated," said LLNL physicist Rob Hoffman, coauthor on the paper. "Imagine a spherical star approaching a black hole. As it approaches the black hole, tidal forces begin to compress the star in a direction perpendicular to the orbital plane, reigniting it. But within the orbital plane, these gravitational forces stretch the star and tear it apart. It's a competing effect."

Prior research has simulated tidal forces on white dwarf stars, but the calculations in this study are the first fully relativistic simulations that model nucleosynthesis in reigniting white dwarf stars. They also are the highest resolution simulations to date of nucleosynthesis inside the core of a tidally disrupted white dwarf star, where the strongest reactions occur.

"This entire project was made possible by our summer students and postdocs," Anninos said. "We're all about training the next generation of physicists, and this sort of project allows early career researchers the chance to spread their wings and run some heavy simulations."

Explore further: Researcher reports new information about black holes

More information: Peter Anninos et al. Relativistic Tidal Disruption and Nuclear Ignition of White Dwarf Stars by Intermediate-mass Black Holes, The Astrophysical Journal (2018). DOI: 10.3847/1538-4357/aadad9

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Surveillance_Egg_Unit
1.4 / 5 (10) Oct 31, 2018

The team ran supercomputer simulations of dozens of different close encounter scenarios to test this theory. Not only did they find that a close encounter would reignite the once-dead star, but they saw evidence that the process could create significant electromagnetic and gravitational wave energies that might be visible from detectors in near-Earth orbit.


Which would mean that the "dead Star" wasn't quite dead yet, and that taking a turn around an alleged Black Hole should enable the nearly-dearly departed Star to accumulate much needed Hydrogen gas and dust in the region to reignite itself in its travels. The poor thing was starving after using up most of its Helium and other Matter, and required food to continue as before.

This may show that Star death is also caused by a lack of enough Matter in its disk to attract to sustain it.
This also may show scientists that for future space travel by humans, planets orbiting a starving Star should be avoided.

SkyLight
5 / 5 (8) Nov 01, 2018
Nowhere in the article is a disk mentioned, nor indeed in the paper referenced by the article. The paper can be seen here: https://arxiv.org...5664.pdf

As the article suggests, the white dwarf (WD) gets so close to the BH, that it undergoes differential tidal compression which, in certain scenarios, would be enough to reignite (in a fusion sense) the materials *already* in the WD.

So, no disk - did I mention that? Try reading the paper - educate yourself.
billpress11
1 / 5 (3) Nov 01, 2018
Could there possibly be other explanations? For one thing I would like to know when this dormant white dwarf was first observed, previously or just when it reignited? If when it reignited, could it possible be mistaken for a dormant white dwarf when it is really a neutron star whose neutrons have been decaying to the point where a sufficient quantity of hydrogen reignites? If so this could be either a permanent or intermittent ignition. I hope there are many follow up articles on this star.
SkyLight
5 / 5 (6) Nov 01, 2018
could it possible be mistaken for a dormant white dwarf when it is really a neutron star whose neutrons have been decaying
The neutrons within a neutron star (NS) would "like to" decay as if they were in a free state, but such decays are very highly suppressed by the fact that the populations of both the protons and electrons, which are also present in the NS, are in a highly degenerate state.

What does this mean? The high degree of degeneracy in the populations of protons and electrons means that very few of the energy levels or "slots" permissible by quantum mechanics for those particles are free to be used, or filled. So, a neutron which decays in a NS has to be "lucky" enough to be able to decay into a proton and an electron such that BOTH of those particles are able to find an unoccupied energy "slot" (a neutrino would also be emitted, but that would just fly away unhindered). This is a rare event at the very high degeneracy levels obtaining in a NS.
billpress11
1 / 5 (2) Nov 01, 2018
SkyLight, I agree with you about the degenerate state of the neutrons. But wouldn't it be possible over 10's of thousands or millions of years for the surface neutron to gain the energy levels needed to decay from the radiation and neutrinos bombarding them from surrounding stars and galaxies?
antialias_physorg
5 / 5 (6) Nov 01, 2018
from the radiation and neutrinos bombarding them from surrounding stars and galaxies?

Neutron stars aren't "bombarded by radiation and neutrinos from surrounding stars and galaxies"any more thany, say, the Earth is (why would they?).
And no: Neutrons don't decay because of neutrinos and other radiation to any great degree on Earth (and here it would be a lot easier for them to do so because there's a lot more of these 'slots' that SkyLight mentions )...so, no: on neutron starts - or anywhere else - that isn't a thing.

Another thing is that neutrons can't gain energy slowly. The energy has to come in at one go
(That's a result of what Einstein got his Nobel Prize for, BTW: The photoelectric effect)
billpress11
2.3 / 5 (3) Nov 01, 2018
Does anyone really know what energy levels are needed to precipitate the decay of a neutron? After all isn't the decay of a neutron thought to be completely random? Another thing energy levels in the form of heat happen in minute or possibly continuous levels don't they?
SkyLight
5 / 5 (4) Nov 02, 2018
Does anyone really know what energy levels are needed to precipitate the decay of a neutron?
Neutron decay occurs by means of the weak nuclear force, and doesn't depend on outside influences like heat being applied, or other "energy levels" as you put it.
energy levels in the form of heat happen in minute or possibly continuous levels don't they?
No - energy levels in systems like atoms or molecules are always discrete, and the energy levels in those systems are set by quantum mechanical rules.

When one talks about energy levels in a macroscopically large system - a body of gas, or a solid body, which is composed of very many atoms or molecules, then one can say that the energy levels of such aggregate systems are continuous. That's because the discrete energy levels of each of the atoms or molecules in the system add together to form a continuum. And it's only in such aggregate systems that a scalar measure like temperature can be defined.

SkyLight
5 / 5 (3) Nov 02, 2018
Another point about stuff being "bombarded" by neutrinos: neutrinos are pretty "ghostly" particles - they interact so extremely rarely with matter that it's often said that a beam of neutrinos propagating through a LIGHT-YEAR (nearly six trillion miles) of solid lead would emerge at the other end with half of the original number of neutrinos still intact. So, a "bombardment" of neutrinos would be like trying to destroy Mt Everest by a baby puffing through a straw.
antialias_physorg
5 / 5 (3) Nov 02, 2018
Another thing energy levels in the form of heat happen in minute or possibly continuous levels don't they?

No. Think about why we have the word "quantum" in quantum theory.

Another point about stuff being "bombarded" by neutrinos: neutrinos are pretty "ghostly" particles

Here's a fun calculation:
"How close do you have to be to a supernova to get a lethal dose of neutrino radiation"
https://what-if.xkcd.com/73/

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