Quark Nova Spotted in Cas A?

Two elements deep within Cassiopeia A, hint the supernova remnant underwent a quark nova — a theoretical second explosion that leaves behind a quark star — just days after the original supernova.

In this image, NuSTAR data, which show high-energy X-rays from radioactive material, are colored blue. Lower-energy X-rays from non-radioactive material, imaged previously with NASA's Chandra X-ray Observatory, are shown in red, yellow and green. NASA/JPL-Caltech/CXC/SAO

Cas A as seen by NuSTAR (blue) and Chandra (red, yellow and green).
NASA/JPL-Caltech/CXC/SAO

Massive stars are thought to end their lives in cataclysmic explosions, leaving behind neutron stars or black holes as their corpses.

But there are even stranger possibilities.

Cassiopeia A exploded some 300 years ago and is now a beautiful cascade of gases surrounding a neutron star (or so we think). Dozens of ground- and space-based telescopes have collected the remnant’s light over the years. And recently, NuSTAR — NASA’s latest high-energy X-ray satellite — stared at Cas A for 13.8 days straight, shedding light deep into this remnant.

Now, astronomers from the University of Calgary in Alberta, Canada, have proposed a more exotic scenario: days after the first supernova explosion, the neutron star exploded once more, creating a quark star instead.

“It’s hard not to imagine that nature wouldn’t make use of this stage between a neutron star and a black hole,” says lead author Rachid Ouyed, one of the first scientists to suggest the concept. “A quark nova can be thought as a bridge between these two.”

Quarks are the fundamental building blocks of matter. They normally associate in groups of two or three, producing familiar protons and neutrons. But, like any new ideas in theoretical physics, they’re enshrouded in controversy and debate.

Some physicists think that extremely dense matter settles into a soup of individual quarks, while others think it isn’t possible for quarks to exist on their own. The energy required to separate quarks would create new quarks, which would immediately bind with any quarks being separated.

While no earthbound experiment has produced an individual quark, the remnants of extremely hot and massive stars present a great laboratory to test the theory and settle the debate.

When the core of a massive star runs out of energy, it collapses to form an incredibly dense neutron star or black hole. Bring a teaspoon of neutron star to Earth, and it would outweigh Mount Everest at about a billion tons. If a neutron star’s density continued to increase — because it received additional mass or slowed its spin rate drastically — the core would collapse even further. That extra pressure could set quarks free from their original protons and neutrons.

At this stage, the only mechanism keeping these particles separated is a law that forbids them from occupying the same quantum states. Some quarks will be forced into much higher energy states, so high they actually convert to heavier quarks.

“Large amounts of energy can be liberated in a short time when this transformation happens,” says quark nova expert Jan Staff (Macquarie University, Australia). “This energy will tear off the crust of the neutron star, creating ejecta consisting of the heavy elements of the crust in addition to many neutrons. This explosion is the quark nova.”

But a quark nova remains theoretical. Though it would likely occur a few days after the supernova, the first explosion’s expanding gases would hide the second detonation. So instead astronomers look for hints in the messy aftermath of gases: the resulting supernova remnant.

Elements form one after another within the star before it explodes, leading to onion-like shells within the supernova remnant. Two elements in particular, titanium and iron, form in nearly the same location less than a second before the star explodes.

Chandra's hot iron map compared to NuSTAR's radiocative titanium map. NASA

Chandra's hot iron map compared to NuSTAR's radiocative titanium map.
NASA

Yet by comparing NuSTAR’s titanium map with Chandra’s previous map of heated iron, it’s clear that in Cas A the two elements are found nowhere near each other. In addition, it appears that there is more titanium than expected and too little iron.

Ouyed and colleagues think the high-energy neutrons from the quark nova would destroy the iron and form lighter elements, one of which is titanium. The quark nova is crucial in this picture, as it provides the neutrons necessary to form the lighter element, says Staff.

There’s one caveat: since Chandra only sees iron atoms shocked to temperatures high enough to emit X-rays, it’s possible there’s cooler iron hiding within the remnant.

Even so, the fact that titanium and iron don’t align is troubling.

Brian Grefenstette (California Institute of Technology) and colleagues took a detailed look at these results earlier this year. They argued that titanium’s uneven distribution results from sloshing within the star itself just before its demise.

“But this doesn’t help explain why the iron and titanium spatial maps are so different,” says Grefenstette. In the quark nova explanation, Ouyed and colleagues “really try to explain much of the morphology that we see in Cas A ... and make several predictions that may be testable over the next couple of years.”

While the quark nova theory will need to be confirmed, Staff thinks these initial results are highly important.

“I feel we have found the door — the quark nova — to enter the quark world like never before,” says Ouyed. “It’s going to be a very exciting time for astrophysics if we can confirm a quark nova in Cas A.”

Reference:
R. Ouyed et al. “Evidence for a Second Explosion (a Quark Nova) in Cassiopeia A Supernova,” Submitted.

9 thoughts on “Quark Nova Spotted in Cas A?

  1. Peter WilsonPeter Wilson

    Some quarks will be forced into much higher energy states, so high they actually convert to heavier quarks. “Large amounts of energy can be liberated in a short time when this transformation happens,” says quark nova expert Jan Staff.

    The energy liberated is gravitational, as in the first blast. The more massive quarks take up the same volume as their lighter cousins, so more mass can be crammed into less space, and more gravitational energy released. This seems to imply the quark nova produces a black hole? Wiki says the remnant is either a neutron star or black hole. Would the detection of BH confirm the quark nova hypothesis?

  2. Anthony Barreiro

    This is the first time I’ve heard of quark novas and quark stars. How long have theorists been speculating that such things might exist? They’re not mentioned in recent introductory astronomy textbooks. Do most people who study massive stars and supernovas believe quark stars might exist, or is this a brand new idea proposed by one team?

    Neutron stars emit characteristic radiation, and the mass and density of neutron stars and black holes can be measured by observing things that are orbiting them. Is there a direct observational method that can reliably differentiate between quark stars and neutron stars on the one hand and black holes on the other? The titanium and iron distributions reported in the Cas A remnant seem open to multiple explanations, at least to this uninformed layperson.

    1. Peter WilsonPeter Wilson

      Agree, the distributions do not seem to call for an exotic explanation. And why would the quark nova occur a few days after the supernova?

      1. Kyp

        I think the supernova has to go off first which leaves you with a neutron star, then depending on the leftover mass it may go quark nova. It takes a few days because the neutron star needs to spin down. I don’t think there is any satisfactory answer for why the titanium is interior to the iron (titanium is lighter so it should be way above), so an “exotic” explanation may be necessary.

    2. Kyp

      The quark nova theory has been around for a decade or so, and quark stars have been talked about for much longer. Lots of work has been done on quark novae, just search ADS for quark nova (http://adswww.harvard.edu/). They probably aren’t mentioned in text books because it is still a relatively new idea and this kind of thing takes a long time to be accepted (just look at how long it took for neutron stars or black holes to be accepted; the latter in my opinion is MUCH stranger than the idea of quark stars).

      Neutron stars and quark stars are probably pretty similar, they are almost the same size and mass so telling them apart from millions of light years away may be difficult.

    3. Lou

      Having developed an interest in astronomy through Chandra, I’ve been subjected to quite a lot of ideas concerning quark stars (although this is the first time that I’ve heard of a “quark nova”).

      Here are some notes clipped from my personal file concerning quark stars:

      Theorists have speculated since the 1980s that if a Neutron Star gained enough mass, the pressure at its core could crush even neutrons themselves into quarks

      The extreme pressures would convert Up and Down quarks into Strange quarks

      Suspected Quark Stars:
      PSR J0205+6449 (within SN remnant 3C58, 10,000 LY away) – 1 Million K (Chandra) – should be twice as hot – more dense = faster cooling
      RX J1856-3754 (400 LY away) – 700,000°K (Chandra) and 11 km diameter (HST) – too small to be a Neutron Star

      1. Anthony Barreiro

        Thanks Lou. So people have been hypothesizing quark stars for around 30 years and there are at least three anomalous objects that could be quark stars. (I developed an interest in astronomy through seeing pretty bright lights in the sky and wanting to learn their names.)

  3. bwana

    Neutron star, black hole, quark star… Interesting theories / discoveries in the past 50+ years. My Intro to Astronomy only hinted at the 1st two. I don’t think anyone had really got their head around what they really were and were just starting to search for them in the heavens.

    Wonderful time to be alive!

    bwa

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