Neutron Star at Core of Cas A Has Carbon Atmosphere

by Nancy Atkinson on November 4, 2009

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A Chandra X-ray Observatory image of the supernova remnant Cassiopeia A. Credit: Chandra image: NASA/CXC
A Chandra X-ray Observatory image of the supernova remnant Cassiopeia A. Credit: NASA/CXC

Supernova remnant Cassiopeia A (Cas A) has always been an enigma. While the explosion that created this supernova was obviously a powerful event, the visual brightness of the outburst that occurred over 300 years ago was much less than a normal supernova, — and in fact, was overlooked in the 1600′s — and astronomers don’t know why. Another mystery is whether the explosion that produced Cas A left behind a neutron star, black hole, or nothing at all. But in 1999, astronomers discovered an unknown bright object at the core of Cas A. Now, new observations with the Chandra X-Ray Observatory show this object is a neutron star. But the enigmas don’t end there: this neutron star has a carbon atmosphere. This is the first time this type of atmosphere has been detected around such a small, dense object.

A Chandra X-ray Observatory image of the supernova remnant Cassiopeia A, with an artist's impression of the neutron star at the center of the remnant. The discovery of a carbon atmosphere on this neutron star resolves a ten-year old mystery surrounding this object.  Credit: Chandra image: NASA/CXC/Southampton/W.Ho; illustration: NASA/CXC/M.Weiss

A Chandra X-ray Observatory image of the supernova remnant Cassiopeia A, with an artist's impression of the neutron star at the center of the remnant. The discovery of a carbon atmosphere on this neutron star resolves a ten-year old mystery surrounding this object. Credit: Chandra image: NASA/CXC/Southampton/W.Ho; illustration: NASA/CXC/M.Weiss


The object at the core is very small – only about 20 km wide, which was key to identifying it as a neutron star, said Craig Heinke from the University of Alberta. Heinke is co-author with Wynn Ho of the University of Southampton, UK on a paper which appears in the Nov. 5 edition of Nature.

“The only two kinds of stars that we know of that are this small are neutron stars and black holes,” Heinke told Universe Today. “We can rule out that this is a black hole, because no light can escape from black holes, so any X-rays we see from black holes are actually from material falling down into the black hole. Such X-rays would be highly variable, since you never see the same material twice, but we don’t see any fluctuations in the brightness of this object.”

Heinke said the Chandra X-ray Observatory is the only telescope that has sharp enough vision to observe this object inside such a bright supernova remnant.

But the most unusual aspect of this neutron star is its carbon atmosphere. Neutron stars are mostly made of neutrons, but they have a thin layer of normal matter on the surface, including a thin–10 cm–very hot atmosphere. Previously studied neutron stars all have hydrogen atmospheres, which is expected, as the intense gravity of the neutron star stratifies the atmosphere, putting the lightest element, hydrogen, on top.

But not so with this object in Cas A.

“We were able to produce models for the X-ray radiation of a neutron star with several different possible atmospheres,” Heinke said in an email interview. “Only the carbon atmosphere can explain all the data we see, so we are pretty sure this neutron star has a carbon atmosphere, the first time we’ve seen a different atmosphere on a neutron star.”

An artist's impression of the neutron star in Cas A showing the tiny extent of the carbon atmosphere. The Earth's atmosphere is shown at the same scale as the neutron star.  Credit: NASA/CXC/M.Weiss

An artist's impression of the neutron star in Cas A showing the tiny extent of the carbon atmosphere. The Earth's atmosphere is shown at the same scale as the neutron star. Credit: NASA/CXC/M.Weiss


An artist’s impression of the neutron star in Cas A showing the tiny extent of the carbon atmosphere. The Earth’s atmosphere is shown at the same scale as the neutron star. Credit: NASA/CXC/M.Weiss

So how does Heinke and his team explain the lack of hydrogen and helium on this neutron star? Think of Cas A as being a baby.

“We think we understand that as due to the really young age of this object–we see it at the tender age of only 330 years old, compared to other neutron stars that are thousands of years old,” he said. “During the supernova explosion that created this neutron star (as the core of the star collapses down to a city-sized object, with an incredibly high density higher than atomic nuclei), the neutron star was heated to high temperatures, up to a billion degrees. It’s now cooled down to a few million degrees, but we think its high temperatures were sufficient to produce nuclear fusion on the neutron star surface, fusing the hydrogen and helium to carbon.”

Because of this discovery, researchers now have access to the complete life cycle of a supernova, and will learn more about the role exploding stars play in the makeup of the universe. For example, most minerals found on Earth are the products of supernovae.

“This discovery helps us understand how neutron stars are born in violent supernova explosions,” said Heinke.

Source: Interview with Craig Heinke

About 

Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

Jon Hanford November 4, 2009 at 12:34 PM

Interesting to read that conditions are right for FUSION reactions to occur on the surface of the neutron star. I wonder does this have a tie-in to the thin carbon ‘atmosphere’.

I also find it remarkable that a 20km wide pulsar is directly related to a debris shell estimated to be at least 20 light-years wide !

DrFlimmer November 4, 2009 at 1:29 PM

An atmosphere that is 10cm high…. A human being on that scale would be about 1 µm. I don’t think that this is a fair estimante, but interesting.

If you wonder: I took the atmosphere of our planet (10^5m) and the length of the human body, as physicist do it, as 10^0m. So we have 5 orders of magnitude difference.

I wonder if such a µ-man will ever take a look “up”?

Dark Gnat November 4, 2009 at 1:47 PM

See what unchecked carbon atmospheres can do? BOOM!!!!

(Just kidding.)

It actually makes sense for a dense object in the middle of a gas cloud to retain an atmosphere. These are probably more common than we thought.

Jon brings up a good point about optimal fusion conditions. That might be worth investigating.

Lawrence B. Crowell November 4, 2009 at 2:10 PM

I think it was R. Forward who conjectured on how the surface of neutron stars might hold chemical-like structures, but with nuclei. I think the idea is probably wrong, for as I understand the surface is iron in a degenerate electron state. But, as the conjecture goes this nucleon-chemistry might be life-like, giving rise to the micron man.

If a supernova occurs close enough we might be able to detect nuclear fusion on the surface of the neutron star.

LC

Jon Hanford November 4, 2009 at 7:53 PM

The relevant paper on this article was recently posted on the arXiv site here: http://arxiv.org/PS_cache/arxiv/pdf/0911/0911.0672v1.pdf . Some intriguing speculation here.

Jon Hanford November 4, 2009 at 8:25 PM

A second paper pertaining to cosmic rays from M 82 also recently appeared here: http://arxiv.org/ftp/arxiv/papers/0911/0911.0873.pdf . Definitely this AGN galaxy is probably associated with UHE cosmic rays.

Don Alexander November 4, 2009 at 10:12 PM

@LBC “If a supernova occurs close enough we might be able to detect nuclear fusion on the surface of the neutron star.”

Oh, we do all the time, they are called Type-I X-ray bursts. They occur in binary systems where a donor star feeds matter onto a NS companion.

Hannes November 5, 2009 at 4:18 AM

The thin carbon atmosphere might also be a reason why Cas A does not yet show normal pulsar-like behaviour.

Before the magnetic character of iron will become visible, something must align its magnetic domains. You need to have a current flowing in the carbon layer to establish that. Maybe the nuclear burning is fuelling that process.

Iron and carbon mixtures have complex properties.
At first the remaining NS after the SN may have an iron layer with a very low carbon percentage. It will generate a powerfull magnetic field, but only for a limited time. These are possibly young “magnetars”, actually periodically active NS.

If there is nuclear burning on the surface of a young NS there would be enough heat to prevent it from cooling.
But it won’t take long. When the current temporary stops flowing for some reason, the domains may immediately return to their original sizes and shapes and the iron may become nonmagnetic again.

A reason might be that the crystalline structure and hardness of the iron layer changes through time.

When iron on earth is diffused with more than 1% carbon it is difficult to magnetize and demagnetize. Above 3% it is again easily magnetized and demagnetized.

When the carbon level is again above a certain level then it will be producing magnetic fields for the second time but now with extreme intensity (800 trillion Gauss). These are possibly the old magnetars.

Lawrence B. Crowell November 5, 2009 at 8:31 AM

I thought the magnetic field of a neutron star was set up from the magnetic field of the original star, but where the magnetic field lines are squeezed through the small volume of the collapsed core.

LC

Hannes November 5, 2009 at 12:59 PM

http://arxiv.org/PS_cache/arxiv/pdf/0911/0911.0672v1.pdf :

….In this case, either neutron star
magnetic fields develop by a dynamo mechanism, or else a
strong field (produced during the collapse of the progenitor
star) is buried and has not yet emerged.

So there are (at least) two possible options for the magnetic field of a NS, indeed.

Lawrence B. Crowell November 5, 2009 at 6:46 PM

Don Alexander, thanks for the information on Type-I X-ray bursts. I have heard of these, after my memory was jogged.

Hannes: So it appears the nature of an NS magnetic field is a subject of research. To be honest I always thought it was curious that a dense gas of neutrons could hold such enormous magnetic field.

LC

Hon. Salacious B. Crumb November 8, 2009 at 7:02 PM

Where’s that jackass ANaconda when you need him?

You get finally physical evidence of dense stars, and you can’t see the EU’s for dust!

Stellar evolution, in the end, for them will be the death nell of their quite crazy and unsubstantiated views. Bring more stories like this one on, please!

Hon. Salacious B. Crumb November 8, 2009 at 7:11 PM

LBC said;

Hannes: So it appears the nature of an NS magnetic field is a subject of research. To be honest I always thought it was curious that a dense gas of neutrons could hold such enormous magnetic field.

It doesn’t. Surely the forces by a violent Fermi Sea of electrons from the collapse generate the strong magnetic fields? Considering the angular momentum contained in a small neutron star is so enormous, such fields are expected.

I am far more curious why one would make a statement relating neutrons to electric fields?

Also curious is the shapes of the neutron stars. It is not the spherical model at all (as shown in the graphic), but is more like a flat accretion disk, which is spinning around 800 to 1000 times per second. Why does everyone seem to misrepresent this view with a common misconception.

Clearly angular momentum seems to be forgotten component of the dense star phenomena – neutron stars included!

Hon. Salacious B. Crumb November 8, 2009 at 7:21 PM

The caption says;

An artist’s impression of the neutron star in Cas A showing the tiny extent of the carbon atmosphere. The Earth’s atmosphere is shown at the same scale as the neutron star. Credit: NASA/CXC/M.Weiss

NASA. please get another artist, or just talk to an astrophysicist specialises with neutron stars. This single graphic is so wrong on so many levels.

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