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They came into existence violently… Born at the death of a massive star. They are composed almost entirely of neutrons, barren of electrical charge and with a slightly larger mass than protons. They are quantum degenerates with an average density typically more than one billion tons per teaspoonful – a state which can never be created here on Earth. And they are absolutely perfect for study of how matter and exotic particles behave under extreme conditions. We welcome the extreme neutron star…
In 1934 Walter Baade and Fritz Zwicky proposed the existence of the neutron star, only a year after the discovery of the neutron by Sir James Chadwick. But it took another 30 years before the first neutron star was actually observed. Up until now, neutron stars have had their mass accurately measured to about 1.4 times that of Sol. Now a group of astronomers using the Green Bank Radio Telescope found a neutron star that has a mass of nearly twice that of the Sun. How can they make estimates so precise? Because the extreme neutron star in question is actually a pulsar – PSR J1614-2230. With heartbeat-like precision, PSR J1614-2230 sends out a radio signal each time it spins on its axis at 317 times per second.
According to the team; “What makes this discovery so remarkable is that the existence of a very massive neutron star allows astrophysicists to rule out a wide variety of theoretical models that claim that the neutron star could be composed of exotic subatomic particles such as hyperons or condensates of kaons.”
The presence of this extreme star poses new questions about its origin… and its nearby white dwarf companion. Did it become so extreme from pulling material from its binary neighbor – or did it simply become that way through natural causes? According to Professor Lorne Nelson (Bishop’s University) and his colleagues at MIT, Oxford, and UCSB, the neutron star was likely spun up to become a fast-rotating (millisecond) pulsar as a result of the neutron star having cannibalized its stellar companion many millions of years ago, leaving behind a dead core composed mostly of carbon and oxygen. According to Nelson, “Although it is common to find a high fraction of stars in binary systems, it is rare for them to be close enough so that one star can strip off mass from its companion star. But when this happens, it is spectacular.”
Through the use of theoretical models, the team hopes to gain insight as to how binary systems evolve over the entire lifetime of the Universe. With today’s extreme super-computing powers, Nelson and his team members were able to calculate the evolution of more than 40,000 plausible starting cases for the binary and determine which ones were relevant. As they describe at this week’s CASCA meeting in Ontario, Canada, they found many instances where the neutron star could evolve higher in mass at the expense of its companion, but as Nelson says, “It isn’t easy for Nature to make such high-mass neutron stars, and this probably explains why they are so rare.”
Original story source at Physorg.com.
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Um… there’s a double “are” in the first line. Also, at the second paragraph, in the fourth line, “measure” should be past tense measured.
Er… I’ll get my coat and see myself out…
aw, take your shoes offf and sit a spell. it just goes to show what happens when one relies on electronic assistance for editing! 🙂
It is too bad we can’t observe the internal state of matter in neutron stars. At huge energies (temperatures) and high density there is a quark-gluon plasma state. However, at lower temperatures there are the quark liquid state and the color-flavor locked states and intermediate and extreme pressures respectively. In that situation gluons becomes massive and form BCS pairs of quarks. There is evidence of a low viscosity fluid state as well at high energy, but low pressure from RHIC and heavy ion data at LHC.
How can we detect the physics inside a neutron star?
LC
Btw, that lecture of Thorne I link to above? It has some on that. After a lot of slides showing NS processes, he sums:
“• There is a rich variety of ways that a NS can radiate GWs.
• The emitted waves will carry rich information about NS
physics and nuclear physics.
• Coordinated GW & EM observations have great potential”
[“Neutron-Star Dynamics”, slide 35.]
These gravity waves are emitted during the transient phase of core collapse in a supernova. These might give a snapshot of data about the internal configuration of a neutron star. If we are lucky we might get such data. This is in contrast to a stationary rotating neutron star, which will not emit any gravity waves.
LC
If the neutron star is spinning at 317 revolutions per second, taking a rough guestimate of the expected diameter of that neutron star, what would the surface speed be, approximately?
Does the speed of the rotation cause the equator of the star to bulge? Or is its gravity too intense to distort the surface?
If the neutron star is spinning at 317 revolutions per second, taking a rough guestimate of the expected diameter of that neutron star, what would the surface speed be, approximately?
Does the speed of the rotation cause the equator of the star to bulge? Or is its gravity too intense to distort the surface?
The periodicity is 317 rotations per seconds, so the frequency is 317 sec^{-1}. Now multiply this be 2? to get the angular frequency ? = 1992sec{-1}. Neutron star have a radius of around 15k and so the tangent velocity at the surface is v = r? = 29876km/sec or about 30 thousand km/sec. This is about 1/10th the speed of light.
This is however not enough to pull the neutron star apart. The centripetal acceleration force on a particle on the equatorial surface is a = r?^2 which is a huge a = 59507100km/sec^2 or 5.95×10^{10}m/sec^2. The surface gravity for this body, assuming it is 1.5 solar masses, is using Newtonian gravity as an approximation
a = GM/r^2 = (6.67×10{-11}m^3/kg-sec^2)(2.985×10^{30}/15
= 1.33×10^{16}m/sec^2
So only 1 millionth of the gravity force is involved with keeping the mass on this rotating frame. So the body is nowhere near any point of ripping itself apart due to this rotation
LC
Yes, they are expected to develop a bulge; indeed, such a bulge is how they radiate gravitational waves, and the radiation of such prevents them from spinning-up to infinity (i.e. the bulge acts as a brake).
Uhmmm, no. Gravitational radiation is produced by a quadrupole moment oscillation, such as the highly eccentric orbit between two neutron stars. The Hulst-Taylor observation of such did find an orbital decay commensurate with energy loss by gravitational radiation. The simple rotation of a stationary body will not radiate gravitational waves. If the neutron star undergoes some massive shift in its mass redistribution there will be a gravity wave pulse, though it will be very weak. Starquakes of neutron stars might do this, as will huge mass redistributions which are thought to occur on magnetars.
LC
So this article is wrong?:
http://www.newscientist.com/article/dn8576-fastspinning-neutron-star-smashes-speed-limit.html
Off hand one would tend to say it is likely wrong; New Scientist has been shunned for distorting main theories and promoting “alternatives” by scientists. Physicists since the early 00’s perhaps, biologists from the middle of 00’s perhaps.
I would only use it if I’m sure of the topic.
However, in this case I can’t see how it is wrong in the basics. [Disclaimer: Haven’t studied GR.] A rotating bulge would set up a quadrupole moment, much as a rotating binary.
It is used as an example of gravitational power radiation in Wikipedia [Gravitational wave”]: “A spinning non-axisymmetric planetoid — say with a large bump or dimple on the equator — will radiate”, in MTW [“Gravitation”, Misner, Thorne, Wheeler]: “In this pulsar phase, gravitational radiation is important only if the star is somewhat deformed from axial symmetry (axial symmetry → constant quadrupole moment → no gravitational waves)”, and in Thorne’s lectures on Gravitational Radiation: “Neutron-Star Dynamics … Solid Crust can support deformations from axisymmetry, with(quadrupole moment)/(star?s moment of inertia) ? ? < 10-5 … [later slide] There is a rich variety of ways that a NS can radiate GWs.”
I think a problem is that even if the competing processes of NS gravitational radiation has died down later, no one knows if these bulges behave like assumed (“Solid Crust”)?
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I tend to share Larsson’s skepticism. There has to be a decent quadrupole moment for the emission of gravity waves. A spinning neutron star would require some matter distribution which deviates from azimuthal symmetry, such as a spinning top. It is not clear to me how this could exist.
LC
Nothing in the article mentions gravitational radiation, it is simply the announcement of a neutron star of significantly greater mass than is typical.
My favorite extreme Neutron star is still Geminiga… That one’s a ‘doozy’! Don’t get me wrong, PSR J1614-2230’s trick spin is interesting, but Geminiga’s faster trajectory indicates an even more complicated history? Regardless, magnetar’s ROCK!