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Quark Stars

by John G. Cramer

Alternate View Column AV-114
Keywords: collapsed, stars, neutron, quark, star, Chandra, X-ray, Observatory
Published in the November-2002 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 04/06/2002 and is copyrighted 2002 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without the explicit permission of the author.

 

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A new type of star and a new kind of matter may have been discovered.  Data on two unusual stars, RXJ1856 and 3C58, recorded by NASA’s orbiting Chandra X-Ray Observatory suggest that in the collapse of a massive star there may be a third alternative to forming a neutron star or a black hole.  This might also be “quark stars” that are smaller and denser than neutron stars.  In this column, I want to discuss these results and the possible role of quark matter in the collapse of a star.

Stars are fusion engines.  In young stars, their energy and luminosity comes from fusing hydrogen into helium in the stellar core.  After a time period that ranges from a few hundred thousand years for very massive stars to billions of years for small stars, the hydrogen-burning fusion phases out as the hydrogen is used up.  Then the stellar evolution process reaches a branch point.  If the star is less than a few times more massive than our sun, its core shrinks and becomes hotter, while the outer regions expand, so that the star becomes a red giant.  After a time the red giant cools and shrinks to become a white dwarf star, and eventually a cold non-luminous black dwarf.

However, more massive stars take a different path.  The gravitational crunch of their core collapse creates enough temperature and pressure to produce helium-burning fusion, with three helium nuclei fusing to form carbon.  This process continues with successively heavier elements, oxygen, neon, magnesium, silicon, and so on up to iron participating in the fusion process, which synthesizes the medium-weight elements up to iron as the “ash” of helium burning.

Eventually the helium fuel is also consumed, and the star goes into another collapse phase, producing a spectacular explosion, a Type II supernova.  The burned-out star still has a very large amount of gravitational energy.  As soon as its nuclear-fusion power plant switches off, the gravitational energy is "cashed in" as the outer part of the star falls inward.

The star's mantle is no longer held in place by the heat and pressure of nuclear fusion.  As it falls in, the density of matter in the interior grows progressively larger.  Finally the density rises to about 1013 grams/cm3, about three times the density of an atomic nucleus.  For reference, a teaspoon of such super-dense matter would weigh as much as all the cars, trucks, and busses on Earth.  At this density, unless the star is so massive that the collapse continues on to form a black hole, the in-falling material literally halts and bounces.  The density has become so large that the "Pauli repulsion", the intrinsic repulsion of one neutron for another, one proton for another, or one electron for another takes over and halts the collapse of the star.

Suddenly all the protons in the star's interior are able to gain energy by eating a nearby electron and spitting out a neutrino.  In the process they become neutrons, and the inner part of the star becomes a neutron star. While this is going on the outer part of the star is being blown outward by the recoil energy of the bounce. That outflow of energy and matter is boosted by the additional energy carried out to the shock wave by the flood of neutrinos from neutron star formation in the core.

About 100 times more energy from the supernova goes into the neutrinos than into the blast shockwave, and 100 times more energy goes into the shockwave than into visible light.  Nevertheless, the visible light from the supernova can outshine the net light output of an entire galaxy.

When the outer part of the star lifts off, it leaves behind a cooling and rapidly spinning neutron star.  The neutron star can be thought of as a gigantic spherical 15-kilometer-diameter nucleus.  It has more mass than our Sun, is made almost entirely of neutrons, and is held together by the force of gravity.  Its collapse has been halted by the mutual strong-force repulsion between neutrons, so that it hovers in a delicate balance between the pull of gravity and the push of the strong force.  This balance would be overcome if the neutron star were more massive and had more gravitational attraction.

Radio astronomers first definitively observed neutron stars as pulsars, sources of periodic radio pulses that repeated a few hundred times a second.  We now understand that these pulses are the result of a "lighthouse effect" of the radio beam emitted along the star's tilted magnetic axis, a beam that scans periodically in our direction as the star rotates, producing pulses.  These pulses at intervals of a few milliseconds can also be seen in the visible light emission from neutron stars.

Neutron stars also produce X-rays, particularly when they pass through an interstellar region that has plenty of hydrogen that can fall in on the neutron star and produce million-degree temperatures at its surface.  NASA’s Chandra X-Ray Observatory, which was launched on July 23, 1999, is designed with a grazing-angle telescope mirror that can focus X-rays.  It can pinpoint such regions of X-ray mission, produce images of extended X-ray objects, and measure the energy spectrum of their emissions.  In the almost three years since it initial operation, Chandra has mapped the X-ray sky and performed detailed studies of many interesting X-ray objects.

Two of these objects seem to break the rules for neutron stars.  Data from Chandra on a neutron star with the memorable name of RXJ1856.5-3754 (or RXJ1856 for short) shows a smooth X-ray spectrum and the lack of any significant pulsar-like activity in its time structure.  From these data combined with observations from the Hubble Space Telescope, theoretical models suggest that the object radiates like a solid body with a temperature of 700,000 degrees Celcius and has a diameter of only 114 kilometers, less than half the diameter expected for a “normal” neutron star.  Observations also indicate that probably RXJ1856 has recently moved into a relatively matter-free region from a dark molecular cloud, where it would have been reheated by in-falling matter.

Another neutron star, 3C58, has a rather different history, in that we know its birthday.  Ancient Japanese and Chinese astronomers recorded its birth in a supernova explosion in 1181 AD.  Chandra observations of the X-rays from 3C58 show that the star has cooled off much faster than would be expected for a normal neutron star.

Thus we have two puzzles: a star that is too small and a star that is too cool.  The behavior of both of these unusual neutron stars can be explained with a single new assumption:  that these stars have made the transition from dense neutron matter to an even denser “quark matter”, in which where are no longer triplets of quarks forming neutrons, but instead a kind of matter composed of quarks in direct contact with many other quarks.  In this scenario, RXJ1856 may have started as a neutron star, but later, in its passage through the dark molecular cloud, it accumulated enough extra mass to make the transition to a quark star, with the neutrons dissolving under the intense force of gravity so that the star became sphere of quarks.  3C58 would have an inner core of quark matter, but an outer shell, where the pressure is lower, that is still a neutron star.

What is the difference between a quark star and a neutron star?  The standard model of elementary particles describes the neutron as a combination of two “down” quarks (quantum numbers: spin=, charge=-1/3, strangeness=0) and one “up” quark (spin=, charge= +2/3, strangeness=0), locked together by color forces and confined within a spherical “bag” with a radius of about 10-15 meters.  The matter of a neutron star is a close-packed medium of such neutrons, each in its own bag, with the Pauli Exclusion Principle keeping them some distance apart.

The matter of a quark star, on the other hand, has no bags of quarks.  Instead, equal numbers of up and down quarks, plus an equal number of “strange” quarks (spin=, charge=-1/3, strangeness=1) are compressed together in the medium.  The Pauli Exclusion Principle also keeps these quarks separated, but since there are three varieties with no bags, the packing is closer and the matter should be two or three times more dense.  The strange quark in its “normal” state as a constituents of strange mesons and baryons is considerably more massive that the up and down quarks that form neutrons and protons.  However, there are theoretical reasons to believe that at the energy densities present in a quark star the strange quark may become lighter.  Even so, converting half of the down quarks in neutrons to strange quarks would require energy, and this may be the reason for the unexpectedly rapid cooling of 3C58.  Thus, the small size of RXJ1856 and the rapid cooling of 3C58 are both expected consequences of the formation of quark matter.


Assuming this evidence stands the test of time, how common should quark stars be in our galaxy?  The answer depends on some unknown factors, but there is a possibility that they may be fairly common.  Even if few massive stars undergoing supernovas fall in the right mass range to go past the neutron star stage and halt at the quark star stage, many neutron stars, either have companions that feed them extra material, or like RXJ1856, pass through regions of the galaxy with relatively large densities of dust and molecular material.  Such neutron stars should grow in mass, and at some stage should make the transition to the quark star state, as 3C58 appears to be doing.

Does the neutron-to-quark star transition make a new supernova-like explosion?  The answer to this question requires modeling and is not settled at the moment.  A major factor in supernova explosions is the bounce and outgoing shock wave arising from neutron star formation and neutrino emission.  It is not clear whether quark star formation involves a major explosion or a quiet burp.

What is clear is that we need better theoretical models of this astrophysical process, more observations of quark star candidates, and more information on the behavior of the kind of quark-dominated matter that would be present in a quark star.  Fortunately, the SPS accelerator at CERN and the new RHIC collider at Brookhaven National Laboratory are bringing heavy nuclei into high-energy high-pressure high-density collisions that should produce just this kind of matter in the laboratory.  In the coming years, we should learn much more about quark matter and quark stars.


AV Columns On-line: Electronic reprints of over 110 "The Alternate View" columns by John G. Cramer, previously published in Analog, are available on-line at: http://www.npl.washington.edu/av.


References

Observations of RXJ1856:
"Is RXJ1856.5-3754 a Quark Star?”, Jeremy J. Drake, et al., Astrophysical Journal (in press), Preprint astro-ph/0204159, available at http://arxiv.org .

Observations of 3C58:
"New Constraints on Neutron Star Cooling from Chandra Observations of 3C58”, Patrick Slane, David J. Hefland, and Stephen S. Murray, Astrophysical Journal Letters (accepted for publication), Preprint astro-ph/0204151, available at http://arxiv.org .


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