Quark droplets envisioned in neutron stars.

The immense pressures inside the collapsed, extremely dense objects known as neutron stars (pulsars) create an extraordinary environment for matter. In principle, such extreme pressures can compress ordinary nuclear matter enough to smear neutrons and protons into their constituent quarks.

Now researchers have demonstrated theoretically that a significant fraction of a neutron star's interior may consist of nuclear matter interleaved with sheets, strands, or droplets of quark matter. Physicist Christopher J. Pethick of NORDITA in Copenhagen, Denmark, and his collaborators describe their model in the March 8 Physical Review Letters.

"Should the quark-droplet phase exist in neutron stars, it could have important observational consequences," the researchers note. For example, the sudden movement or cracking of rigid shells of quark matter trapped within nuclear matter could account for the glitches, or sudden frequency shifts, that sometimes disturb emissions from pulsars.

Until recently most theorists had assumed that any quark matter in a neutron star would lie at its core, separated from the overlying nuclear matter by a sharp boundary Last year, Norman K. Glendenning of the Lawrence Berkeley (Calif.) Laboratory challenged this notion and presented theoretical arguments suggesting that quark and nuclear matter can intermingle and coexist in equilibrium.

"Instead of having a sharp boundary between them, they can form a mixed region, and the mixed region can have a very interesting microscopic structure," Glendenning says.

Inspired by Glendenning's work, Pethick and his colleagues investigated what this mixture would look like. They found that at low densities, the mixed region consists of droplets of quark matter immersed in nuclear matter.

At higher densities, the proportion of quark matter increases and its geometrical structure becomes more complicated. The quark matter adopts shapes resembling rods and plates rather than spheres. The researchers label these the "spaghetti" and "lasagna" stages.

As the density increases further, quark matter becomes the predominant constituent, and any nuclear matter present forms into rods or spherical droplets. These droplets would have roughly the same diameter as atomic nuclei.

"These new details are interesting," Glendenning says. "One can easily imagine how the presence of this mixed region would affect the rate at which the star cools and how well it conducts electricity which would affect the rate at which the star's magnetic field decays."

Whether neutron stars actually contain quark matter remains uncertain. In their calculations, Pethick and his colleagues adopted what they considered to be reasonable estimates of such properties as the surface tension of quark droplets. But no one has ever measured or even precisely calculated what this surface tension would be.

Nonetheless, the possibility that a mixed region can form across a large region of a neutron star's interior increases the likelihood that such a star contains quark matter.