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  Current Research Highlight :: November 2006 all highlights

Neutron star seismology challenges strange star models

The nature of matter in the ultra-dense cores of neutron stars is a key question for nuclear astrophysics. Neutrons may even dissolve into a mixture of up, down and strange quarks - forming what is known as a strange star. But do strange stars really exist? A new study by scientists at the Max Planck Institute for Astrophysics and Los Alamos National Laboratory in the US, using stellar seismology, casts doubt on this possibility.

Fig. 1: The top panel shows a cross section through the star, with the solid crust in yellow and the fluid core in grey. The patterns on the surface show movement due to the vibrations. The lower panels illustrate different types of crust vibrations. In panel (a) the motion in the crust is primarily horizontal. This is the type of movement associated with the torsional shear modes. In panel (b) the motion is radial - these types of vibrations should be much harder to excite.

Fig. 2: Frequencies of torsional shear modes for two different strange crust models: one in which there is a thin crust of normal nuclear material (top panel), and one in which strange matter exists in nuggets (lower panel). In the top plot the black line shows the value predicted for the lowest frequency mode (the fundamental), with the blue dashed line showing the observed value. The red line shows the value predicted for the first radial overtone, and the green dashed line the observed value. For the strange nugget crust (lower panel) the models predict a range of values, shown as shaded areas, depending on the strange quark mass. For the thin nuclear crust, the fundamental is always higher than the measured value; for the nugget crust it is always lower. For the harmonic, the values predicted by our models are far higher than the observed value. Varying the stellar mass, radius and temperature does not alter the conclusions.

The technique exploits observations of some of the most violent events in the Universe, giant gamma-ray flares from a group of stars known as Soft Gamma Repeaters. These are thought to be magnetars - neutron stars with very strong magnetic fields. This field decays and twists, becoming periodically unstable. Eventually the field lines snap and shift, launching bursts of gamma-rays in the process.

The magnetic field lines are, however, fixed to the crust of the neutron star and as they break they should also crack the crust. It had long been suspected that this might trigger global seismic vibrations in the star. Calculations showed that a particular type of vibrations, torsional shear modes, were the most likely to be excited (Figure 1). Observational evidence to support this idea came in late 2004, when high frequency oscillations were discovered in a particularly energetic flare from the Soft Gamma Repeater SGR 1806-20. Similar phenomena were then uncovered in data of an earlier giant flare from a different star. The frequencies of the vibrations were close to those predicted for the torsional shear modes of a neutron star crust.

The frequencies of seismic vibrations can tell us about the composition and depth of the crust (see Research Highlight, May 2006). However they can also tell us about the matter deep in the core of the star. The pressures are so high that neutrons will be squeezed out of atomic nuclei, and may themselves dissolve into their constituent quarks. Calculations in the 1980s by US scientist Ed Witten suggested that the quarks might then react to form a stable fluid of up, down and strange quarks which due to its enhanced stability would convert the entire star into strange quark matter - making a strange star. Much effort has since gone into working out how observations might distinguish strange stars from neutron stars, but the question has yet to be settled decisively.

One of the major differences between the two types of stars is crust composition, which would modify the frequencies of seismic vibrations. In addition strange star crusts should be much thinner, which has a large effect on higher frequency harmonics. These differences led Anna Watts (MPA) and her colleague Sanjay Reddy (LANL) to wonder whether the magnetar observations might rule out strange star models. They have now completed the first computation of torsional shear mode frequencies for strange star crusts. Despite many poorly constrained parameters, no strange star crust model matches the observations - whereas neutron star models do (Figure 2).

"This is for us a very exciting result.", said Watts. "Our calculations predict a dramatic difference between the frequencies of a strange star crust and a neutron star crust. If the seismic interpretation of the data is correct - and no other explanation comes close at present - we may have found a robust way to rule out strange stars".

Anna Watts and Sanjay Reddy


A.L.Watts & S. Reddy:
"Magnetar QPOs pose challenges for strange stars",
Physical Review Letters, submitted; linkPfeilExtern.gifastro-ph/0609364

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