Thickness of the strangelet-crystal crust of a strange star

Abstract

It has recently been pointed out that if the surface tension of quark matter is low enough, the surface of a strange star will be a crust consisting of a crystal of charged strangelets in a neutralizing background of electrons. This affects the behavior of the surface, and must be taken into account in efforts to observationally rule out strange stars. We calculate the thickness of this "mixed phase" crust, taking into account the effects of surface tension and Debye screening of electric charge. Our calculation uses a generic parametrization of the equation of state of quark matter. For a reasonable range of quark matter equations of state, and surface tension of order a few MeV/fm 2 , we find that the preferred crystal structure always involves spherical strangelets, not rods or slabs of quark matter. We find that for a star of radius 10 km and mass 1.5M⊙, the strangelet-crystal crust can be from zero to hundreds of meters thick, the thickness being greater when the strange quark is heavier, and the surface tension is smaller. For smaller quark stars the crust will be even thicker.

Strange quark matter fragmentation in astrophysical events

“…It has been suggested [41], however, that strange stars may present a strangelet crust embedded in a electron background. If this is the case, then during a merger event strangelets would already be ejected with a mass spectrum with baryonic number of a few hundreds and one should expect a considerable amount of strangelets in the cosmic ray flux, an idea so far not favored by experiments.…”

Strangelet dwarfs

“…It is generally assumed that strange stars are compact objects, with sizes in the 10 kilometer range [4], ending at a sharp surface of thickness ∼ 1 Fermi, perhaps with a very thin electrostatically suspended nuclear matter crust [9][10][11]. However, if the surface tension σ of the interface between quark matter and the vacuum is less than a critical value σ crit (of order a few MeV/fm 2 in typical models of quark matter) then large strangelets are unstable against fission into smaller ones [12][13][14], and the energetically preferred state is a crystal of strangelets: a mixed phase consisting of nuggets of positively-charged strange matter in a neutralizing background of electrons.…”

Nonradial oscillation modes of compact stars with a crust

“…In any case, effects of the quark BCS gap on the oscillation spectrum of homogeneous strange stars have been studied recently [40]. Fig.1 shows the mass-radius relationship for homogeneous quark stars for different values of the Bag constant B. Crust EoS: A two-component structure for strange quark stars was suggested in [30] and developed further in subsequent works [31,46]. Models of strange stars that consist entirely of homogeneous strange quark matter or a thin nuclear crust suspended on top have large quark density as the quark surface is approached and the pressure goes to zero.…”

“…The analysis in [31] shows that for surface tension below a critical value, there is an optimal strangelet size R * ≈ yλ D with λ D =1/ 4πα e χ Q where α e is the fine structure constant and the dimensionless parameter y is in the range 1.60-2.77. Since the phase fraction f of the strangelets turns out to be quite low, we work in the approximation of isolated strangelets and ignore the possibility of Wigner-Seitz cells and lattice structures for the mixed phase, but in principle these can be studied using the results of [46]. The energy density of the mixed phase crust is contributed mainly by the strangelets…”