Preon stars: a new class of cosmic compact objects (original) (raw)

Abstract

In the context of the standard model of particle physics, there is a definite upper limit to the density of stable compact stars. However, if a more fundamental level of elementary particles exists, in the form of preons, stability may be re-established beyond this limiting density. We show that a degenerate gas of interacting fermionic preons does allow for stable compact stars, with densities far beyond that in neutron stars and quark stars. In keeping with tradition, we call these objects “preon stars”, even though they are small and light compared to white dwarfs and neutron stars. We briefly note the potential importance of preon stars in astrophysics, e.g., as a candidate for cold dark matter and sources of ultra-high energy cosmic rays, and a means for observing them.

Introduction

The three different types of compact objects traditionally considered in astrophysics are white dwarfs, neutron stars (including quark and hybrid stars), and black holes. The first two classes are supported by Fermi pressure from their constituent particles. For white dwarfs, electrons provide the pressure counterbalancing gravity. In neutron stars, the neutrons play this role. For black holes, the degeneracy pressure is overcome by gravity and the object collapses indefinitely, or at least to the Planck density.

The distinct classes of degenerate compact stars originate directly from the properties of gravity, as was made clear by a theorem of Wheeler and collaborators in the mid 1960s [1]. The theorem states that for the solutions to the stellar structure equations, whether Newtonian or relativistic, there is a change in stability of one radial mode of normal vibration whenever the mass reaches a local maximum or minimum as a function of central density. The theorem assures that distinct classes of stars, such as white dwarfs and neutron stars, are separated in central density by a region in which there are no stable configurations.

In the Standard Model of particle physics (SM), the theory of the strong interaction between quarks and gluons predicts that with increasing energy and density, the coupling between quarks asymptotically fades away [2], [3]. As a consequence of this “asymptotic freedom”, matter is expected to behave as a gas of free fermions at sufficiently high densities. This puts a definite upper limit to the density of stable compact stars, since the solutions to the stellar equations end up in a never-ending sequence of unstable configurations, with increasing central density. Thus, in the light of the standard model, the densest stars likely to exist are neutron stars, quark stars or the potentially more dense hybrid stars [4], [5], [6]. However, if there is a deeper layer of constituents, below that of quarks and leptons, asymptotic freedom will break down at sufficiently high densities, as the quark matter phase dissolves into the preon subconstituent phase.

There is a general consensus among the particle physics community, that something new should appear at an energy-scale of around one TeV. The possibilities are, e.g., supersymmetric particles, new dimensions and compositeness. In this Letter we consider “preon models” [7], [8], i.e., models in which quarks and leptons, and sometimes some of the gauge bosons, are composite particles built out of more elementary preons. If fermionic preons exist, it seems reasonable that a new type of astrophysical compact object, a preon star, could exist. The density in preon stars should far exceed that inside neutron stars, since the density of preon matter must be much higher than the density of nuclear and deconfined quark matter. The sequence of compact objects, in order of increasing compactness, would thus be: white dwarfs, neutron stars, preon stars and black holes.

Section snippets

Mass–radius relations

Assuming that a compact star is composed of non-interacting fermions with mass mf, the non-general relativistic (Chandrasekhar) expression for the maximum mass is [9], [10]: M≃1mf2(ℏcG)3/2. This expression gives a correct order of magnitude estimate for the mass of a white dwarf and a neutron star. For quark stars, this estimate cannot be used literally, since the mass of quarks cannot be defined in a similar way as for electrons and neutrons. However, making the simplifying assumption that

Stability analysis

A necessary, but not sufficient, condition for stability of a compact star is that the total mass is an increasing function of the central density dM/dρc>0 [14]. This condition implies that a slight compression or expansion of a star will result in a less favourable state, with higher total energy. Obviously, this is a necessary condition for a stable equilibrium configuration. Equally important, a star must be stable when subject to radial oscillations. Otherwise, any small perturbation would

Potential astrophysical consequences and detection

If preon stars do exist, and are as small as 10−1–10−4m, it is plausible that primordial preon stars (or “nuggets”) formed from density fluctuations in the early universe. As this material did not take part in the ensuing nucleosynthesis, the abundance of preon nuggets is not constrained by the hot big bang model bounds on baryonic matter. Also, preon nuggets are immune to Hawking radiation [18] that rapidly evaporates small primordial black holes, making it possible for preon nuggets to

Conclusions

In this Letter we argue that if there is a deeper layer of fermionic constituents, so-called preons, below that of quarks and leptons, a new class of stable compact stars could exist. Since no detailed theory yet exists for the interaction between preons, we assume that the mass–energy contribution from preon interactions can be accounted for by a ‘bag constant’. By fitting the bag constant to the energy density of a composite electron, the maximum mass for preon stars can be estimated to ∼102M⊕

Acknowledgments

F. Sandin acknowledges support from the Swedish National Graduate School of Space Technology. We thank S. Fredriksson for several useful discussions and for reading the manuscript.

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