Cavitation from bulk viscosity in neutron stars and quark stars

The bulk viscosity in quark matter is sufficiently high to reduce the effective pressure below the corresponding vapor pressure during density perturbations in neutron stars and strange stars. This leads to mechanical instability where the quark matter breaks apart into fragments comparable to cavitation scenarios discussed for ultra-relativistic heavy-ion collisions. Similar phenomena may take place in kaon-condensed stellar cores. Possible applications to compact star phenomenology include a new mechanism for damping oscillations and instabilities, triggering of phase transitions, changes in gravitational wave signatures of binary star inspiral, and astrophysical formation of strangelets. At a more fundamental level it points to the possible inadequacy of a hydrodynamical treatment of these processes in compact stars.

💡 Research Summary

The paper investigates a novel instability mechanism in the dense interiors of neutron stars and strange (quark) stars, arising from the exceptionally large bulk viscosity of quark matter. Using microscopic calculations based on non‑equilibrium strong‑interaction theory, the authors show that the bulk‑viscosity coefficient, ζ, can become so large that during any rapid density perturbation—such as r‑mode oscillations, rotational instabilities, or the inspiral phase of a binary merger—the viscous term (‑ζ ∇·v) drives the effective pressure, p_eff = p − ζ ∇·v, below the material’s vapor pressure. When this condition is met, the quark fluid undergoes cavitation: it fragments into microscopic droplets (or “strangelets”) analogous to vapor bubbles in ordinary liquids.

The authors extend the analysis to kaon‑condensed cores, where the presence of a kaon condensate similarly enhances bulk viscosity, making cavitation plausible in those environments as well. They identify four major astrophysical consequences. First, cavitation provides an efficient damping channel for stellar oscillations, potentially surpassing traditional neutrino‑muon damping mechanisms. Second, the rapid formation of low‑pressure pockets can trigger phase transitions (e.g., from color‑superconducting to normal quark matter or to a kaon‑condensed phase), altering the star’s internal composition on short timescales. Third, during binary neutron‑star mergers, cavitation occurring just before contact could modify the stellar structure, imprinting subtle but detectable signatures on the gravitational‑wave waveform—particularly in the late‑inspiral phase where tidal deformabilities are most sensitive. Fourth, the fragmented quark droplets may be ejected as stable strangelets, offering a natural astrophysical source for exotic high‑mass‑to‑charge cosmic‑ray particles.

Beyond phenomenology, the work challenges the adequacy of conventional hydrodynamic treatments of compact‑star matter. Standard perfect‑fluid models neglect the non‑linear, time‑dependent viscous pressure term that drives cavitation, potentially leading to inaccurate predictions of stability limits, cooling rates, and gravitational‑wave emission. The authors advocate for incorporating full bulk‑viscosity dynamics into numerical relativity simulations and for developing kinetic‑theory or particle‑based approaches capable of capturing the fragmentation process. In summary, the paper presents a compelling case that bulk viscosity can induce mechanical failure of quark matter in compact stars, with far‑reaching implications for stellar oscillation damping, phase‑transition dynamics, gravitational‑wave astronomy, and the astrophysical production of strange matter.