Bulk viscosity of baryonic matter with trapped neutrinos

Abstract

We study bulk viscosity arising from weak current Urca processes in dense baryonic matter at and beyond nuclear saturation density. We consider the temperature regime where neutrinos are trapped and therefore have non-zero chemical potential. We model the nuclear matter in a relativistic density functional approach, taking in to account the trapped neutrino component. We find that the resonant maximum of the bulk viscosity would occur at or below the neutrino trapping temperature, so in the neutrino trapped regime the bulk viscosity decreases with temperature as T −2 , this decrease being interrupted by a drop to zero at a special temperature where the proton fraction becomes density-independent and the material scale-invariant. The bulk viscosity is larger for matter with lower lepton fraction, i.e., larger isospin asymmetry. We find that bulk viscosity in the neutrinotrapped regime is smaller by several orders than in the neutrino-transparent regime, which implies that bulk viscosity in neutrino-trapped matter is probably not strong enough to affect the evolution of neutron star mergers.

Damping of density oscillations in neutrino-transparent nuclear matter

“…There are several directions in which this topic could be further developed. In parallel with this work, an analogous calculation of the bulk viscosity for nuclear matter with trapped neutrinos was conducted [58]. This is appropriate for nuclear matter at temperatures well above 5 MeV, and is relevant to mergers because temperatures up to 80 or even 100 MeV are predicted by simulations.…”

Bulk Viscous Damping of Density Oscillations in Neutron Star Mergers

“…In this contribution, we extend recent work [7] to compute the timescales associated with bulk-viscous damping of oscillations. We use two models of the equation of state (EoS) and associated composition of dense matter which are based on the relativistic density functional theory of nuclear matter.…”

“…The non-equilibrium perturbations δn j (t) drag matter out of chemical equilibrium by leading to a small chemical potential shift (4) which can be written in terms of particle densities as µ ∆ (t) = A n δn n (t) + A ν δn ν (t) − A p δn p (t) − A e δn e (t), (7) where A n = A nn − A pn , A p = A pp − A np , and A e = A ee , A ν = A νν with…”

“…The recent detections of gravitational waves by the LIGO-Virgo collaboration, in particular, the multimessenger binary-neutron star (BNS) merger event GW170817 [1], motivate studies of the transport properties of dense nuclear matter at temperatures and densities relevant to BNS mergers [2][3][4][5][6][7]. The mass of the post-merger object typically would exceed the maximum mass of a neutron star and, as a consequence, it would collapse to a black hole on the timescales ranging from tens of milliseconds up to seconds depending on the mass of the post-merger object [8][9][10].…”

“…It is expected that dissipation or damping of matter flows in the merged stars could influence the gravitational waves emitted during the post-merger phase. Recent estimates of the role of the thermal conduction and shear and bulk viscosities indicate that, of these, damping of density oscillations via bulk viscosity has the strongest influence [2,6,7]. After a brief introduction to the problem of computation of the bulk viscosity, we present an extension of our recent work [7] on the bulk viscosity of nucleonic matter which includes the estimates of timescales of the damping of the oscillations by the bulk viscosity.…”