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Heat Conduction in Rotating Relativistic Stars

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Heat Conduction in Rotating Relativistic Stars MNRAS 479, 4207–4215 (2018) doi:10.1093/mnras/sty1725 Advance Access publication 2018 June 29 Downloaded from https://academic.oup.com/mnras/article-abstract/479/3/4207/5046729 by Southampton Oceanography Centre National Oceanographic Library user on 13 September 2018 Heat conduction in rotating relativistic stars S. K. Lander1‹ and N. Andersson2 1Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, PL-00-716 Warsaw, Poland 2Mathematical Sciences and STAG Research Centre, University of Southampton, Southampton SO17 1BJ, UK Accepted 2018 June 26. Received 2018 June 20; in original form 2018 March 30 ABSTRACT In the standard form of the relativistic heat equation used in astrophysics, information propa- gates instantaneously, rather than being limited by the speed of light as demanded by relativity. We show how this equation none the less follows from a more general, causal theory of heat propagation in which the entropy plays the role of a fluid. In deriving this result, however, we see that it is necessary to make some assumptions which are not universally valid: the dynamical time-scales of the process must be long compared with the explicitly causal physics of the theory, the heat flow must be sufficiently steady, and the space–time static. Generalizing the heat equation (e.g. restoring causality) would thus entail retaining some of the terms we neglected. As a first extension, we derive the heat equation for the space–time associated with a slowly-rotating star or black hole, showing that it only differs from the static result by an additional advection term due to the rotation, and as a consequence demonstrate that a hotspot on a neutron star will be seen to be modulated at the rotation frequency by a distant observer. Key words: accretion, accretion discs – conduction – gravitation – stars:- neutron – stars: rotation. of entropy is important, or in situations where the space–time is 1 INTRODUCTION highly dynamic – like the merger of compact objects. In its full Treating heat propagation in general relativity (GR) is a necessary form it is, however, likely to be too computationally complex for ingredient for the quantitative modelling of compact objects. It be- many practical purposes. comes important during the collapse of a massive star into either a Here we show how the usual form of the relativistic heat equation, neutron star or black hole (Misner & Sharp 1964; Govender, Ma- for a spherical space–time, may be recovered from the multifluid haraj & Maartens 1998; Woosley, Heger & Weaver 2002; Sekiguchi framework in a natural way. Along the way we show that it is 2010). It is needed to understand neutron-star cooling from birth, necessary to make a few assumptions – probably safe ones for through the rapid proto-neutron-star phase (Burrows & Lattimer secular processes in a neutron-star or black hole environment, but 1986) to the slower, secular evolution of mature neutron stars (Van not necessarily in every astrophysical situation. This therefore gives Riper 1991; Aguilera, Pons & Miralles 2008). It is also important a diagnostic of when the familiar relativistic heat equation is not in modelling short-time-scale cooling following outbursts from ac- applicable: if any of these terms is not negligible. Having established creting neutron stars (Cumming et al. 2017), for accretion physics the non-rotating result, we generalize our approach to find the heat around black holes (Yuan & Narayan 2014; Ressler et al. 2015), equation governing a slowly-rotating star, keeping the terms of and in the very late stages of a binary neutron-star inspiral (Shibata, linear order in the rotation rate (which include frame dragging). We Taniguchi & Uryu 2005). drop second-order rotational terms (which cause the space–time to The standard relativistic heat equation is acausal, predicting that deviate from sphericity), but note that these have been explored information propagates instantaneously, and thus violating a basic in the context of neutron-star cooling by Miralles, Van Riper & tenet of relativity that nothing can travel faster than light. It is now Lattimer (1993) and Negreiros, Schramm & Weber (2017). well established that causality can be restored in a natural way, by This paper is aimed at an astrophysics audience, and so is intended treating entropy as a fluid whose dynamics couple to those of the to assume no specialist knowledge of the reader. For this reason, medium (e.g. the various fluid species of a neutron star) (Lopez- we begin with a brief review of the theory of relativistic thermal Monsalvo & Andersson 2011). Relativistic thermal dynamics is dynamics and the foliation of space–time for numerical simulations, naturally expressed in this multifluid framework, and can be applied in order to make our discussion self-contained. We then derive, in in problems on short time-scales where the finite-speed propagation turn, Fourier’s law and the heat equation; in each case beginning with the non-rotating result and then exploring the effect of slow rotation. We conclude with a discussion of the rotational modulation of a neutron-star hotspot seen by a distant observer. E-mail: [email protected] C 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society 4208 S. K. Lander and N. Andersson 2 RELATIVISTIC THERMAL DYNAMICS usual fluid particles with the entropy ‘particles’ (e.g. phonons). The Downloaded from https://academic.oup.com/mnras/article-abstract/479/3/4207/5046729 by Southampton Oceanography Centre National Oceanographic Library user on 13 September 2018 third, and least familiar, interaction is entrainment. This is a non- 2.1 The problem of causality in heat conduction dissipative coupling between two fluid species, which depends on the relative velocity between the two. In normal fluid dynamics, the The heat equation is so familiar that it is easy to forget one concep- momentum and velocity of a fluid are parallel; in the presence of tually unsatisfactory feature of it. Although this paper is concerned entrainment the two can be misaligned, a phenomenon sometimes with heat conduction in GR, let us begin in Newtonian gravity. The known as the Andreev–Bashkin effect (Andreev & Bashkin 1975; heat equation states that a temperature distribution T evolves as Alpar, Langer & Sauls 1984) in the context of mixed Fermi liquids. ∂T 1 Within the multifluid framework, it is certainly permitted to allow =− ∇·Q, (1) ∂t CV for entrainment between the entropy fluid and other species, but the physical reason for doing so is not initially obvious. Indeed, in the where CV is the volumetric heat capacity. The heat flux Q is, in turn, given by Fourier’s law: original incarnation of his multifluid model, Carter dropped these terms for simplicity. It was quickly seen, however, that the resulting Q =−κ∇T, (2) theory suffered from serious instabilities (Olson & Hiscock 1990), casting doubt on the usefulness of the framework until Priou (1991) where κ is the thermal conductivity. Substituting (2) into (1), how- showed that the theory was indeed stable once the entrainment ever, we see that the resulting heat equation is parabolic – and so terms were restored. In fact, the resulting model can be shown to be the characteristic propagation speed is infinite. Even in Newtonian equivalent to the Israel-Stewart theory up to first order in deviations physics, it is unreasonable to expect some physical quantity to be from thermal equilibrium (Lopez-Monsalvo & Andersson 2011). transmitted instantaneously, and efforts to rectify this deficiency Despite its superficially obscure nature, entrainment between the in the heat equation go back decades. A natural reference point is entropy fluid and the matter fluid(s) in a system is also the key the Cattaneo equation, in which causality is introduced – albeit in ingredient which allows one to derive a Cattaneo-type form of a phenomenological manner – through a time-dependent term in Fourier’s law, and thus restore causality to heat conduction within Fourier’s law: the multifluid approach – both in Newtonian gravity and GR (Lopez- ∂ Q t + Q =−κ∇T, (3) Monsalvo 2011). Because it acts over time-scales much shorter than ∂t those of many relativistic astrophysics problems, it might appear to where t is some small positive number, which can be thought of as a be irrelevant for these. Let us sound two notes of caution, though. relaxation time-scale for the medium (Herrera & Santos 1997). Now First, the standard heat equation will not be adequate for mod- plugging this back into equation (1), we have a hyperbolic equation, elling every hot relativistic system – especially not those involving known as the telegrapher’s equation; the problem of instantaneous dynamic space–times or processes with short characteristic time- propagation has been removed, but at the expense of introducing a scales. Secondly, even when it should be valid, there is still a risk term not clearly linked to any underlying microphysics. that the neglect of entrainment could lead to instabilities, as for The relativistic heat equation used for neutron stars and black Carter’s original approach. For these reasons, whilst we will in- hole space–times emerges from the same derivation as in Newto- deed drop entropy entrainment during the course of deriving the nian gravity, but with all quantities redshifted, as they are seen by heat equation, we will keep it for the first steps, to show where the a distant observer (Misner & Sharp 1965; Thorne 1967;VanRiper explicitly causal terms lie. 1991). The causality problem is therefore still present, but is now even more serious; it is fundamentally unacceptable in GR for any information to propagate beyond the speed of light, let alone at in- finite speed. The first successful (and not ad hoc) resolution to the 2.2 The equations of entropy dynamics problem was the Israel–Stewart approach (Israel & Stewart 1979), which posits an expansion of the entropy flux through a set of terms Our starting point will be Andersson et al.
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