Neutron Star Cooling: I
This chapter presents a basic, but detailed, introduction to the physical and astro-physical issues involved in the study of the thermal evolution of isolated neutron stars. Results of numerical calculations,1 for both minimal and enhanced cooling scenari
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Neutron Star Cooling: I Dany Page
11.1 Introduction This chapter presents a basic, but detailed, introduction to the physical and astrophysical issues involved in the study of the thermal evolution of isolated neutron stars. Results of numerical calculations,1 for both minimal and enhanced cooling scenarios, are presented and compared with observational data. The first conjectures about the possible existence of stellar neutron cores by Landau [37] and Baade and Zwicky [6] and the pioneering work of Oppenheimer and Volkoff [48] pointed to very mysterious, exotic, small and dense objects. Forty years after the actual discovery of neutron stars [28] these early thoughts have been fully confirmed: neutron stars are demonstrably very small and dense, they very probably enclose some exotic form(s) of matter, and they are still mysterious.2 A theorist view of the interior of a neutron star is depicted in Fig. 11.1: the central region, marked as “?”, is the mysterious part and it is the main goal of the study of neutron star cooling, necessarily complemented with the study of many other facets of neutron star phenomenology, to elucidate it. However, any information about this central part which we can glean by observing the surface is conditioned by our understanding and correct modeling of the outer parts of the star. The core, where neutrons and protons form a homogeneous quantum liquid, is distinguished from the crust, where the nucleons cluster and matter is hence inhomogeneous at D. Page Departamento de Astrof´ısica Te´orica, Instituto de Astronom´ıa, Universidad Nacional Aut´onoma de M´exico, M´exico D.F 04510, M´exico e-mail: [email protected] 1
A 1D (i.e., assuming spherical symmetry) cooling code, NSCool, with which most calculations presented in this chapter were performed, is available at http://www.astroscu.unam.mx/ neutrones/NS-Cooler/. 2 The existence of pulsars with periods around 1.5 ms implies, by causality, that they have radii smaller than 75 km and, if they are bound by gravity, that their average density is, at least, of the order of 1014 g cm−3 . W. Becker (ed.), Neutron Stars and Pulsars, Astrophysics and Space Science Library 357, c Springer-Verlag Berlin Heidelberg 2009
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Sw ch iss ee se La sa g Sp na ag he tt i
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B Crust:
Core:
nuclei + neutron superfluid
homogeneous matter
Atmosphere Envelope Crust Outer core Inner core
C
A Neutron superfluid Neutron superfluid + proton superconductor Neutron vortex Neutron vortex Nuclei in a lattice Magnetic flux tube
Fig. 11.1 A pictorial vision of the inside of a neutron star (drawing by the author, from [58])
the microscopic level. This crust–core separation is currently estimated [44] to be located at a density ρcc 1.6 × 1014 g cm−3 , i.e., about 60% of nuclear matter density, ρnuc 2.8 × 1014 g cm−3 . (ρnuc refers to symmetric nuclear matter, i.e., made of 50% neutrons and 50% protons, at zero pressure and is deduced from the central density of heavy nuclei, while at ρcc in a neutron star, pressure is non zero and matte
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