Multipole Structure of Compact Objects

Physical Sciences and Technology Vol. 3 (No. 1), 2016: 42-49 UDC 53.02 Multipole structure of compact objects Quevedo H. Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, AP 70543, México, DF 04510, Mexico Dipartimento di Fisica and ICRA, Università di Roma "La Sapienza", I-00185 Roma, Italy e-mail: [email protected] We analyze the applications of general relativity in relativistic astrophysics in order to solve the problem of describing the geometric and physical properties of the interior and exterior gravitational and electromagnetic fields of compact objects. We focus on the interpretation of exact solutions of Einstein's equations in terms of their multipole moments structure. In view of the lack of physical interior solutions, we propose an alternative approach in which higher multipoles should be taken into account. Key words: Exact solutions of Einstein equations, compact objects, multipole moments PACS numbers: 04.20.Jb, 95.30.Sf 1 Introduction considered to contain collectively planet-like objects, white dwarfs, neutron stars, other exotic dense stars, Einstein's equations [1] and black holes. The problem of describing the gravitational field of compact objects can be split 1 into two related problems, namely, the exterior and R Rg= T , (1) 2 the interior field, each of them represented by a i e particular metric g and g , respectively. The relate the geometric structure of spacetime, which is surface of the compact object represents the 1 hypersurface at which the interior and the exterior given by the Einstein tensor G= R Rg , 2 fields must coincide. The exterior field corresponds with the matter content of spacetime, which is given to a vacuum spacetime Tμv = 0, for which Einstein's by the energy-momentum tensor Tμv. Moreover, the equations reduce to equivalence principle allows us to establish a R =0, (2) relationship between the geometric structure of spacetime and the gravitational field. This implies whereas for the interior field it is necessary to choose that the curvature of spacetime, interpreted as a particular energy-momentum tensor that would four-dimensional differential Riemannian manifold, take into account all the physical properties of the can be considered as a measure of the gravitational internal structure of the compact object. A interaction. For instance, if the spacetime is flat, Rμv particularly simple choice is the perfect-fluid = 0, the Einstein tensor vanishes, leading to the energy-momentum tensor [2] condition that the matter content of the spacetime vanishes too. The opposite is not true. If Tμv = 0, the T =( p ) u u pg , (3) Ricci tensor Rμv and the curvature scalar R vanish as well, but the curvature tensor Rμv can be different where ρ and p are the density and the pressure of the from zero. fluid, respectively, and uμ is the 4-velocity. The case Einstein's general relativity is thus a theory of the of fluids with anisotropic pressures is also used in the gravitational interaction. As such, it should be able to literature. describe all physical situations in which the Most compact objects, however, are gravitational field is involved. Consider, for instance, characterized by the presence of internal and external the case of astrophysical compact objects, i.e., electromagnetic fields. This implies that in general objects that are small for their mass. In general, the we should consider the Einstein-Maxwell theory. In class of astrophysical compact objects is often this case, the exterior field should be described by an 42 Quevedo H. Phys. Sci. Technol., Vol. 3 (No. 1), 2016: 42-49 exterior metric which satisfies the electrovacuum perfect-fluid interior solutions of Einstein and equations Einstein-Maxwell equations. We propose an alternative approach that implies the inclusion of the 1 quadrupole as an additional degree of freedom. This R = FF g FF , F; =0, (4) could contribute to solve the general set of 4 Einstein-Maxwell equations. Section 5 contains the conclusions. We use throughout the paper geometric where Fμv is the Faraday electromagnetic tensor. On units with G = c = 1. the surface of the compact object, the exterior metric must be matched with the interior metric which 2 Line element satisfies the general equations It is very difficult to find physically relevant 1 solutions of Einstein's equations. To simplify the R Rg = [( p ) u u pg ] 2 resulting system of differential equations, one , (5) 1 usually assumes that certain conditions are satisfied FF g FF 4 which follow from the physical properties of the system under consideration. In the case of compact objects, two assumptions are made, namely, F ; =0. (6) stationarity and axial symmetry. The first condition means that the field does not depend explicitly on the In this work, we present a brief review of the time coordinate, say t. This is in accordance with our main exact solutions of Einstein-Maxwell equations experience since the shape and rotation of isolated which can be used to describe the exterior and compact objects have not been observed to change interior field of astrophysical compact objects. We over long periods of time. The second condition is interpret the solutions in terms of their multipole also based on observations. Indeed, all compact solutions. To this end, we use the Geroch-Hansen [5, objects observed in Nature are characterized by a 6] procedure which provides a relativistic and particular rotation with respect to an axis located coordinate-invariant definition of multipole inside the object. The rotation axis determines a moments. According to this definition, any privileged direction with respect to which compact stationary, vacuum, asymptotically flat solution of objects are usually symmetric. It is therefore Eistein's equations can be uniquely characterized by physically meaningful to assume that the two sets of multipoles Mn and Jn (n = 0.1, ...), the first gravitational field and, consequently, the spacetime of which represents the field generated by the mass metric are stationary and axially symmetric. Under distribution whereas the second one is due to the these conditions, one can show that the line element rotation of the distribution. In the case of reduces to the Weyl-Papapetrou-Lewis form [2] electrovacuum fields, two more sets En and Hn must be included, representing the electric and magnetic ds22=( f dt d ) multipoles of the electromagnetic field, respectively. 12 2 2 2 2, (7) The concrete calculations necessary to find the f e() d dz d explicit expressions of the multipole moments for particular solutions are not easy to be carried out. where f, ω, μ, and γ are functions which depend on Some auxiliary procedures can be used which are the spatial coordinates ρ and z only. Notice that the based on particular representations of the solutions. metric does not depend on the coordinates t and φ as Here, we will use the Ernst representation and the a consequence of the stationarity and axial symmetry concrete formulas derived in [7, 8]. assumptions, respectively. This paper is organized as follows. First, in Sec. The corresponding field equations are, in general, 2, we present the general line element that can be a highly non-linear and complicated system of used to investigate the structure of the exterior and second-order partial differential equations. Several interior field equations. In Sec. 3, we present the Kerr representations for the field equations are known in metric with its multipole structure and its the literature [2, 3]. The particular form of the line generalizations which include the electric charge and element (7) is convenient for the analysis of the field higher multipole moments. In Sec. 4, we mention the equations structure. For instance, in the limiting case main problems associated with the search for of static vacuum field (ω = 0), the function f turns out 43 Multipole structure of compact objects Phys. Sci. Technol., Vol. 3 (No. 1), 2016: 42-49 22222 to satisfy a second-order linear differential equation, 22(asin )2 a sin ( ra ) whereas all the non-linearities of Einstein's equations ds = dt dtd ,(9) are contained in the metric function γ. In the case of 2 22 2 2 ()ra asin 2 2 22 electrovacuum spacetimes, the function μ turns out to sin d dr d satisfy a harmonic differential equation so that it can be chosen as μ = ρ to simplify the form of the remaining field equations. In the general case of where interior perfect-fluid metrics with electromagnetic field, the functions f, ω and μ satisfy a system of three =r2 2 mr a 2 ,= r 22 a cos2 . (10) coupled, non-linear, second-order, partial differential equations and the function γ is determined by a set of The parameter α stands for the angular two first-order differential equations which can be momentum per unit mass, J/m, as measured by a integrated by quadratures. distant observer. This can also be seen at the level of As we will see below, for the representation of the multipole moments which in this case can be particular exact solutions, however, the expressed as1 cylindrical-like coordinates of the line element (7) are not very convenient. In fact, the physical M= ( 1)kk ma2 , significance of particular solutions can be understood 2k MJ=0, =0, (11) more easily by using spherical-like coordinates. 21kk 2 Also, the motion of test particles in the gravitational kk21 J = ( 1) ma . field of compact objects have been analyzed mostly 21k in spherical-like coordinates [4]. Notice that the odd mass multipoles and the even An additional condition must be imposed in order angular-momentum multipoles vanish identically as for a particular solution to describe the exterior field a result of the additional symmetry of the Kerr of compact objects, namely, asymptotic flatness.

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