Thin Disks As Sources of Stationary Axisymmetric Electrovacuum
Total Page:16
File Type:pdf, Size:1020Kb
Thin disks as sources of stationary axisymmetric electrovacuum spacetimes TOMAS LEDVINKA Department of Theoretical Physics Faculty of Mathematics and Physics Charles University V Holesovickach Praha Czech Republic A disertation submitted to the Faculty of Mathematics and Physics Charles University in accordance with the regulations for admission to the degree of Do ctor of Physics Branch of the do ctoral study F Theoretical physics astronomy and astrophysics Praha Octob er Acknowledgements I would like to thank professor Jir Bicak for his help He prop osed an inter esting problem encouraged me in my work and he has always had the time to discuss my work with me and has made valuable comments on the content of the thesis Contents Intro duction Relativistic disks as sources of the Kerr metric Thin layers as sources of the stationary axisymmetric spacetimes Total mass angular momentum and charge Energy conditions and the interpretation of the surface stressenergy tensor of the disk Disks formed of counterrotating surface streams of charged particles KerrNewman disks Schwarzschild disks Extreme Reissner Nordstrom disks Kerr disks Charged KerrNewman disks TomimatsuSato spacetimes Concluding remarks App endix A Radial prop erties of Kerr disks App endix B Radial prop erties of KerrNewman disks App endix C Radial prop erties of TomimatsuSato disks Intro duction Unlike in Newtonian gravity there is a lack of complete exact solutions repre senting selfgravitating rotating compact ob jects in general relativity due to its essential nonlinearity On the other hand a great progress has b een made within a class of the stationary axisymmetric vacuum or electrovacuum asymptotically at spacetimes where p owerful metho ds were applied to obtain analytic solutions of Einsteins equations Among these solutions the most imp ortant role still plays the spacetime discovered by Kerr One of the reasons consists in the fact that compact astrophysical ob jects very often p osses large sp ecic angular momentum and a Kerr black hole is thus a natural candidate for the spinning remnant of gravitational collapse Indeed assuming that the the cosmic cen sorship conjecture holds the blackhole uniqueness theorems guarantee that the Kerr metric represents the unique solution describing all rotating vacuum black holes cf eg If one takes into account that the prop erties of the interior Schwarzschild solution describing a static spherically symmetric star surrounded by its gravitational eld are even now sixty years after its discovery inspiring astrophysicists in a deep er understanding of the p eculiarities of general relativity there is no doubt that an exact interior Kerr solution would b e even more exciting This of course has b een realized by many workers As the prop erties of Kerrs spinning particle metric were analyzed the search for such solution started eg Since that time several authors have b een able to construct interior solutions that can b e matched with the Kerr vacuum geometry as an external eld but the material of various balls or shells has very strange prop erties and allows no reasonable interpretation For example quite recent work that app eared in gives a toroidal source consisting of a toroidal shell a disk and an annulus of matter interior to the torus The masses of the disk and annulus are negative The review on the Sources for the Kerr Metric written in contains references and concludes with Destructive statements denying the existence of a material source for the Kerr metric should b e rejected until if ever they are reasonably justied To summarize in Hermann Bondis way the sources suggested so far for the Kerr metric are not the easiest materials to buy in the shops In this situation less complicated sources should b e studied the most natu ral choice seems to b e an axisymmetric thin disk Even among such sources the most realistic one the thin disk of a nite diameter without a radial pressure was excluded from a list of p ossible sources of external parts of Kerr spacetime The situation is somewhat dierent in the sp ecial case of the extreme Kerr metric where there is a denite relationship b etween mass and angular momentum The numerical study of uniformly rotating disks indicated how the extreme Kerr geometry forms around the disk in the ultrarelativistic limit These numerical results were supp orted recently by the imp ortant analytical work If a re quirement of the nite diameter is relaxed it can b e shown that such disks can play a role of the interior Kerr solution with any angular momentum see also In this thesis b oth our rst results concerning Kerr disks and their gener alization to stationary axisymmetric electrovacuum spacetimes are presented The section is based on letter published in The basic idea which enables one to construct physically acceptable thin disks as well as completely unreasonable shells is then presented in section in greater detail Because we are interested in the interpretation of the stressenergy tensor of the disk using the usual matter we study only disks without radial pressure Even though the radius of these disks is innite they have nite integral quantities such as mass or angular momentum which means that their resp ective densities as dened in section have reasonable asymptotic b ehavior In the central region of the disk the situation is quite complicated and the prop erties of the stressenergy tensor must b e carefully studied using covariant characteristics the energy conditions which are intro duced in section It is shown that for a general axisymmetric stationary electrovacuum asymptotically at spacetime which admits WeylPapap etrou co ordinates the reasonable disk like source may exist and that the surface stressenergy tensor of such a disk may consist of two counterrotating streams of charged electro geo desic particles at each radius if certain conditions studied in section are met Although in last years several galaxies were found to have the disks built from two nearly identical counterrotationg stellar comp onents see eg the presence of counter streaming comp onent apparently makes such disks not so realictic astrophysically Wee shall see however that the Kerr disks exist with the mass of the corotating comp onent exceeding of the total mass of the disk It is clear that physically realistic disks can b e constructed only within certain limits of the values of total charge angular momentum eective radius of the disk and other parameters which enter into the metric The survey of the pa rameter space of the KerrNewman disks is presented in section The analogous survey of TomimatsuSato disk sources is given in section In these two sections the prop erties of disks generating considered spacetimes are studied with an emphasis on p ossible interpretations of the surface stressenergy tensor of the disk and on various relativistic phenomena such as the existence of ergoregions or of closed timelike curves in the vicinity of the disks In app endices A B and C the plots of relevant disk quantities as functions of the circumferential radius are given Relativistic disks as sources of the Kerr metric This section is based on letter with several notes app ended Bicak LyndenBell and Katz BLK in the following have shown that most vacuum static Weyl solutions can arise as the metrics of counterrotating relativistic disks see also for other references on relativistic disks The simple idea which inspired the work of BLK is commonly used in Newtonian galactic dynamics the idea rst app eared in Kuzmins work Imagine a p oint mass M placed at a distance b b elow the centre of a plane z see Figure This gives a solution of Laplaces equation ab ove the plane Then consider the p otential obtained by reecting this z p otential in z so that a symmetrical solution b oth ab ove and b elow the plane is obtained It is continuous but has a discontinuous normal derivative on z the jump in which gives a p ositive surface density on the plane In galactic dynamics one considers general line distributions of mass along the negative z axis and employing the metho d describ ed ab ove one nds the p otentialdensity pairs for general axially symmetric disks In fact Bicak LyndenBell and Pichon found an innite numb er of new static solutions of Einsteins equations starting from realistic p otentials used to describ e at galaxies as given recently by Evans and de Zeeuw z M b b M Figure Gravitational eld of a Kuzmin disk illustrated using lines of force Here we wish to demonstrate that a similar metho d works also for axisymmet ric reection symmetric stationary spacetimes It is imp ortant to realize that although now no metric function solves Laplaces equation as in the static case we may view the pro cedure describ ed ab ove as the identication of the hyp er surface z b with z b The eld then remains continuous but the jump of its normal derivative induces a matter distribution in the disk which arises due to the identication What remains to b e seen is whether the material can b e b ought in the shops This idea can b e employed for all known asymptotically at stationary vacuum spacetimes eg for the TomimatsuSato solutions for the rotating Curzon solution or for other metrics cf for references Here we shall illustrate the pro cedure for the simplest but most interesting case the Kerr metric A stationary axisymmetric