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Attraction and Rejection: On the love–hate relationship between stars and black holes
Stockholm University, Faculty of Science, Department of Astronomy. Oskar Klein Centre; Isaac Newton Group of Telescopes; Nordic Optical Telescope.ORCID iD: 0000-0003-0781-6638
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Solitary stars wandering too close to the supermassive black hole at the centre of their galaxy may become tidally disrupted, if the tidal forces due to the black hole overcome the self-gravity holding the star together. Depending on the strength of the encounter, the star may be partially disrupted, resulting in a surviving stellar core and two tidal arms, or may be completely disrupted, resulting in a long and thin tidal stream expected to fall back and circularize into an accretion disc (the two cases are illustrated on the cover of this thesis).

While some aspects of a tidal disruption can be described analytically with reasonable accuracy, such an event is the highly non-linear outcome of the interplay between the stellar hydrodynamics and self-gravity, tidal accelerations from the black hole, radiation, potentially magnetic fields and, in extreme cases, nuclear reactions. In the vicinity of the black hole, general relativistic effects become important in determining both the fate of the star and the subsequent evolution of the debris stream.

In this thesis we present a new approach for studying the relativistic regime of tidal disruptions. It combines an exact relativistic description of the hydrodynamical evolution of a test fluid in a fixed curved spacetime with a Newtonian treatment of the fluid's self-gravity. The method, though trivial to incorporate into existing Newtonian codes, yields very accurate results at minimal additional computational expense.

Equipped with this new tool, we set out to systematically explore the parameter space of tidal disruptions, focusing on the effects of the impact parameter (describing the strength of the disruption) and of the black hole spin on the morphology and energetics of the resulting debris stream. We also study the effects of general relativity on partial disruptions, in order to determine the range of impact parameters at which partial disruptions occur for various black hole masses, and the effects of general relativity on the velocity kick imparted to the surviving core. Finally, we simulate the first part of a tidal disruption with our code and then use the resulting debris distribution as input for a grid-based, general relativistic magnetohydrodynamics code, with which we follow the formation and evolution of the resulting accretion disc.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University , 2019. , p. 80
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-167197ISBN: 978-91-7797-582-3 (print)ISBN: 978-91-7797-583-0 (electronic)OAI: oai:DiVA.org:su-167197DiVA, id: diva2:1340196
Public defence
2019-09-18, sal FA31, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2019-08-26 Created: 2019-08-02 Last updated: 2019-08-19Bibliographically approved
List of papers
1. Tidal disruptions by rotating black holes: effects of spin and impact parameter
Open this publication in new window or tab >>Tidal disruptions by rotating black holes: effects of spin and impact parameter
2019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 487, no 4, p. 4790-4808Article in journal (Refereed) Published
Abstract [en]

We present the results of relativistic smoothed particle hydrodynamics simulations of tidal disruptions of stars by rotating supermassive black holes, for a wide range of impact parameters and black hole spins. For deep encounters, we find that: relativistic precession creates debris geometries impossible to obtain with the Newtonian equations; part of the fluid can be launched on plunging orbits, reducing the fallback rate and the mass of the resulting accretion disc; multiple squeezings and bounces at periapsis may generate distinctive X-ray signatures resulting from the associated shock breakout; disruptions can occur inside the marginally bound radius, if the angular momentum spread launches part of the debris on non-plunging orbits. Perhaps surprisingly, we also find relativistic effects important in partial disruptions, where the balance between self-gravity and tidal forces is so precarious that otherwise minor relativistic effects can have decisive consequences on the stellar fate. In between, where the star is fully disrupted but relativistic effects are mild, the difference resides in a gentler rise of the fallback rate, a later and smaller peak, and longer return times. However, relativistic precession always causes thicker debris streams, both in the bound part (speeding up circularization) and in the unbound part (accelerating and enhancing the production of separate transients). We discuss various properties of the disruption (compression at periapsis, shape and spread of the energy distribution) and potential observables (peak fallback rate, times of rise and decay, duration of super-Eddington fallback) as a function of the impact parameter and the black hole spin.

Keywords
black hole physics, hydrodynamics, relativistic processes, methods: numerical, galaxies: nuclei
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-167196 (URN)10.1093/mnras/stz1530 (DOI)000475888500025 ()
Available from: 2019-03-22 Created: 2019-03-22 Last updated: 2019-08-27Bibliographically approved
2. Tidal disruptions by rotating black holes: relativistic hydrodynamics with Newtonian codes
Open this publication in new window or tab >>Tidal disruptions by rotating black holes: relativistic hydrodynamics with Newtonian codes
2017 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 469, no 4, p. 4483-4503Article in journal (Refereed) Published
Abstract [en]

We propose an approximate approach for studying the relativistic regime of stellar tidal disruptions by rotating massive black holes. It combines an exact relativistic description of the hydrodynamical evolution of a test fluid in a fixed curved space-time with a Newtonian treatment of the fluid's self-gravity. Explicit expressions for the equations of motion are derived for Kerr space-time using two different coordinate systems. We implement the new methodology within an existing Newtonian smoothed particle hydrodynamics code and show that including the additional physics involves very little extra computational cost. We carefully explore the validity of the novel approach by first testing its ability to recover geodesic motion, and then by comparing the outcome of tidal disruption simulations against previous relativistic studies. We further compare simulations in Boyer-Lindquist and Kerr-Schild coordinates and conclude that our approach allows accurate simulation even of tidal disruption events where the star penetrates deeply inside the tidal radius of a rotating black hole. Finally, we use the new method to study the effect of the black hole spin on the morphology and fallback rate of the debris streams resulting from tidal disruptions, finding that while the spin has little effect on the fallback rate, it does imprint heavily on the stream morphology, and can even be a determining factor in the survival or disruption of the star itself. Our methodology is discussed in detail as a reference for future astrophysical applications.

Keywords
accretion, accretion discs, black hole physics, relativistic processes, methods: numerical, galaxies: nuclei
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-147132 (URN)10.1093/mnras/stx1089 (DOI)000406837900052 ()
Available from: 2017-09-29 Created: 2017-09-29 Last updated: 2019-08-19Bibliographically approved
3. Magnetohydrodynamical simulations of a deep tidal disruption in general relativity
Open this publication in new window or tab >>Magnetohydrodynamical simulations of a deep tidal disruption in general relativity
Show others...
2016 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 458, no 4, p. 4250-4268Article in journal (Refereed) Published
Abstract [en]

We perform hydro- and magnetohydrodynamical general-relativistic simulations of a tidal disruption of a 0.1 M-circle dot red dwarf approaching a 10(5) M-circle dot non-rotating massive black hole on a close (impact parameter beta = 10) elliptical (eccentricity e = 0.97) orbit. We track the debris self-interaction, circularization and the accompanying accretion through the black hole horizon. We find that the relativistic precession leads to the formation of a self-crossing shock. The dissipated kinetic energy heats up the incoming debris and efficiently generates a quasi-spherical outflow. The self-interaction is modulated because of the feedback exerted by the flow on itself. The debris quickly forms a thick, almost marginally bound disc that remains turbulent for many orbital periods. Initially, the accretion through the black hole horizon results from the self-interaction, while in the later stages it is dominated by the debris originally ejected in the shocked region, as it gradually falls back towards the hole. The effective viscosity in the debris disc stems from the original hydrodynamical turbulence, which dominates over the magnetic component. The radiative efficiency is very low because of low energetics of the gas crossing the horizon and large optical depth that results in photon trapping. Although the parameters of the simulated tidal disruption are probably not representative of most observed events, it is possible to extrapolate some of its properties towards more common configurations.

Keywords
accretion, accretion discs, black hole physics, relativistic processes, methods: numerical
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-132058 (URN)10.1093/mnras/stw589 (DOI)000375799500068 ()
Available from: 2016-07-14 Created: 2016-07-06 Last updated: 2019-08-19Bibliographically approved
4. Relativistic effects on tidal disruption kicks of solitary stars
Open this publication in new window or tab >>Relativistic effects on tidal disruption kicks of solitary stars
Show others...
2015 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 449, no 1, p. 771-780Article in journal (Refereed) Published
Abstract [en]

Solitary stars that wander too close to their galactic centres can become tidally disrupted, if the tidal forces due to the supermassive black hole residing there overcome the self-gravity of the star. If the star is only partially disrupted, so that a fraction survives as a self-bound object, this remaining core will experience a net gain in specific orbital energy, which translates into a velocity 'kick' of up to similar to 10(3) km s(-1). In this paper, we present the result of smoothed particle hydrodynamics simulations of such partial disruptions, and analyse the velocity kick imparted on the surviving core. We compare gamma = 5/3 and gamma = 4/3 polytropes disrupted in both a Newtonian potential, and a generalized potential that reproduces most relativistic effects around a Schwarzschild black hole either exactly or to excellent precision. For the Newtonian case, we confirm the results of previous studies that the kick velocity of the surviving core is virtually independent of the ratio of the black hole to stellar mass, and is a function of the impact parameter beta alone, reaching at most the escape velocity of the original star. For a given beta, relativistic effects become increasingly important for larger black holemasses. In particular, we find that the kick velocity increases with the black hole mass, making larger kicks more common than in the Newtonian case, as low-beta encounters are statistically more likely than high-beta encounters. The analysis of the tidal tensor for the generalized potential shows that our results are robust lower limits on the true relativistic kick velocities, and are generally in very good agreement with the exact results.

Keywords
black hole physics, hydrodynamics, relativistic processes, methods: numerical, galaxies: nuclei
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-118995 (URN)10.1093/mnras/stv350 (DOI)000355345600056 ()
Available from: 2015-07-28 Created: 2015-07-24 Last updated: 2019-08-19Bibliographically approved

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