Tidal disruption of a star by a supermassive black hole Page: 2 of 5
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TIDAL DISRUPTION OF A STAR BY A
SUPERMASSTVE BLACK HOLE
Pablo Laguna. Warnei A. Miller and Wojciech H. Zurek
Theoretical Division, Los Alamos National Laboratory
Los Alamos. New Mexico 37545, USA
The analysis of stars in galactic nuclei that are captured and tidally dis-
rupted by a black hole of mass > lO*.Vf0 requires the inclusion of general rela-
tivistic effects. We present the first numerical study of tidal breakup of a 1 V/r
main sequence star by a 107.Vf3 black hole. We use a smoothed particle code
to solve the hydrodynamic equations for a relativistic fluid in a static curved
spacetime geometry to analyze, among other things, the fraction of the debris
captured by the hole and the velocity of fragments escaping the hole.
Stellar disruption by tidal forces of supermassive black holes is one of the
most important unsolved problems in the astrophysics of active galactic nuclei
(AGN').1 Studies in the 1970s suggested that tidal distortion and disruption of
stars passing near massive black holes could perhaps h; an essential mechanism
for fueling AGN.2 3 However, it was shown2 4 that tidal disruption alone could
not achieve the observed luminosity because it requires such a high concentra-
tion of stars that collisions among the stars would dominate the process. When
stellar disruption occurs, part of the debris ir accreted by the black hole, result-
ing in a burst of activity, ihe remaining material may vrell be ejected at speeds
much larger than the mean velocity of the stellar system. Stellar disruption
could also then have important observational ramifications since its signature
may provide direct evidence for the presence of supermassive black holes.5
When a star gets close enough to a black hole, it can no longer be treated
as a point mass particle. There is a characteristic radius within which the
star becomes subject to tidal distortions and disruptions. This tidal radius
(Rt) is defined as the distance where the surface gravity of the star equals
the tidal acceleration.2'* For a l.V/0 star, the tidal radius is given by (R,/R,) ss
0 5h3(where R, = 2MH is the black hole’s Schwarzschild radius
with .W* its mass, and the parameter r> is the strength of the tidal encounter. For
\ft, > 10*.Vf,T>, R, < 37R,. Encounters of stars in this regime ire close enough to
the hole that Newtonian approximations are inadequate. Black holes with >
10*.V? have R, < R,. The star gets swallowed before being disrupted. Therefore,
there is a window (10* - 10*.W,3) where the analysis of tidal stellar disruption by
black holes requires a general relativistic numerical simulation. Our goal is then
to provide eno:igh features of stellar disruptions by supermtissive black holes
in support of a possible observation. To this end, we are analyzing in detail
what happens to the gas after the star has been disrupted, ru.mely, amount of
material ejected and accreted, and the duration of the mass-accretion phase.
We have implemented a general relativistic smoothed particle hydrody-
namic (SPH) code.7 The code uses a discretized Lagrangian version of the hy-
drodynamic equations for a relativistic fluid in a static curved spacetime geom-
etry ( Schwarzschild or Kerr metric) using SPH methods.* We have developed
an algorithm that transforms relativistic hydrodynarnical contact interactions
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Laguna, P.; Miller, W.A. & Zurek, W.H. Tidal disruption of a star by a supermassive black hole, article, January 1, 1991; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1096816/m1/2/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.