233 research outputs found
On the distribution of stellar remnants around massive black holes: slow mass segregation, star cluster inspirals and correlated orbits
We study the long term dynamical evolution of stellar mass black holes (BHs)
at the Galactic center (GC) and put constraints on their number and central
mass distribution. Models of the GC are considered that have not yet achieved a
steady state under the influence of random gravitational encounters. Contrary
to some recent claims that mass-segregation can rapidly rebuild a density cusp
in the stars, we find that time scales associated with cusp regrowth are longer
than the Hubble time. These results cast doubts on standard models that
postulate high densities of BHs near the GC and motivate studies that start
from initial conditions which correspond to well-defined physical models. For
the first time, we consider the distribution of BHs in a dissipationless
formation model for the Milky Way nuclear cluster (NC), in which massive
stellar clusters merge in the GC to form a nucleus. We simulate the successive
inspiral of massive clusters containing an inner dense cluster of BHs. The
pre-existing mass segregation is not completely erased as the clusters are
disrupted by the massive black hole tidal field. As a result, after 12 inspiral
events a NC forms in which the BHs have higher central densities than the
stars. After evolving the model for 5-10 Gyr, the BHs do form a steep central
cusp, while the stellar distribution maintains properties that resemble those
of the Milky Way NC. Finally, we investigate the effect of BH perturbations on
the motion of the GC S-stars, as a means of constraining the number of the
perturbers. We find that reproducing the S-star orbital distribution requires
>~1000 BHs within 0.1 pc of Sgr A*. A dissipationless formation scenario for
the Milky Way NC is consistent with this lower limit and therefore could
reconcile the need for high central densities of BHs (to explain the orbits of
the S-stars), with the missing-cusp problem of the GC giant star population.Comment: 23 pages, 21 Figures. Accepted for publication in Ap
Dynamical friction and the evolution of Supermassive Black hole Binaries: the final hundred-parsec problem
The supermassive black holes originally in the nuclei of two merging galaxies
will form a binary in the remnant core. The early evolution of the massive
binary is driven by dynamical friction before the binary becomes "hard" and
eventually reaches coalescence through gravitational wave emission. { We
consider the dynamical friction evolution of massive binaries consisting of a
secondary hole orbiting inside a stellar cusp dominated by a more massive
central black hole.} In our treatment we include the frictional force from
stars moving faster than the inspiralling object which is neglected in the
standard Chandrasekhar's treatment. We show that the binary eccentricity
increases if the stellar cusp density profile rises less steeply than
. In cusps shallower than the
frictional timescale can become very long due to the deficit of stars moving
slower than the massive body. Although including the fast stars increases the
decay rate, low mass-ratio binaries () in sufficiently
massive galaxies have decay timescales longer than one Hubble time. During such
minor mergers the secondary hole stalls on an eccentric orbit at a distance of
order one tenth the influence radius of the primary hole (i.e., for massive ellipticals). We calculate the expected number of
stalled satellites as a function of the host galaxy mass, and show that the
brightest cluster galaxies should have of such satellites orbiting
within their cores. Our results could provide an explanation to a number of
observations, which include multiple nuclei in core ellipticals, off-center
AGNs and eccentric nuclear disks.Comment: 18 pages, 13 Figures. Accepted for publication in Ap
Massive binary star mergers in galactic nuclei: implications for blue stragglers, binary S-stars and gravitational waves
Galactic nuclei are often found to contain young stellar populations and, in
most cases, a central supermassive black hole (SMBH). Most known massive stars
are found in binaries or higher-multiplicity systems, and in a galactic nucleus
the gravitational interaction with the SMBH can affect their long-term
evolution. In this paper, we study the orbital evolution of stellar binaries
near SMBHs using high precision -body simulations, and including tidal
forces and Post-Newtonian corrections to the motion. We focus on the
Lidov-Kozai (LK) effect induced by the SMBH on massive star binaries. We
investigate how the properties of the merging binaries change with varying the
SMBH mass, the slope of the initial mass function, the distributions of the
binary orbital parameters and the efficiency in energy dissipation in
dissipative tides. We find that the fraction of merging massive binary stars is
in the range -- regardless of the details of the initial
distributions of masses and orbital elements. For a Milky Way-like nucleus, we
find a typical rate of binary mergers
yr. The merger products of massive binaries can be rejuvenated
blue-straggler stars, more massive than each of their original progenitors, and
G2-like objects. Binary systems that survive the LK cycles can be source of
X-rays and gravitational waves, observable with present and upcoming
instruments.Comment: 13 pages, 7 figures, 1 table, accepted by MNRA
The final-parsec problem in the collisionless limit
A binary supermassive black hole loses energy via ejection of stars in a
galactic nucleus, until emission of gravitational waves becomes strong enough
to induce rapid coalescence. Evolution via the gravitational slingshot requires
that stars be continuously supplied to the binary, and it is known that in
spherical galaxies the reservoir of such stars is quickly depleted, leading to
stalling of the binary at parsec-scale separations. Recent N-body simulations
of galaxy mergers and isolated nonspherical galaxies suggest that this stalling
may not occur in less idealized systems. However, it remains unclear to what
degree these conclusions are affected by collisional relaxation, which is much
stronger in the numerical simulations than in real galaxies. In this study, we
present a novel Monte Carlo method that can efficiently deal with both
collisional and collisionless dynamics, and with galaxy models having arbitrary
shapes. We show that without relaxation, the final-parsec problem may be
overcome only in triaxial galaxies. Axisymmetry is not enough, but even a
moderate departure from axisymmetry is sufficient to keep the binary shrinking.
We find that the binary hardening rate is always substantially lower than the
maximum possible, "full-loss-cone" rate, and that it decreases with time, but
that stellar-dynamical interactions are nevertheless able to drive the binary
to coalescence on a timescale <=1 Gyr in any triaxial galaxy.Comment: 17 pages, 10 figures; matches published versio
Dynamical processes near the super massive black hole at the galactic center
Observations of the stellar environment near the Galactic center provide the strongest empirical evidence for the existence of massive black holes in the Universe. Theoretical models of the Milky Way nuclear star cluster fail to explain numerous properties of such environment, including the presence of very young stars close to the super massive black hole (SMBH) and the more recent discovery of a parsec-scale core in the central distribution of the bright late-type (old) stars. In this thesis we present a theoretical study of dynamical processes near the Galactic center, strongly related to these issues. Using different numerical techniques we explore the close environment of a SMBH as catalyst for stellar collisions and mergers. We study binary stars that remain bound for several revolutions around the SMBH, finding that in the case of highly inclined binaries the Kozai resonance can lead to large periodic oscillations in the internal binary eccentricity and inclination. Collisions and mergers of the binary elements are found to increase significantly for multiple orbits around the SMBH. In collisions involving a low-mass and a high-mass star, the merger product acquires a high core hydrogen abundance from the smaller star, effectively resetting the nuclear evolution clock to a younger age. This process could serve as an important source of young stars at the Galactic center. We then show that a core in the old stars can be naturally explained in a scenario in which the Milky Way nuclear star cluster (NSC) is formed via repeated inspiral of globular clusters into the Galactic center. We present results from a set of N-body simulations of this process, which show that the fundamental properties of the NSC, including its mass, outer density profile and velocity structure, are also reproduced. Chandrasekhar’s dynamical friction formula predicts no frictional force on a test body in a low-density core, regardless of its density, due to the absence of stars moving more slowly than the local circular velocity. We have tested this prediction using large-scale N-body experiments. The rate of orbital decay never drops precisely to zero, because stars moving faster than the test body also contribute to the frictional force. When the contribution from the fast-moving stars is included in the expression for the dynamical friction force, and the changes induced by the massive body on the stellar distribution are taken into account, Chandrasekhar’s theory is found to reproduce the rate of orbital decay remarkably well. However, this rate is still substantially smaller than the rate predicted by Chandrasekhar’s formula in its most widely-used forms, implying longer time scales for inspiral. Motivated by recent observations that suggest a parsec-scale core around the Galactic center SMBH, we investigated the evolution of a population of stellar-mass black holes (BHs) as they spiral in to the center of the Galaxy. After ! 10 Gyr, we find that the density of BHs can remain substantially less than the density in stars at all radii; we conclude that it would be unjustified to assume that the spatial distribution of BHs at the Galactic center is well described by steady-state models
Dynamical constraints on the origin of hot and warm Jupiters with close friends
Gas giants orbiting their host star within the ice line are thought to have
migrated to their current locations from farther out. Here we consider the
origin and dynamical evolution of observed Jupiters, focusing on hot and warm
Jupiters with outer friends. We show that the majority of the observed Jupiter
pairs (twenty out of twenty-four) will be dynamically unstable if the inner
planet was placed at >~1AU distance from the stellar host. This finding is at
odds with formation theories that invoke the migration of such planets from
semi-major axes >~1AU due to secular dynamical processes (e.g., secular chaos,
Lidov-Kozai oscillations) coupled with tidal dissipation. In fact, the results
of N-body integrations show that the evolution of dynamically unstable systems
does not lead to tidal migration but rather to planet ejections and collisions
with the host star. This and other arguments lead us to suggest that most of
the observed planets with a companion could not have been transported from
further out through secular migration processes. More generally, by using a
combination of numerical and analytic techniques we show that the high-e
Lidov-Kozai migration scenario can only account for less than 10% of all gas
giants observed between 0.1-1 AU. Simulations of multi-planet systems support
this result. Our study indicates that rather than starting on highly eccentric
orbits with orbital periods above one year, these "warm" Jupiters are more
likely to have reached the region where they are observed today without having
experienced significant tidal dissipation.Comment: Accepted to AAS journals (AJ). 15 pages, 9 figure
An Instability in Triaxial Stellar Systems
The radial-orbit instability is a collective phenomenon that has heretofore
only been observed in spherical systems. We find that this instability occurs
also in triaxial systems, as we checked by performing extensive N-body
simulations whose initial conditions were obtained by sampling a
self-consistent triaxial model of a cuspy galaxy composed of luminous and dark
matter. N-body simulations show a time evolution of the galaxy that is not due
to the development of chaotic motions but, rather, to the collective
instability induced by an excess of box-like orbits. The instability quickly
transforms such models into a more prolate configuration, with 0.64<b/a<0.77
and 0.6<c/a<0.7 for the dark halo and 0.64<b/a<0.77 and 0.59<c/a<0.67 for the
luminous matter. Stable triaxial, cuspy galaxies with dark matter halos are
obtained when the contribution of radially-biased orbits to the solution is
reduced. These results constitute the first evidence of the radial-orbit
instability in triaxial galaxy models.Comment: 4 pages, to appear in "Galactic and Stellar Dynamics in the Era of
Large-Scale Surveys," ed. C. Boil
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