75 research outputs found
On the number of young stellar discs in the Galactic Centre
Observations of the Galactic Centre show evidence of disc-like structures of
very young stars orbiting the central super-massive black hole within a
distance of a few 0.1 pc. While it is widely accepted that about half of the
stars form a relatively flat disc rotating clockwise on the sky, there is a
substantial ongoing debate on whether there is a second, counter-clockwise disc
of stars.
By means of N-body simulations using our bhint code, we show that two highly
inclined stellar discs with the observed properties cannot be recognised as two
flat circular discs after 5 Myr of mutual interaction. Instead, our
calculations predict a significant warping of the two discs, which we show to
be apparent among the structures observed in the Galactic Centre. While the
high eccentricities of the observed counter-clockwise orbits suggest an
eccentric origin of this system, we show the eccentricity distribution in the
inner part of the more massive clockwise disc to be perfectly consistent with
an initially circular disc in which stellar eccentricities increase due to both
non-resonant and resonant relaxation.
We conclude that the relevant question to ask is therefore not whether there
are two discs of young stars, but whether there were two such discs to begin
with.Comment: 6 pages, 7 figures, accepted for publication in MNRAS Letter
Constraining the initial mass function of stars in the Galactic Centre
(abridged) Here we discuss the question whether the extreme circumstances in
the centre of the Milky Way may be the reason for a significant variation of
the IMF. By means of stellar evolution models using different codes we show
that the observed luminosity in the central parsec is too high to be explained
by a long-standing top-heavy IMF, considering the limited amount of mass
inferred from stellar kinematics in this region. In contrast, continuous star
formation over the Galaxy's lifetime following a canonical IMF results in a
mass-to-light ratio and a total mass of stellar black holes (SBHs) consistent
with the observations. Furthermore, these SBHs migrate towards the centre due
to dynamical friction, turning the cusp of visible stars into a core as
observed in the Galactic Centre. For the first time here we explain the
luminosity and dynamical mass of the central cluster and both the presence and
extent of the observed core, since the number of SBHs expected from a canonical
IMF is just enough to make up for the missing luminous mass. We conclude that
the Galactic Centre is consistent with the canonical IMF and do not suggest a
systematic variation as a result of the region's properties such as high
density, metallicity, strong tidal field etc.Comment: MNRAS, accepted, 8 pages, 4 figure
Influence of a stellar cusp on the dynamics of young stellar discs and the origin of the S-stars in the Galactic Centre
Observations of the Galactic Centre show evidence of one or two disc-like
structures of very young stars orbiting the central super-massive black hole
within a distance of a few 0.1 pc. A number of analyses have been carried out
to investigate the dynamical behaviour and consequences of these discs,
including disc thickness and eccentricity growth as well as mutual interaction
and warping. However, most of these studies have neglected the influence of the
stellar cusp surrounding the black hole, which is believed to be 1-2 orders of
magnitude more massive than the disc(s).
By means of N-body integrations using our bhint code, we study the impact of
stellar cusps of different compositions. We find that although the presence of
a cusp does have an important effect on the evolution of an otherwise isolated
flat disc, its influence on the evolution of disc thickness and warping is
rather mild in a two-disc configuration. However, we show that the creation of
highly eccentric orbits strongly depends on the graininess of the cusp (i.e.
the mean and maximum stellar masses): While Chang (2009) recently found that
full cycles of Kozai resonance are prevented by the presence of an analytic
cusp, we show that relaxation processes play an important role in such highly
dense regions and support short-term resonances. We thus find that young disc
stars on initially circular orbits can achieve high eccentricities by resonant
effects also in the presence of a cusp of stellar remnants, yielding a
mechanism to create S-stars and hyper-velocity stars.
Furthermore, we discuss the underlying initial mass function (IMF) of the
young stellar discs and find no definite evidence for a non-canonical IMF.Comment: 10 pages, 7 figures, 1 table, accepted for publication in MNRA
The origin of S-stars and a young stellar disk: distribution of debris stars of a sinking star cluster
Within the distance of 1 pc from the Galactic center (GC), more than 100
young massive stars have been found. The massive stars at 0.1-1 pc from the GC
are located in one or two disks, while those within 0.1 pc from the GC,
S-stars, have an isotropic distribution. How these stars are formed is not well
understood, especially for S-stars. Here we propose that a young star cluster
with an intermediate-mass black hole (IMBH) can form both the disks and
S-stars. We performed a fully self-consistent -body simulation of a star
cluster near the GC. Stars escaped from the tidally disrupted star cluster were
carried to the GC due to an 1:1 mean motion resonance with the IMBH formed in
the cluster. In the final phase of the evolution, the eccentricity of the IMBH
becomes very high. In this phase, stars carried by the 1:1 resonance with the
IMBH were dropped from the resonance and their orbits are randomized by a
chaotic Kozai mechanism. The mass function of these carried stars is extremely
top-heavy within 10". The surface density distributions of young massive stars
has a slope of -1.5 within 10" from the GC. The distribution of stars in the
most central region is isotropic. These characteristics agree well with those
of stars observed within 10" from the GC.Comment: 10 pages, 5 figures, accepted for ApJ
Resonant relaxation and the warp of the stellar disc in the Galactic centre
Observations of the spatial distribution and kinematics of young stars in the
Galactic centre can be interpreted as showing that the stars occupy one, or
possibly two, discs of radii ~0.05-0.5 pc. The most prominent (`clockwise')
disc exhibits a strong warp: the normals to the mean orbital planes in the
inner and outer third of the disc differ by ~60 deg. Using an analytical model
based on Laplace-Lagrange theory, we show that such warps arise naturally and
inevitably through vector resonant relaxation between the disc and the
surrounding old stellar cluster.Comment: 24 pages, 8 figures, accepted by MNRA
Secular Dynamical Anti-Friction in Galactic Nuclei
We identify a gravitational-dynamical process in near-Keplerian potentials of
galactic nuclei that occurs when an intermediate-mass black hole (IMBH) is
migrating on an eccentric orbit through the stellar cluster towards the central
supermassive black hole (SMBH). We find that, apart from conventional dynamical
friction, the IMBH experiences an often much stronger systematic torque due to
the secular (i.e., orbit-averaged) interactions with the cluster's stars. The
force which results in this torque is applied, counterintuitively, in the same
direction as the IMBH's precession and we refer to its action as
"secular-dynamical anti-friction" (SDAF). We argue that SDAF, and not the
gravitational ejection of stars, is responsible for the IMBH's eccentricity
increase seen in the initial stages of previous N-body simulations. Our
numerical experiments, supported by qualitative arguments, demonstrate that (1)
when the IMBH's precession direction is artificially reversed, the torque
changes sign as well, which decreases the orbital eccentricity, (2) the rate of
eccentricity growth is sensitive to the IMBH migration rate, with zero
systematic eccentricity growth for an IMBH whose orbit is artificially
prevented from inward migration, and (3) SDAF is the strongest when the central
star cluster is rapidly rotating. This leads to eccentricity growth/decrease
for the clusters rotating in the opposite/same direction relative to the IMBH's
orbital motion.Comment: 8 pages, 7 figures, Published in Ap
The coupling of a young stellar disc with the molecular torus in the Galactic centre
The Galactic centre hosts, according to observations, a number of early-type
stars. About one half of those which are orbiting the central supermassive
black hole on orbits with projected radii 0.03 pc form a coherently
rotating disc. Observations further reveal a massive gaseous torus and a
significant population of late-type stars. In this paper, we investigate, by
means of numerical N-body computations, the orbital evolution of the stellar
disc, which we consider to be initially thin. We include the gravitational
influence of both the torus and the late-type stars, as well as the
self-gravity of the disc. Our results show that, for a significant set of
system parameters, the evolution of the disc leads, within the lifetime of the
early-type stars, to a configuration compatible with the observations. In
particular, the disc naturally reaches a specific - perpendicular - orientation
with respect to the torus, which is indeed the configuration observed in the
Galactic centre. We, therefore, suggest that all the early-type stars may have
been born within a single gaseous disc.Comment: Accepted for publication in MNRAS; 9 pages, 4 figures, 1 tabl
Dynamical evolution of the young stars in the Galactic center: N-body simulations of the S-stars
We use N-body simulations to study the evolution of the orbital
eccentricities of stars deposited near (<0.05 pc) the Milky Way massive black
hole (MBH), starting from initial conditions motivated by two competing models
for their origin: formation in a disk followed by inward migration; and
exchange interactions involving a binary star. The first model predicts modest
eccentricities, lower than those observed in the S-star cluster, while the
second model predicts higher eccentricities than observed. The N-body
simulations include a dense cluster of 10 M_sun stellar black holes (SBHs),
expected to accumulate near the MBH by mass segregation. Perturbations from the
SBHs tend to randomize the stellar orbits, partially erasing the dynamical
signatures of their origin. The eccentricities of the initially highly
eccentric stars evolve, in 20 Myr (the S-star lifespan), to a distribution that
is consistent at the ~95 % level with the observed eccentricity distribution.
In contrast, the eccentricities of the initially more circular orbits fail to
evolve to the observed values in 20 Myr, arguing against the disk migration
scenario. We find that 20 % - 30 % of the S-stars are tidally disrupted by the
MBH over their lifetimes, and that the S-stars are not likely to be ejected as
hypervelocity stars outside the central 0.05 pc by close encounters with
stellar black holes.Comment: 6 pages, 2 figures. Minor corrections, Sumitted to Ap
The Anisotropic Spatial Distribution of Hypervelocity Stars
We study the distribution of angular positions and angular separations of
unbound hypervelocity stars (HVSs). HVSs are spatially anisotropic at the
3-sigma level. The spatial anisotropy is significant in Galactic longitude, not
in latitude, and the inclusion of lower velocity, possibly bound HVSs reduces
the significance of the anisotropy. We discuss how the observed distribution of
HVSs may be linked to their origin. In the future, measuring the distribution
of HVSs in the southern sky will provide additional constraints on the spatial
anisotropy and the origin of HVSs.Comment: 4 pages, accepted to ApJ Letter
The Effectiveness of the Kozai Mechanism in the Galactic Centre
I examine the effectiveness of Kozai oscillations in the centres of galaxies
and in particular the Galactic centre using standard techniques from celestial
mechanics. In particular, I study the effects of a stellar bulge potential and
general relativity on Kozai oscillations, which are induced by stellar discs.
Lockmann et al (2008) recently suggested that Kozai oscillations induced by the
two young massive stellar discs in the Galactic centre drives the orbits of the
young stars to large eccentricity (). If some of these young
eccentric stars are in binaries, they would be disrupted near pericentre,
leaving one star in a tight orbit around the central SMBH and producing the
S-star population. I find that the {\it spherical} stellar bulge suppresses
Kozai oscillations, when its enclosed mass inside of a test body is of order
the mass in the stellar disc(s). Since the stellar bulge in the Galactic centre
is much larger than the stellar discs, Kozai oscillations {\it due to the
stellar discs} are likely suppressed. Whether Kozai oscillations are induced
from other nonspherical components to the potential (for instance, a flattened
stellar bulge) is yet to be determined.Comment: 6 pages, 3 figures, accepted to MNRA
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