799 research outputs found
Dynamics around supermassive black holes
The dynamics of galactic nuclei reflects the presence of supermassive black
holes (SBHs) in many ways. Single SBHs act as sinks, destroying a mass in stars
equal to their own mass in roughly one relaxation time and forcing nuclei to
expand. Formation of binary SBHs displaces a mass in stars roughly equal to the
binary mass, creating low-density cores and ejecting hyper-velocity stars.
Gravitational radiation recoil can eject coalescing binary SBHs from nuclei,
resulting in offset SBHs and lopsided cores. We review recent work on these
mechanisms and discuss the observable consequences.Comment: Invited talk. To appear in "2007 STScI Spring Symposium: Black
Holes", eds. M. Livio & A. M. Koekemoer. (Cambridge University Press, in
press) 26 pages, 12 figure
Hypervelocity stars from star clusters hosting Intermediate-Mass Black Holes
Hypervelocity stars (HVSs) represent a unique population of stars in the
Galaxy reflecting properties of the whole Galactic potential. Determining their
origin is of fundamental importance to constrain the shape and mass of the dark
halo. The leading scenario for the ejection of HVSs is an encounter with the
supermassive black hole in the Galactic Centre. However, new proper motions
from the \textit{Gaia} mission indicate that only the fastest HVSs can be
traced back to the Galactic centre and the remaining stars originate in the
disc or halo. In this paper, we study HVSs generated by encounters of stellar
binaries with an intermediate-mass black hole (IMBH) in the core of a star
cluster. For the first time, we model the effect of the cluster orbit in the
Galactic potential on the observable properties of the ejected population. HVSs
generated by this mechanism do not travel on radial orbits consistent with a
Galactic centre origin, but rather point back to their parent cluster, thus
providing observational evidence for the presence of an IMBH. We also model the
ejection of high-velocity stars from the Galactic population of globular
clusters, assuming that they all contain an IMBH, including the effects of the
cluster's orbit and propagation of the star in the Galactic potential up to
detection. We find that high-velocity stars ejected by IMBHs have distinctive
distributions in velocity, Galactocentric distance and Galactic latitude, which
can be used to distinguish them from runaway stars and stars ejected from the
Galactic Centre.Comment: 15 pages, 16 Figures, 1 Tabl
Is NGC6752 hiding a double black hole binary in its core ?
NGC6752 hosts in its halo PSR J1911-5958A, a newly discovered binary
millisecond pulsar which is the most distant pulsar ever known from the core of
a globular cluster. Interestingly, its recycling history seems in conflict with
a scenario of ejection resulting from ordinary stellar dynamical encounters. A
scattering event off a binary system of two black holes with masses in the
range of 3-50 solar masses that propelled PSR J1911-5958A into its current
peripheral orbit seems more likely. It is still an observational challenge to
unveil the imprint(s) left from such a dark massive binary on cluster's stars:
PSR J1911-5958A may be the first case.Comment: 2 pages, newpasp.sty. To appear in "New Horizons in Globular Cluster
Astronomy", eds. G. Piotto, G. Meylan, G.Djorgovski, M. Riell
Unveiling black holes ejected from globular clusters
Was the black hole in XTE J1118+480 ejected from a globular cluster or kicked
away from the galactic disk?Comment: 2 pages, newpasp.sty. To appear in "New Horizons in Globular Cluster
Astronomy", eds. G. Piotto, G. Meylan, G.Djorgovski, M. Riell
Perturbations induced by a molecular cloud on the young stellar disc in the Galactic Centre
The Galactic centre (GC) is a crowded environment: observations have revealed
the presence of (molecular, atomic and ionized) gas, of a cusp of late-type
stars, and of ~100 early-type stars, about half of which lying in one or
possibly two discs. In this paper, we study the perturbations exerted on a thin
stellar disc (with outer radius ~0.4 pc) by a molecular cloud that falls
towards the GC and is disrupted by the supermassive black hole (SMBH). The
initial conditions for the stellar disc were drawn from the results of previous
simulations of molecular cloud infall and disruption in the SMBH potential. We
find that most of the gas from the disrupted molecular cloud settles into a
dense and irregular disc surrounding the SMBH. If the gas disc and the stellar
disc are slightly misaligned (~5-20 deg), the precession of the stellar orbits
induced by the gas disc significantly increases the inclinations of the stellar
orbits (by a factor of ~3-5 in 1.5 Myr) with respect to the normal vector to
the disc. Furthermore, the distribution of orbit inclinations becomes
significantly broader. These results might be the clue to explain the broad
distribution of observed inclinations of the early-type stars with respect to
the normal vector of the main disc. We discuss the implications for the
possibility that fresh gas was accreted by the GC after the formation of the
disc(s) of early-type stars.Comment: 12 pages, 12 figures, 2 tables, accepted for publication in MNRA
Three-body encounters in the Galactic centre: the origin of the hypervelocity star SDSS J090745.0+024507
Hills (1988) predicted that runaway stars could be accelerated to velocities
larger than 1000 km/s by dynamical encounters with the supermassive black hole
(SMBH) in the Galactic center. The recently discovered hypervelocity star SDSS
J090745.0+024507 (hereafter HVS) is escaping the Galaxy at high speed and could
be the first object in this class. With the measured radial velocity and the
estimated distance to the HVS, we trace back its trajectory in the Galactic
potential. Assuming it was ejected from the center, we find that a 2
mas/yr proper motion is necessary for the star to have come within a few
parsecs of the SMBH. We perform three-body scattering experiments to constrain
the progenitor encounter which accelerated the HVS. As proposed by Yu &
Tremaine (2003), we consider the tidal disruption of binary systems by the SMBH
and the encounter between a star and a binary black hole, as well as an
alternative scenario involving intermediate mass black holes. We find that the
tidal disruption of a stellar binary ejects stars with a larger velocity
compared to the encounter between a single star and a binary black hole, but
has a somewhat smaller ejection rate due to the greater availability of single
stars.Comment: 6 pages, 7 figures, 1 table, accepted for publication in MNRA
Supernovae in the Central Parsec: A Mechanism for Producing Spatially Anisotropic Hypervelocity Stars
Several tens of hyper-velocity stars (HVSs) have been discovered escaping our
Galaxy. These stars share a common origin in the Galactic centre and are
distributed anisotropically in Galactic longitude and latitude. We examine the
possibility that HVSs may be created as the result of supernovae occurring
within binary systems in a disc of stars around Sgr A* over the last 100 Myr.
Monte Carlo simulations show that the rate of binary disruption is ~10^-4
yr^-1, comparable to that of tidal disruption models. The supernova-induced HVS
production rate (\Gamma_HVS) is significantly increased if the binaries are
hardened via migration through a gaseous disc. Moderate hardening gives
\Gamma_HVS ~ 2*10^-7 yr^-1 and an estimated population of ~20 HVSs in the last
100 Myr. Supernova-induced HVS production requires the internal and external
orbital velocity vectors of the secondary binary component to be aligned when
the binary is disrupted. This leaves an imprint of the disc geometry on the
spatial distribution of the HVSs, producing a distinct anisotropy.Comment: 7 pages, 4 figures. Accepted for publication in the Astrophysical
Journa
Eccentric disc instability in stellar discs formed from inspiraling gas clouds in the Galactic Centre
The inspiral of a turbulent molecular cloud in the Galactic Centre may result
in the formation of a small, dense and moderately eccentric gas disc around the
supermassive black hole (SMBH). Such a disc is unstable to fragmentation and
may lead to the formation of young massive stars in the central parsec of the
Galaxy. Here we perform high-accuracy direct summation N-body simulations of a
ring of massive stars (with initial semi-major axes 0.1 < a/pc < 0.4 and
eccentricities 0.2 < e < 0.4), subject to the potential of the SMBH, a stellar
cusp, and the parent gas disc, to study how the orbital elements of the ring
evolve in time. The initial conditions for the stellar ring are drawn from the
results of previous simulations of molecular cloud infall and disruption in the
SMBH potential. While semi-major axes do not evolve significantly, the
distribution of eccentricities spreads out very fast (~1 Myr) as a consequence
of cusp precession. In particular, stellar orbits with initial eccentricity
e>0.3 (e<0.3) tend to become even more (less) eccentric, resulting in a bimodal
eccentricity distribution. The distribution is qualitatively consistent with
that of the massive stars observed in the Galactic Centre's clockwise disc.Comment: 7 pages, 8 figures, accepted for publication in MNRA
A Semi-Analytic dynamical friction model that reproduces core stalling
We present a new semi-analytic model for dynamical friction based on
Chandrasekhar's formalism. The key novelty is the introduction of physically
motivated, radially varying, maximum and minimum impact parameters. With these,
our model gives an excellent match to full N-body simulations for isotropic
background density distributions, both cuspy and shallow, without any
fine-tuning of the model parameters. In particular, we are able to reproduce
the dramatic core-stalling effect that occurs in shallow/constant density
cores, for the first time. This gives us new physical insight into the
core-stalling phenomenon. We show that core stalling occurs in the limit in
which the product of the Coulomb logarithm and the local fraction of stars with
velocity lower than the infalling body tends to zero. For cuspy backgrounds,
this occurs when the infalling mass approaches the enclosed background mass.
For cored backgrounds, it occurs at larger distances from the centre, due to a
combination of a rapidly increasing minimum impact parameter and a lack of slow
moving stars in the core. This demonstrates that the physics of core-stalling
is likely the same for both massive infalling objects and low-mass objects
moving in shallow density backgrounds. We implement our prescription for
dynamical friction in the direct summation code NBODY6 as an analytic
correction for stars that remain within the Roche volume of the infalling
object. This approach is computationally efficient, since only stars in the
inspiralling system need to be evolved with direct summation. Our method can be
applied to study a variety of astrophysical systems, including young star
clusters orbiting near the Galactic Centre; globular clusters moving within the
Galaxy; and dwarf galaxies orbiting within dark matter halos.Comment: 16 pages, 21 figures, Accepted for publication in MNRA
A semi-analytic dynamical friction model for cored galaxies
We present a dynamical friction model based on Chandrasekhar's formula that
reproduces the fast inspiral and stalling experienced by satellites orbiting
galaxies with a large constant density core. We show that the fast inspiral
phase does not owe to resonance. Rather, it owes to the background velocity
distribution function for the constant density cores being dissimilar from the
usually-assumed Maxwellian distribution. Using the correct background velocity
distribution function and the semi-analytic model from Petts et al. (2015), we
are able to correctly reproduce the infall rate in both cored and cusped
potentials. However, in the case of large cores, our model is no longer able to
correctly capture core-stalling. We show that this stalling owes to the tidal
radius of the satellite approaching the size of the core. By switching off
dynamical friction when rt(r) = r (where rt is the tidal radius at the
satellite's position) we arrive at a model which reproduces the N-body results
remarkably well. Since the tidal radius can be very large for constant density
background distributions, our model recovers the result that stalling can occur
for Ms/Menc << 1, where Ms and Menc are the mass of the satellite and the
enclosed galaxy mass, respectively. Finally, we include the contribution to
dynamical friction that comes from stars moving faster than the satellite. This
next-to-leading order effect becomes the dominant driver of inspiral near the
core region, prior to stalling.Comment: 13 pages, 12 figures, resubmitted to MNRAS after responding to
feedback from the refere
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