1,172 research outputs found
Synchronization Properties of Network Motifs
We address the problem of understanding the variable abundance of 3-node and
4-node subgraphs (motifs) in complex networks from a dynamical point of view.
As a criterion in the determination of the functional significance of a n-node
subgraph, we propose an analytic method to measure the stability of the
synchronous state (SSS) the subgraph displays. We show that, for undirected
graphs, the SSS is correlated with the relative abundance, while in directed
graphs the correlation exists only for some specific motifs.Comment: 7 pages, 3 figure
Self-regulated gravitational accretion in protostellar discs
We present a numerical model for the evolution of a protostellar disc that
has formed self-consistently from the collapse of a molecular cloud core. The
global evolution of the disc is followed for several million years after its
formation. The capture of a wide range of spatial and temporal scales is made
possible by use of the thin-disc approximation. We focus on the role of
gravitational torques in transporting mass inward and angular momentum outward
during different evolutionary phases of a protostellar disc with disc-to-star
mass ratio of order 0.1. In the early phase, when the infall of matter from the
surrounding envelope is substantial, mass is transported inward by the
gravitational torques from spiral arms that are a manifestation of the
envelope-induced gravitational instability in the disc. In the late phase, when
the gas reservoir of the envelope is depleted, the distinct spiral structure is
replaced by ongoing irregular nonaxisymmetric density perturbations. The
amplitude of these density perturbations decreases with time, though this
process is moderated by swing amplification aided by the existence of the
disc's sharp outer edge. Our global modelling of the protostellar disc reveals
that there is typically a residual nonzero gravitational torque from these
density perturbations, i.e. their effects do not exactly cancel out in each
region. In particular, the net gravitational torque in the inner disc tends to
be negative during first several million years of the evolution, while the
outer disc has a net positive gravitational torque. Our global model of a
self-consistently formed disc shows that it is also self-regulated in the late
phase, so that it is near the Toomre stability limit, with a near-uniform
Toomre parameter Q\approx 1.5-2.0. (Abstract abridged).Comment: 9 pages, 9 figures, accepted for publication in MNRA
The Infrared Afterglow of Supermassive Black Hole Mergers
We model the spectra and light curves of circumbinary accretion disks during
the time after the central black holes merge. The most immediate effect of this
merger is the dissipation of energy in the outer regions of the disk due to the
gravitational wave energy and linear momentum flux released at merger. This has
the effect of perturbing the gas in the disk, which then radiates the
dissipated energy over a cooling timescale, giving a characteristic infrared
signal for tens of thousands of years when the total black hole mass is M~10^8
M_sun. On the basis of a simple cosmological merger model in which a typical
supermassive black hole undergoes a few major mergers during its lifetime, we
predict that ~10^4-10^5 of these IR sources should be observable today and
discuss the possibility of identifying them with multi-wavelength surveys such
as SWIRE/XMM-LSS/XBootes and COSMOS.Comment: v2: expanded discussion of optical depth calculations; ApJ in pres
Secular evolution of viscous and self-gravitating circumstellar discs
We add the effect of turbulent viscosity via the \alpha-prescription to
models of the self-consistent formation and evolution of protostellar discs.
Our models are non-axisymmetric and carried out using the thin-disc
approximation. Self-gravity plays an important role in the early evolution of a
disc, and the later evolution is determined by the relative importance of
gravitational and viscous torques. In the absence of viscous torques, a
protostellar disc evolves into a self-regulated state with disk-averaged Toomre
parameter Q \sim 1.5-2.0, non-axisymmetric structure diminishing with time, and
maximum disc-to-star mass ratio \xi = 0.14. We estimate an effective viscosity
parameter \alpha_eff associated with gravitational torques at the inner
boundary of our simulation to be in the range 10^{-4}-10^{-3} during the late
evolution. Addition of viscous torques with a low value \alpha = 10^{-4} has
little effect on the evolution, structure, and accretion properties of the
disc, and the self-regulated state is largely preserved. A sequence of
increasing values of \alpha results in the discs becoming more axisymmetric in
structure, being more gravitationally stable, having greater accretion rates,
larger sizes, shorter lifetimes, and lower disc-to-star mass ratios. For
\alpha=10^{-2}, the model is viscous-dominated and the self-regulated state
largely disappears by late times. (Abridged)Comment: 13 pages, 11 figures, accepted for publication in MNRA
Obscuring and feeding supermassive black holes with evolving nuclear star clusters
Recently, high resolution observations with the help of the near-infrared
adaptive optics integral field spectrograph SINFONI at the VLT proved the
existence of massive and young nuclear star clusters in the centres of a sample
of Seyfert galaxies. With the help of high resolution hydrodynamical
simulations with the PLUTO-code, we follow the evolution of such clusters,
especially focusing on mass and energy feedback from young stars. This leads to
a filamentary inflow of gas on large scales (tens of parsec), whereas a
turbulent and very dense disc builds up on the parsec scale. Here, we
concentrate on the long-term evolution of the nuclear disc in NGC 1068 with the
help of an effective viscous disc model, using the mass input from the large
scale simulations and accounting for star formation in the disc. This two-stage
modelling enables us to connect the tens of parsec scale region (observable
with SINFONI) with the parsec scale environment (MIDI observations). At the
current age of the nuclear star cluster, our simulations predict disc sizes of
the order of 0.8 to 0.9 pc, gas masses of 1.0e6 Msun and mass transfer rates
through the inner boundary of 0.025 Msun/yr in good agreement with values
derived from observations.Comment: 6 pages, 3 figures, to appear in the proceedings of the IAU General
Assembly 2009, Rio de Janeiro, S267 Co-evolution of Central Black Holes and
Galaxie
ALMA Observations of the Young Substellar Binary System 2M1207
We present ALMA observations of the 2M1207 system, a young binary made of a
brown dwarf with a planetary-mass companion at a projected separation of about
40 au. We detect emission from dust continuum at 0.89 mm and from the rotational transition of CO from a very compact disk around the young brown
dwarf. The small radius found for this brown dwarf disk may be due to
truncation from the tidal interaction with the planetary-mass companion. Under
the assumption of optically thin dust emission, we estimated a dust mass of 0.1
for the 2M1207A disk, and a 3 upper limit of for dust surrounding 2M1207b, which is the tightest upper
limit obtained so far for the mass of dust particles surrounding a young
planetary-mass companion. We discuss the impact of this and other
non-detections of young planetary-mass companions for models of planet
formation, which predict the presence of circum-planetary material surrounding
these objects.Comment: 10 pages, 6 figures, accepted for publication in A
Misaligned snowplough effect and the electromagnetic counterpart to black hole binary mergers
We estimate the accretion rates produced when a circumprimary gas disc is pushed into the primary supermassive black hole (SMBH) by the tidal force of the decaying secondary during a SMBH merger. Using the 3D Smoothed Particle Hydrodynamics (SPH) code PHANTOM, we extend previous investigations of co-planar discs to the case where the disc and binary orbital planes are misaligned. We consider a geometrically thin disc with inclination angles varying from 1 18 to 180 18 and a binary with mass ratio q = 10 123. We find that discs with small inclination angles (<10 18) produce an increase in luminosity exceeding the Eddington rate. By contrast, discs with inclinations between 20 18 and 30 18 show a less pronounced rise in the accretion rate, whilst discs inclined by 180 18 show no peak in the mass accretion rate. While previous analytic work predicted that the effective tidal torque drops with increasing inclination angle, we show that the misaligned snowplough effect remains important even for angles larger than the disc aspect ratio. The rise in the accretion rate produced by discs inclined at small angles to the binary orbit can produce an electromagnetic counterpart to the gravitational wave signal emitted from final stages of the binary orbital decay
Planetesimal formation via fragmentation in self-gravitating protoplanetary discs
An unsolved issue in the standard core accretion model for gaseous planet formation is how kilometre-sized planetesimals form from, initially, micron-sized dust grains. Solid growth beyond metre sizes can be difficult both because the sticking efficiency becomes very small, and because these particles should rapidly migrate into the central star. We consider here how metre-sized particles evolve in self-gravitating accretion discs using simulations in which the gravitational influence of the solid particles is also included. Metre-sized particles become strongly concentrated in the spiral structures present in the disc and, if the solid to gas density ratio is sufficiently high, can fragment due to their own self-gravity to form planetesimals directly. This result suggests that planetesimal formation may occur very early in the star formation process while discs are still massive enough to be self-gravitating. The dependence of this process on the surface density of the solids is also consistent with the observation that extrasolar planets are preferentially found around high metallicity stars
Dynamical and spectral properties of complex networks
Dynamical properties of complex networks are related to the spectral
properties of the Laplacian matrix that describes the pattern of connectivity
of the network. In particular we compute the synchronization time for different
types of networks and different dynamics. We show that the main dependence of
the synchronization time is on the smallest nonzero eigenvalue of the Laplacian
matrix, in contrast to other proposals in terms of the spectrum of the
adjacency matrix. Then, this topological property becomes the most relevant for
the dynamics.Comment: 14 pages, 5 figures, to be published in New Journal of Physic
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