152 research outputs found
Stellar dynamical evidence against a cold disc origin for stars in the Galactic Centre
Observations of massive stars within the central parsec of the Galaxy show
that, while most stars orbit within a well-defined disc, a significant fraction
have large eccentricities and / or inclinations with respect to the disc plane.
Here, we investigate whether this dynamically hot component could have arisen
via scattering from an initially cold disc -- the expected initial condition if
the stars formed from the fragmentation of an accretion disc. Using N-body
methods, we evolve a variety of flat, cold, stellar systems, and study the
effects of initial disc eccentricity, primordial binaries, very massive stars
and intermediate mass black holes. We find, consistent with previous results,
that a circular disc does not become eccentric enough unless there is a
significant population of undetected 100--1000 Msun objects. However, since
fragmentation of an eccentric disc can readily yield eccentric stellar orbits,
the strongest constraints come from inclinations. We show that_none_ of our
initial conditions yield the observed large inclinations, regardless of the
initial disc eccentricity or the presence of massive objects. These results
imply that the orbits of the young massive stars in the Galactic Centre are
largely primordial, and that the stars are unlikely to have formed as a
dynamically cold disc.Comment: 5 pages, 6 colour figures. MNRAS Letters in press. (v2: very minor
changes
On the origin of kinematic distribution of the sub-parsec young stars in the Galactic center
Within a half-parsec from the Galactic center (GC), there is a population of
coeval young stars which appear to reside in a coherent disk. Surrounding this
dynamically-cool stellar system, there is a population of stars with a similar
age and much larger eccentricities and inclinations relative to the disk. We
propose a hypothesis for the origin of this dynamical dichotomy. Without
specifying any specific mechanism, we consider the possibility that both
stellar populations were formed within a disk some 6 Myr ago. But this orderly
structure was dynamically perturbed outside-in by an intruding object with a
mass ~10^4 Msun, which may be an intermediate-mass black hole (IMBH) or a dark
stellar cluster hosting an IMBH. We suggest that the perturber migrated inward
to ~0.15-0.3pc from the GC under the action of dynamical friction. Along the
way, it captured many stars in the outer disk region into its mean-motion
resonance, forced them to migrate with it, closely encountered with them, and
induced the growth of their eccentricity and inclination. But stars in the
inner regions of the disk retain their initial coplanar structure. We predict
that some of the inclined and eccentric stars surrounding the disk may have
similar Galactocentric semimajor axis. Future precision determination of their
kinematic distribution of these stars will not only provide a test for this
hypothesis but also evidences for the presence of an IMBH or a dark cluster at
the immediate proximity of the massive black hole at the GC. (abridged)Comment: 14 pages, including 13 figures, typo corrected, reference added, ApJ
in pres
Self-gravitating fragmentation of eccentric accretion disks
We consider the effects of eccentricity on the fragmentation of
gravitationally unstable accretion disks, using numerical hydrodynamics. We
find that eccentricity does not affect the overall stability of the disk
against fragmentation, but significantly alters the manner in which such
fragments accrete gas. Variable tidal forces around an eccentric orbit slow the
accretion process, and suppress the formation of weakly-bound clumps. The
"stellar" mass function resulting from the fragmentation of an eccentric disk
is found to have a significantly higher characteristic mass than that from a
corresponding circular disk. We discuss our results in terms of the disk(s) of
massive stars at ~0.1pc from the Galactic Center, and find that the
fragmentation of an eccentric accretion disk, due to gravitational instability,
is a viable mechanism for the formation of these systems.Comment: 9 pages, 7 figures. Accepted for publication in Ap
Binary formation and mass function variations in fragmenting discs with short cooling times
Accretion discs at sub-pc distances around supermassive black holes are
likely to cool rapidly enough that self-gravity results in fragmentation. Here,
we use high-resolution hydrodynamic simulations of a simplified disc model to
study how the outcome of fragmentation depends upon numerical resolution and
cooling time, and to investigate the incidence of binary formation within
fragmenting discs. We investigate a range of cooling times, from the relatively
long cooling time-scales that are marginally unstable to fragmentation down to
highly unstable cooling on a time-scale that is shorter than the local
dynamical time. The characteristic mass of fragments decreases with reduced
cooling time, though the effect is modest and dependent upon details of how
rapidly bound clumps radiate. We observe a high incidence of capture binaries,
though we are unable to determine their final orbits or probability of
survival. The results suggest that faster cooling in the parent disc results in
an increased binary fraction, and that a high primordial binary fraction may
result from disc fragmentation. We discuss our results in terms of the young
massive stars close to the Galactic Centre, and suggest that observations of
some stellar binaries close to the Galactic Centre remain consistent with
formation in a fragmenting accretion disc.Comment: 10 pages, 5 figures. Accepted for publication in MNRAS. Figures 1 and
3 degraded to meet arXiv size limits - version with high resolution figures
available at http://www.strw.leidenuniv.nl/~rda/publications.htm
Constraints on the Stellar Mass Function from Stellar Dynamics at the Galactic Center
We consider the dynamical evolution of a disk of stars orbiting a central
black hole. In particular, we focus on the effect of the stellar mass function
on the evolution of the disk, using both analytic arguments and numerical
simulations. We apply our model to the ring of massive stars at ~0.1pc from the
Galactic Center, assuming that the stars formed in a cold, circular disk, and
find that our model requires the presence of a significant population of
massive (>100Msun) stars in order to explain the the observed eccentricities of
0.2-0.3. Moreover, in order to limit the damping of the heavier stars'
eccentricities, we also require fewer low-mass stars than expected from a
Salpeter mass function, giving strong evidence for a significantly
``top-heavy'' mass function in the rings of stars seen near to the Galactic
Center. We also note that the maximum possible eccentricities attainable from
circular initial conditions at ages of <10Myr are around 0.4-0.5, and suggest
that any rings of stars found with higher eccentricities were probably not
formed from circular disks.Comment: 9 pages, 5 figures. Accepted for publication in Ap
Expanding Relativistic Shells and Gamma-Ray Burst Temporal Structure
Many models of cosmological gamma-ray bursts involve the sudden release of
erg which produce shells which expand at relativistic speeds
(Lorentz factors of ). We investigate the kinematic limits
on the source size due to the observed time structure in three types of bursts:
short spikes, FREDs (Fast Rise, Exponentail decay), and long complex bursts.
The emitting shell keeps up with the photons it produces reducing apparent
durations by so that source sizes can be very large (c\Delta T\Delta T\Gamma^{-1}$, we show that the
curvature of the shell within that angle creates delays comparable to those
associated with the duration of the event. As a result, most bursts should be
like FREDs with sharp rises related to how long the shell emits and power law
decays related to how long the shell expanded before becoming gamma-ray active.
Few bursts have the long decay phases required for large shells resulting in
unacceptable high densities for ISM objects to cause the observed subpeaks. To
be consistent with the observations, perhaps very thick shells (which act as
parallel slabs) are required to avoid the effects of the curvature, or the
duration is dictated by a central engine.Comment: Tex file, 30 pages, 7 Postscript figures, in press ApJ, Vol 47
Flaring Activity of Sgr A* at 43 and 22 GHz: Evidence for Expanding Hot Plasma
We have carried out Very Large Array (VLA) continuum observations to study
the variability of Sgr A* at 43 GHz (=7mm) and 22 GHz
(=13mm). A low level of flare activity has been detected with a
duration of 2 hours at these frequencies, showing the peak flare
emission at 43 GHz leading the 22 GHz peak flare by to 40 minutes. The
overall characteristics of the flare emission are interpreted in terms of the
plasmon model of Van der Laan (1966) by considering the ejection and
adiabatically expansion of a uniform, spherical plasma blob due to flare
activity. The observed peak of the flare emission with a spectral index
of =1.6 is consistent with the prediction that the peak
emission shifts toward lower frequencies in an adiabatically-expanding
self-absorbed source. We present the expected synchrotron light curves for an
expanding blob as well as the peak frequency emission as a function of the
energy spectral index constrained by the available flaring measurements in
near-IR, sub-millimeter, millimeter and radio wavelengths. We note that the
blob model is consistent with the available measurements, however, we can not
rule out the jet of Sgr A*. If expanding material leaves the gravitational
potential of Sgr A*, the total mass-loss rate of nonthermal and thermal
particles is estimated to be M yr. We
discuss the implication of the mass-loss rate since this value matches closely
with the estimated accretion rate based on polarization measurements.Comment: Revised with new Figures 1 and 2, 17 pages, 4 figures, ApJ (in press
Soft X-ray components in the hard state of accreting black holes
Recent observations of two black hole candidates (GX 339-4 and J1753.5-0127)
in the low-hard state (L_X/L_Edd ~ 0.003-0.05) suggest the presence of a cool
accretion disk very close to the innermost stable orbit of the black hole. This
runs counter to models of the low-hard state in which the cool disk is
truncated at a much larger radius. We study the interaction between a
moderately truncated disk and a hot inner flow. Ion-bombardment heats the
surface of the disk in the overlap region between a two-temperature
advection-dominated accretion flow and standard accretion disk, producing a hot
(kT_e ~70 keV) layer on the surface of the cool disk. The hard X-ray flux from
this layer heats the inner parts of the underlying cool disk, producing a soft
X-ray excess. Together with interstellar absorption these effects mimic the
thermal spectrum from a disk extending to the last stable orbit. The results
show that soft excesses in the low-hard state are a natural feature of
truncated disk models.Comment: 12 pages, 8 figures, accepted by Astronomy & Astrophysics, reference
added, minor typos correcte
On the Prospect of Constraining Black-Hole Spin Through X-ray Spectroscopy of Hotspots
Future X-ray instrumentation is expected to allow us to significantly improve
the constraints derivedfrom the Fe K lines in AGN, such as the black-hole
angular momentum (spin) and the inclination angle of the putative accretion
disk. We consider the possibility that measurements of the persistent,
time-averaged Fe K line emission from the disk could be supplemented by the
observation of a localized flare, or "hotspot", orbiting close to the black
hole. Although observationally challenging, such measurements would recover
some of the information loss that is inherent to the radially-integrated line
profiles. We present calculations for this scenario to assess the extent to
which, in principle, black-hole spin may be measured. We quantify the
feasibility of this approach using realistic assumptions about likely
measurement uncertainties.Comment: 7 pages, 7 figures. Accepted for publication in Ap
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