2,596 research outputs found
Stellar interactions in dense and sparse star clusters
Stellar encounters potentially affect the evolution of the protoplanetary
discs in the Orion Nebula Cluster (ONC). However, the role of encounters in
other cluster environments is less known. We investigate the effect of the
encounter-induced disc-mass loss in different cluster environments. Starting
from an ONC-like cluster we vary the cluster size and density to determine the
correlation of collision time scale and disc-mass loss. We use the NBODY6++
code to model the dynamics of these clusters and analyze the effect of
star-disc encounters. We find that the disc-mass loss depends strongly on the
cluster density but remains rather unaffected by the size of the stellar
population. The essential outcome of the simulations are: i) Even in clusters
four times sparser than the ONC the effect of encounters is still apparent. ii)
The density of the ONC itself marks a threshold: in less dense and less massive
clusters it is the massive stars that dominate the encounter-induced disc-mass
loss whereas in denser and more massive clusters the low-mass stars play the
major role for the disc mass removal. It seems that in the central regions of
young dense star clusters -- the common sites of star formation -- stellar
encounters do affect the evolution of the protoplanetary discs. With higher
cluster density low-mass stars become more heavily involved in this process.
This finding allows for the extrapolation towards extreme stellar systems: in
case of the Arches cluster one would expect stellar encounters to destroy the
discs of most of the low- and high-mass stars in several hundred thousand
years, whereas intermediate mass stars are able to retain to some extant their
discs even under these harsh environmental conditions.Comment: accepted by Astronomy and Astrophysic
Keldysh-Rutherford model for attoclock
We demonstrate a clear similarity between attoclock offset angles and
Rutherford scattering angles taking the Keldysh tunnelling width as the impact
parameter and the vector potential of the driving pulse as the asymptotic
velocity. This simple model is tested against the solution of the
time-dependent Schr\"odinger equation using hydrogenic and screened (Yukawa)
potentials of equal binding energy. We observe a smooth transition from a
hydrogenic to 'hard-zero' intensity dependence of the offset angle with
variation of the Yukawa screening parameter. Additionally we make comparison
with the attoclock offset angles for various noble gases obtained with the
classical-trajectory Monte Carlo method. In all cases we find a close
correspondence between the model predictions and numerical calculations. This
suggests a largely Coulombic origin of the attoclock offset angle and casts
further doubt on its interpretation in terms of a finite tunnelling time
The need for a second black hole at the Galactic center
Deep infra-red observations and long-term monitoring programs have provided
dynamical evidence for a supermassive black hole of mass 3.e6 solar masses
associated with the radio source Sagitarrius A* at the center of our Galaxy.
The brightest stars orbiting within 0.1 parsecs of the black hole appear to be
young, massive main sequence stars, n spite of an environment near the black
hole that is hostile to star formation. We discuss mechanisms by which stars
born outside the central parsec can sink towards the black hole and conclude
that the drag coming from plausible stellar populations does not operate on the
short timescales required by the stellar ages. We propose that these stars were
dragged in by a second black hole of mass of 1.e3-1.e4 solar masses, which
would be classified as an intermediate-mass black hole. We discuss the
implications for the stellar populations and the kinematics in the Galactic
center. Finally we note that continued astrometric monitoring of the central
radio source offers the prospect for a direct detection of such objects.Comment: 5 pages, 2 postscript figures, submitted to ApJ letters The
introduction section has been updated since submission to Ap
Near infrared flares of Sagittarius A*: Importance of near infrared polarimetry
We report on the results of new simulations of near-infrared (NIR)
observations of the Sagittarius A* (Sgr A*) counterpart associated with the
super-massive black hole at the Galactic Center. The observations have been
carried out using the NACO adaptive optics (AO) instrument at the European
Southern Observatory's Very Large Telescope and CIAO NIR camera on the Subaru
telescope (13 June 2004, 30 July 2005, 1 June 2006, 15 May 2007, 17 May 2007
and 28 May 2008). We used a model of synchrotron emission from relativistic
electrons in the inner parts of an accretion disk. The relativistic simulations
have been carried out using the Karas-Yaqoob (KY) ray-tracing code. We probe
the existence of a correlation between the modulations of the observed flux
density light curves and changes in polarimetric data. Furthermore, we confirm
that the same correlation is also predicted by the hot spot model. Correlations
between intensity and polarimetric parameters of the observed light curves as
well as a comparison of predicted and observed light curve features through a
pattern recognition algorithm result in the detection of a signature of
orbiting matter under the influence of strong gravity. This pattern is detected
statistically significant against randomly polarized red noise. Expected
results from future observations of VLT interferometry like GRAVITY experiment
are also discussed.Comment: 26 pages, 38 figures, accepted for publication by A&
Multiwavelength VLBI observations of Sagittarius A*
The compact radio source Sgr\,A*, associated with the super massive black
hole at the center of the Galaxy, has been studied with VLBA observations at 3
frequencies (22, 43, 86\,GHz) performed on 10 consecutive days in May 2007. The
total VLBI flux density of Sgr\,A* varies from day to day. The variability is
correlated at the 3 observing frequencies with higher variability amplitudes
appearing at the higher frequencies. For the modulation indices, we find 8.4\,%
at 22\,GHz, 9.3\,% at 43\,GHz, and 15.5\,% at 86\,GHz. The radio spectrum is
inverted between 22 and 86\,GHz, suggesting inhomogeneous synchrotron
self-absorption with a turnover frequency at or above 86\,GHz. The radio
spectral index correlates with the flux density, which is harder (more inverted
spectrum) when the source is brighter. The average source size does not appear
to be variable over the 10-day observing interval. However, we see a tendency
for the sizes of the minor axis to increase with increasing total flux, whereas
the major axis remains constant. Towards higher frequencies, the position angle
of the elliptical Gaussian increases, indicative of intrinsic structure, which
begins to dominate the scatter broadening. At cm-wavelength, the source size
varies with wavelength as , which is interpreted as the
result of interstellar scatter broadening. After removal of this scatter
broadening, the intrinsic source size varies as . The
VLBI closure phases at 22, 43, and 86\,GHz are zero within a few degrees,
indicating a symmetric or point-like source structure. In the context of an
expanding plasmon model, we obtain an upper limit of the expansion velocity of
about 0.1\,c from the non-variable VLBI structure. This agrees with the
velocity range derived from the radiation transport modeling of the flares from
the radio to NIR wavelengths.}Comment: 14pages, 14 Figures, Accepted for publication in A&
Libration driven multipolar instabilities
We consider rotating flows in non-axisymmetric enclosures that are driven by
libration, i.e. by a small periodic modulation of the rotation rate. Thanks to
its simplicity, this model is relevant to various contexts, from industrial
containers (with small oscillations of the rotation rate) to fluid layers of
terrestial planets (with length-of-day variations). Assuming a multipolar
-fold boundary deformation, we first obtain the two-dimensional basic flow.
We then perform a short-wavelength local stability analysis of the basic flow,
showing that an instability may occur in three dimensions. We christen it the
Libration Driven Multipolar Instability (LDMI). The growth rates of the LDMI
are computed by a Floquet analysis in a systematic way, and compared to
analytical expressions obtained by perturbation methods. We then focus on the
simplest geometry allowing the LDMI, a librating deformed cylinder. To take
into account viscous and confinement effects, we perform a global stability
analysis, which shows that the LDMI results from a parametric resonance of
inertial modes. Performing numerical simulations of this librating cylinder, we
confirm that the basic flow is indeed established and report the first
numerical evidence of the LDMI. Numerical results, in excellent agreement with
the stability results, are used to explore the non-linear regime of the
instability (amplitude and viscous dissipation of the driven flow). We finally
provide an example of LDMI in a deformed spherical container to show that the
instability mechanism is generic. Our results show that the previously studied
libration driven elliptical instability simply corresponds to the particular
case of a wider class of instabilities. Summarizing, this work shows that
any oscillating non-axisymmetric container in rotation may excite intermittent,
space-filling LDMI flows, and this instability should thus be easy to observe
experimentally
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