425 research outputs found
Random Forest Classification of Stars in the Galactic Centre
Near-infrared high-angular resolution imaging observations of the Milky Way's
nuclear star cluster have revealed all luminous members of the existing stellar
population within the central parsec. Generally, these stars are either evolved
late-type giants or massive young, early-type stars. We revisit the problem of
stellar classification based on intermediate-band photometry in the K-band,
with the primary aim of identifying faint early-type candidate stars in the
extended vicinity of the central massive black hole. A random forest
classifier, trained on a subsample of spectroscopically identified stars,
performs similarly well as competitive methods (F1=0.85), without involving any
model of stellar spectral energy distributions. Advantages of using such a
machine-trained classifier are a minimum of required calibration effort, a
predictive accuracy expected to improve as more training data becomes
available, and the ease of application to future, larger data sets. By applying
this classifier to archive data, we are also able to reproduce the results of
previous studies of the spatial distribution and the K-band luminosity function
of both the early- and late-type stars.Comment: accepted for publication in MNRA
Dynamics of gas and dust clouds in active galactic nuclei
We analyse the motion of single optically thick clouds in the potential of a
central mass under the influence of an anisotropic radiation field
~|cos(\theta)|, a model applicable to the inner region of active galactic
nuclei. Resulting orbits are analytically soluble for constant cloud column
densities. All stable orbits are closed, although they have non-trivial shapes.
Furthermore, there exists a stability criterion in the form of a critical
inclination, which depends on the luminosity of the central source and the
column density of the cloud.Comment: 4 pages, 3 figures; language corrections, minor formatting change
Nucleosynthesis and Clump Formation in a Core Collapse Supernova
High-resolution two-dimensional simulations were performed for the first five
minutes of the evolution of a core collapse supernova explosion in a 15 solar
mass blue supergiant progenitor. The computations start shortly after bounce
and include neutrino-matter interactions by using a light-bulb approximation
for the neutrinos, and a treatment of the nucleosynthesis due to explosive
silicon and oxygen burning. We find that newly formed iron-group elements are
distributed throughout the inner half of the helium core by Rayleigh-Taylor
instabilities at the Ni+Si/O and C+O/He interfaces, seeded by convective
overturn during the early stages of the explosion. Fast moving nickel mushrooms
with velocities up to about 4000 km/s are observed. This offers a natural
explanation for the mixing required in light curve and spectral synthesis
studies of Type Ib explosions. A continuation of the calculations to later
times, however, indicates that the iron velocities observed in SN 1987 A cannot
be reproduced because of a strong deceleration of the clumps in the dense shell
left behind by the shock at the He/H interface.Comment: 8 pages, LaTeX, 2 postscript figures, 2 gif figures, shortened and
slightly revised text and references, accepted by ApJ Letter
Unrecognized Astrometric Confusion in the Galactic Centre
The Galactic Centre is a highly crowded stellar field and frequent
unrecognized events of source confusion, which involve undetected faint stars,
are expected to introduce astrometric noise on a sub-mas level. This confusion
noise is the main non-instrumental effect limiting the astrometric accuracy and
precision of current near-infrared imaging observations and the long-term
monitoring of individual stellar orbits in the vicinity of the central
supermassive black hole. We self-consistently simulate the motions of the known
and the yet unidentified stars to characterize this noise component and show
that a likely consequence of source confusion is a bias in estimates of the
stellar orbital elements, as well as the inferred mass and distance of the
black hole, in particular if stars are being observed at small projected
separations from it, such as the star S2 during pericentre passage.
Furthermore, we investigate modeling the effect of source confusion as an
additional noise component that is time-correlated, demonstrating a need for
improved noise models to obtain trustworthy estimates of the parameters of
interest (and their uncertainties) in future astrometric studies.Comment: accepted for publication in MNRA
Are Comfort Predictions From ISO 2631-1 and Self-Reported Comfort Values During Occupational Exposure to Whole-Body Vehicular Vibration Related?
On the apparent horizon in fluid-gravity duality
This article develops a computational framework for determining the location
of boundary-covariant apparent horizons in the geometry of conformal
fluid-gravity duality in arbitrary dimensions. In particular, it is shown up to
second order and conjectured to hold to all orders in the gradient expansion
that there is a unique apparent horizon which is covariantly expressible in
terms of fluid velocity, temperature and boundary metric. This leads to the
first explicit example of an entropy current defined by an apparent horizon and
opens the possibility that in the near-equilibrium regime there is preferred
foliation of apparent horizons for black holes in asymptotically-AdS
spacetimes
The effect of weight loss on serum concentrations of nitric oxide induced by short - term exercise in obese women
Objective: The aim of present study was to examine the effect of weight loss comprising regular moderate physical activity on resting serum concentrations of nitric oxide metabolites and exercise induced NO release. Materials and Methods: The study was carried out in 43 obese women without additional diseases (age 41.8±11.9y, body weight 94.5±15.1kg, BMI 36.5±4.6kg/m2). All obese patients participated in a 3-month weight reduction programme that consisted of 1) a group instruction in behavioural and dietary methods of weight control every two weeks; 2) 1000-1400kcal/day balanced diet, and 3) moderate physical exercises (30 minutes, 3 times a week). Before and after treatment body mass and height were measured, body mass index (BMI) was calculated. Body composition was determined by impedance analysis using a Bodystat analyser. The serum concentration of nitric oxide metabolites before and after exercise was measured using spectrophotometry method by Griess. The serum concentrations of lactate before and after exercise were measured with the use of strip test (ACCUSPORT analyzer). Serum concentration of insulin was measured with the use of RIA. Plasma glucose, cholesterol, HDL cholesterol and triglicerydes were determined by enzymatic procedure. Results: The mean weight loss during treatment was 8.3±4.3 kg. We did not observe differences between resting serum concentrations of NO and lactate before and after weight loss. During exercise serum NO concentrations increased significantly both before and after weight loss treatment. After the weight reduction treatment, the time of exercise test increased significantly P<0.005, but there were no significant differences between the value of NO before and after weight loss. Conclusion: 3 – month regular physical activity and weight loss did not influence exercise-induced nitric oxide production
3D AMR hydrosimulations of a compact source scenario for the Galactic Centre cloud G2
The nature of the gaseous and dusty cloud G2 in the Galactic Centre is still
under debate. We present three-dimensional hydrodynamical adaptive mesh
refinement (AMR) simulations of G2, modeled as an outflow from a "compact
source" moving on the observed orbit. The construction of mock
position-velocity (PV) diagrams enables a direct comparison with observations
and allow us to conclude that the observational properties of the gaseous
component of G2 could be matched by a massive () and slow ()
outflow, as observed for T Tauri stars. In order for this to be true, only the
material at larger () distances from the source must be
actually emitting, otherwise G2 would appear too compact compared to the
observed PV diagrams. On the other hand, the presence of a central dusty source
might be able to explain the compactness of G2's dust component. In the present
scenario, 5-10 years after pericentre the compact source should decouple from
the previously ejected material, due to the hydrodynamic interaction of the
latter with the surrounding hot and dense atmosphere. In this case, a new
outflow should form, ahead of the previous one, which would be the smoking gun
evidence for an outflow scenario.Comment: resubmitted to MNRAS after referee report, 16 pages, 11 figure
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