1,844 research outputs found
Apoastron Shift Constraints on Dark Matter Distribution at the Galactic Center
The existence of dark matter (DM) at scales of few pc down to pc around the centers of galaxies and in particular in the Galactic
Center region has been considered in the literature. Under the assumption that
such a DM clump, principally constituted by non-baryonic matter (like WIMPs)
does exist at the center of our galaxy, the study of the -ray emission
from the Galactic Center region allows us to constrain both the mass and the
size of this DM sphere. Further constraints on the DM distribution parameters
may be derived by observations of bright infrared stars around the Galactic
Center. Hall and Gondolo \cite{hallgondolo} used estimates of the enclosed mass
obtained in various ways and tabulated by Ghez et al.
\cite{Ghez_2003,Ghez_2005}. Moreover, if a DM cusp does exist around the
Galactic Center it could modify the trajectories of stars moving around it in a
sensible way depending on the DM mass distribution. Here, we discuss the
constraints that can be obtained with the orbit analysis of stars (as S2 and
S16) moving inside the DM concentration with present and next generations of
large telescopes. In particular, consideration of the S2 star apoastron shift
may allow improving limits on the DM mass and size.Comment: in press on Phys. Rev.
Evaluation Framework for Action Innovation projects : A series of Case Studies from Germany, The Netherlands, Hungary and Spain
Land-use type effects on soil organic carbon and microbial properties in a semi-arid region of Northeast Brazil.
Published online in Wiley Online Library (wileyonlinelibrary.com)
Probing the Galactic Potential with Next-Generation Observations of Disk Stars
Near-future surveys promise a dramatic improvement in the number and
precision of astrometric, photometric and spectroscopic measurements of stars
in the Milky Way's disk. We examine the impact of such surveys on our
understanding of the Galaxy by "observing" particle realizations of
non-axisymmetric disk distributions orbiting in an axisymmetric halo with
appropriate errors and then attempting to recover the underlying potential
using a Markov Chain Monte Carlo (MCMC) approach. We demonstrate that the
azimuthally averaged gravitational force field in the Galactic plane--and
hence, to a lesser extent, the Galactic mass distribution--can be tightly
constrained over a large range of radii using a variety of types of surveys so
long as the error distribution of the measurements of the parallax, proper
motion and radial velocity are well-understood and the disk is surveyed
globally. One advantage of our method is that the target stars can be selected
non-randomly in real or apparent-magnitude space to ensure just such a global
sample without biasing the results. Assuming we can always measure the
line-of-sight velocity of a star with at least 1 km/s precision, we demonstrate
that the force field can be determined to better than ~1% for Galactocentric
radii in the range R=4-20 kpc We conclude that near-future surveys, like SIM
Lite, Gaia, and VERA, will provide the first precise mapping of the
gravitational force field in the region of the Galactic disk.Comment: 41 pages and 10 figures, accepted for publication in Ap
Soil surface-active fauna in degraded and restored lands of Northeast Brazil.
Land degradation reducing vegetation cover may affect the soil surface-active fauna because both aboveground and belowground invertebrates depend on complex plant communities. In this study, we evaluated the effect of land degradation and restoration on soil fauna in northeast Brazil. Sites differed in degradation status: native vegetation, moderately degraded land, highly degraded land, and land under restoration for 4 years.Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ldr.224
Proper Motion Dispersions of Red Clump Giants in the Galactic Bulge: Observations and Model Comparisons
Red clump giants in the Galactic bulge are approximate standard candles and
hence they can be used as distance indicators. We compute the proper motion
dispersions of RCG stars in the Galactic bulge using the proper motion
catalogue from the second phase of the Optical Gravitational Microlensing
Experiment (OGLE-II, Sumi et al. 2004) for 45 fields. The proper motion
dispersions are measured to a few per cent accuracy due to the large number of
stars in the fields. The observational sample is comprised of 577736 stars.
These observed data are compared to a state-of-the-art particle simulation of
the Galactic bulge region. The predictions are in rough agreement with
observations, but appear to be too anisotropic in the velocity ellipsoid. We
note that there is significant field-to-field variation in the observed proper
motion dispersions. This could either be a real feature, or due to some unknown
systematic effect.Comment: 12 pages, 13 figures, accepted for publication in MNRA
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&
Inconsistent impacts of decomposer diversity on the stability of aboveground and belowground ecosystem functions
The intensive discussion on the importance of biodiversity for the stability of essential processes in ecosystems has prompted a multitude of studies since the middle of the last century. Nevertheless, research has been extremely biased by focusing on the producer level, while studies on the impacts of decomposer diversity on the stability of ecosystem functions are lacking. Here, we investigate the impacts of decomposer diversity on the stability (reliability) of three important aboveground and belowground ecosystem functions: primary productivity (shoot and root biomass), litter decomposition, and herbivore infestation. For this, we analyzed the results of three laboratory experiments manipulating decomposer diversity (1–3 species) in comparison to decomposer-free treatments in terms of variability of the measured variables. Decomposer diversity often significantly but inconsistently affected the stability of all aboveground and belowground ecosystem functions investigated in the present study. While primary productivity was mainly destabilized, litter decomposition and aphid infestation were essentially stabilized by increasing decomposer diversity. However, impacts of decomposer diversity varied between plant community and fertility treatments. There was no general effect of the presence of decomposers on stability and no trend toward weaker effects in fertilized communities and legume communities. This indicates that impacts of decomposers are based on more than effects on nutrient availability. Although inconsistent impacts complicate the estimation of consequences of belowground diversity loss, underpinning mechanisms of the observed patterns are discussed. Impacts of decomposer diversity on the stability of essential ecosystem functions differed between plant communities of varying composition and fertility, implicating that human-induced changes of biodiversity and land-use management might have unpredictable effects on the processes mankind relies on. This study therefore points to the necessity of also considering soil feedback mechanisms in order to gain a comprehensive and holistic understanding of the impacts of current global change phenomena on the stability of essential ecosystem functions
Prospects for testing the nature of Sgr A*'s NIR flares on the basis of current VLT- and future VLTI-observations
Sagittarius A*, the supermassive compact object at the center of the Galaxy,
exhibits outbursts in the near infrared and X-ray domains. These flares are
likely due to energetic events very close to the central object, on a scale of
a few Schwarzschild radii. Optical interferometry will soon be able to provide
astrometry with an accuracy of this order (~10 muas). In this article we use
recent photometric near infrared data observed with the adaptive optics system
NACO at the Very Large Telescope combined with simulations in order to deploy a
method to test the nature of the flares and to predict the possible outcome of
observations with the Very Large Telescope Interferometer. To accomplish this
we implement a hot spot model and investigate its appearance for a remote
observer in terms of light curves and centroid tracks, based on general
relativistic ray tracing methods. First, we use a simplified model of a small
steady source in order to investigate the relativistic effects qualitatively. A
more realistic scenario is then being developed by fitting our model to
existing flare data. While indications for the spin of the black hole and
multiple images due to lensing effects are marginal in the light curves,
astrometric measurements offer the possibility to reveal these high-order
general relativistic effects. This study makes predictions on these astrometric
measurements and leads us to the conclusion that future infrared
interferometers will be able to detect proper motion of hot spots in the
vicinity of Sagittarius A*.Comment: 15 pages, 14 figures, accepted for publication in Ap
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