Apoastron Shift Constraints on Dark Matter Distribution at the Galactic Center

The existence of dark matter (DM) at scales of few pc down to $\simeq 10^{-5}$ 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 $\gamma$-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.

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