Testing fundamental physics with distant star clusters: theoretical models for pressure-supported stellar systems

We investigate the mean velocity dispersion and the velocity dispersion profile of stellar systems in modified Newtonian dynamics (MOND), using the N -body code n-mody , which is a particle-mesh-based code with a numerical MOND potential solver developed by Ciotti, Londrillo & Nipoti. We have calculated mean velocity dispersions for stellar systems following Plummer density distributions with masses in the range of 10⁴ to  10⁹ M_⊙  and which are either isolated or immersed in an external field. Our integrations reproduce previous analytic estimates for stellar velocities in systems in the deep MOND regime  ( a _i, a _e≪ a ₀)  , where the motion of stars is either dominated by internal accelerations  ( a _i≫ a _e)  or constant external accelerations  ( a _e≫ a _i)  . In addition, we derive for the first time analytic formulae for the line-of-sight velocity dispersion in the intermediate regime  ( a _i∼ a _e∼ a ₀)  . This allows for a much-improved comparison of MOND with observed velocity dispersions of stellar systems. We finally derive the velocity dispersion of the globular cluster Pal 14 as one of the outer Milky Way halo globular clusters that have recently been proposed as a differentiator between Newtonian and MONDian dynamics.     

A GIS-Based Multivariate Statistical Analysis for Shallow Landslide Susceptibility Mapping in La Pobla de Lillet Area (Eastern Pyrenees,Spain)

This paper presents a GIS-aided procedure for shallow landslide susceptibility mapping at a regional scale. Most of the input data for the susceptibility assessment have been captured automatically. A total of 13 parameters, related to the slope geometry, have been derived from the digital elevation model (DEM) while vegetation cover and thickness of superficial formations have been obtained from photointerpretation and field work. The susceptibility assessment is based on multivariate statistical techniques (discriminant analysis), which hasbeen tested in a pilot area in La Pobla de Lillet (Eastern Pyreenes, Spain). Theresults obtained using a random sample show that 82% of all the cells, and 90% of cells including slope failures, have been properly classified. A susceptibility map based on the discriminant function has given consistent results. The susceptibilityassessment is very sensitive to the parameters selected. Compared to thetraditional methods, the main advantage of the GIS-aided procedure is the rapidityprovided by the automatic capture of parameters. It also has the capability of coveringlarge areas, and the objectivity and reproducibility of the results. The main drawbackis that, at present, not all regions have DEM accurate enough to cope with small landslides.     

Seasonal ecosystem variability in remote mountain lakes: implications for detecting climatic signals in sediment records

Weather variation and climate fluctuations are the main sources of ecosystem variability in remote mountain lakes. Here we describe the main patterns of seasonal variability in the ecosystems of nine lakes in Europe, and discuss the implications for recording climatic features in their sediments. Despite the diversity in latitude and size, the lakes showed a number of common features. They were ice-covered between 5–9 months, and all but one were dimictic. This particular lake was long and shallow, and wind action episodically mixed the water column throughout the ice-free period. All lakes showed characteristic oxygen depletion during the ice-covered-period, which was greater in the most productive lakes. Two types of lakes were distinguished according to the number of production peaks during the ice-free season. Lakes with longer summer stratification tended to have two productive periods: one at the onset of stratification, and the other during the autumn overturn. Lakes with shorter stratification had a single peak during the ice-free period. All lakes presented deep chlorophyll maxima during summer stratification, and subsurface chlorophyll maxima beneath the ice. Phosphorus limitation was common to all lakes, since nitrogen compounds were significantly more abundant than the requirements for the primary production observed. The major chemical components present in the lakes showed a short but extreme dilution during thawing. Certain lake features may favour the recording of particular climatic fluctuations, for instance: lakes with two distinct productive periods, climatic fluctuations in spring or autumn (e.g., through chrysophycean cysts); lakes with higher oxygen consumption, climatic factors affecting the duration of the ice-cover (e.g., through low-oxygen tolerant chironomids); lakes with higher water retention time; changes in atmospheric deposition (e.g., through carbon or pigment burial); lakes with longer stratification, air temperature changes during summer and autumn (e.g., through all epilimnetic species).     

Management alternatives to offset climate change effects on Mediterranean fire regimes in NE Spain

Fire regime is affected by climate and human settlements. In the Mediterranean, the predicted climate change is likely to exacerbate fire prone weather conditions, but the mid- to long-term impact of climate change on fire regime is not easily predictable. A negative feedback via fuel reduction, for instance, might cause a non-linear response of burned area to fire weather. Also, the number of fires escaping initial control could grow dramatically if the fire meteorology is just slightly more severe than what fire brigades are prepared for. Humans can directly influence fire regimes through ignition frequency, fire suppression and land use management. Here we use the fire regime model FIRE LADY to assess the impacts of climate change and local management options on number of fires, burned area, fraction of area burned in large fires and forest area during the twenty-first century in three regions of NE Spain. Our results show that currently fuel-humidity limited regions could suffer a drastic shift of fire regime with an up to 8 fold increase of annual burned area, due to a combination of fuel accumulation and severe fire weather, which would result in a period of unusually large fires. The impact of climate change on fire regime is predicted to be less pronounced in drier areas, with a gradual increase of burned area. Local fire prevention strategies could reduce but not totally offset climate induced changes in fire regimes. According to our model, a combination of restoring the traditional rural mosaic and classical fire prevention would be the most effective strategy, as a lower ignition frequency reduces the number of fires and the creation of agricultural fields in marginal areas reduces their extent.     

Constraining velocity and density contrasts across the crust–mantle boundary with receiver function amplitudes

This paper investigates the ability of P -wave receiver functions to constrain both the velocity and density contrasts across the Moho. Expressions for receiver function amplitudes corresponding to a layer over a half-space are extended to media with depth-dependent properties by explicitly accounting for free-surface reflection coefficients. Forward numerical computations show that receiver function amplitudes become frequency-dependent for depth-dependent structures and that, for a given frequency, wavelengths associated to Ps converted phases are larger than wavelengths associated to multiply reverberated phases. When consistent measurements of the amplitudes of Ps phase and multiples can be obtained, the extended expressions suffice to account for the observed amplitudes. Receiver function amplitudes are sensitive to both velocity and density contrasts across the Moho, and a two-step grid-search procedure is proposed to recover the contrasts from their measurement. The near-surface velocity is recovered in the first step from the amplitude of the direct P wave and then used in the second step to work out the relative density and S -wave velocity contrast from the amplitude of the secondary phases. Examples from central Spain (PAB) and the Indian Shield (HYB) are discussed and demonstrate that receiver function amplitudes can constrain contrasts across the Moho accurately enough to be utilized in geological interpretation.     

Particle fluxes dynamics in Blanes submarine canyon (Northwestern Mediterranean)

Within the framework of the multidisciplinary RECS project and with the aim of describing the particle flux transfer from the continental shelf to the deep basin, an array of five mooring lines equipped with a total of five pairs of PPS3/3 sequential-sampling sediment traps and RCM-7/8 current meters were deployed 30 m above the bottom from March 2003 to March 2004 inside and outside the Blanes Canyon. One mooring line was located in the upper canyon at 600 m depth, one in the canyon axis at 1700 m depth and other two close to the canyon walls at 900 m depth. A fifth mooring line was deployed in the continental open slope at 1500 m water depth.The highest near-bottom downward particle flux (14.50 g m⁻² d⁻¹) was recorded at the trap located in the upper canyon (M1), where continental inputs associated with the presence of the Tordera River are most relevant. On the other hand, the downward fluxes (4.35 g m⁻² d⁻¹) in the canyon axis (M2) were of the same order as those found in the western flank (M3) of the canyon. Both values were clearly higher than the value (1.95 g m⁻² d⁻¹) recorded at the eastern canyon wall (M4). The open slope (M5) mass flux (5.42 mg m⁻² d⁻¹) recorded by the sediment trap located outside the canyon system was three orders of magnitude lower than the other values registered by the inner canyon stations. The relevance of our data is that it explains how the transport pathway in the canyon occurs through its western flank, where a more active and persistent current toward the open ocean was recorded over the entire year of the experiment.Off-shelf sediment transport along the canyon axis showed clear differences during the period of the study, with some important events leading to strong intensifications of the current coupled with large transport of particle fluxes to the deepest parts of the canyon. Such events are primarily related to increases in river discharge and the occurrence of strong storms and cascading events during the winter.In summary, in this study it is shown that the dynamics of the water masses and the currents in the study area convert the sharp western flank of the Blanes Canyon in a more active region that favors erosion processes than the eastern flank, which has a smoother topography and where the absence of erosional conditions yields to steadier sedimentary processes.     

The white dwarf luminosity function – II. The effect of the measurement errors and other biases

The disc white dwarf luminosity function is an important tool for studying the solar neighbourhood, since it allows the determination of several Galactic parameters, the most important one being the age of the Galactic disc. However, only the     method has been employed so far for observationally determining the white dwarf luminosity function, whereas for other kind of luminosity functions several other methods have been frequently used. Moreover, the procedures to determine the white dwarf luminosity function are not free of biases. These biases have two different origins: they can either be of statistical nature or a consequence of the measurement errors. In a previous paper we carried out an in-depth study of the first category of biases for several luminosity function estimators. In this paper we focus on the biases introduced by the measurement errors and on the effects of the degree of contamination of the input sample used to build the disc white dwarf luminosity function by different kinematical populations. To assess the extent of these biases we use a Monte Carlo simulator to generate a controlled synthetic population and analyse the behaviour of the disc white dwarf luminosity function for several assumptions about the magnitude of the measurement errors and for several degrees of contamination, comparing the performances of the most robust luminosity function estimators under such conditions.     

Corrections to Watts' charts varying with libration

About 169000 observations of lunar occultations of stars are analysed for systematic corrections to Watts' charts of the marginal zone of the Moon. Corrections to the radius and shape varying with libration are derived. These are in good agreement with previous results.     

Tailored data compression using stream partitioning and prediction: application to Gaia

Gaia is the most ambitious space astrometry mission currently envisaged and it will be a technological challenge in all its aspects. Here we describe a proposal for the data compression system of Gaia, specifically designed for this mission but based on concepts that can be applied to other missions and systems as well. Realistic simulations have been performed with our Telemetry CODEC software, which performs a stream partitioning and pre-compression to the science data. In this way, standard compressors such as bzip2 or szip boost their performance and decrease their processing requirements when applied to such pre-processed data. These simulations have shown that a lossless compression factor of 3 can be achieved, whereas standard compression systems were unable to reach a factor of 2.      

Motion close to the Hopf Bifurcation Of the vertical family of periodic orbits of L4

The paper deals with different kinds of invariant motions (periodic orbits, 2D and 3D invariant tori and invariant manifolds of periodic orbits) in order to analyze the Hamiltonian direct Hopf bifurcation that takes place close to the Lyapunov vertical family of periodic orbits of the triangular equilibrium point L₄ in the 3D restricted three-body problem (RTBP) for the mass parameter, μ greater than (and close to) μ_R (Routh’s mass parameter). Consequences of such bifurcation, concerning the confinement of the motion close to the hyperbolic orbits and the 3D nearby tori are also described.