The early expansion of cluster cores

The observed properties of young star clusters, such as the core radius and luminosity profile, change rapidly during the early evolution of the clusters. Here we present observations of six young clusters in M51 where we derive their sizes using Hubble Space Telescope ( HST ) imaging and ages using deep Gemini-North spectroscopy. We find evidence for a rapid expansion of the cluster cores during the first 20 Myr of their evolution. We confirm this trend by including data from the literature of both Galactic and extragalactic embedded and young clusters, and possible mechanisms (rapid gas removal, stellar evolutionary mass loss and internal dynamical heating) are discussed. We explore the implications of this result, focussing on the fact that clusters were more concentrated in the past, implying that their stellar densities were much higher and relaxation times ( t _relax) correspondingly shorter. Thus, when estimating if a particular cluster is dynamically relaxed (i.e. when determining if a cluster's mass segregation is due to primordial or dynamical processes), the current relaxation time is only an upper limit, with t _relax likely being significantly shorter in the past.     

On a monotonicity preserving Eulerian–Lagrangian localized adjoint method for advection–diffusion equations

Eulerian–Lagrangian localized adjoint methods (ELLAMs) provide a general approach to the solution of advection-dominated advection–diffusion equations allowing large time steps while maintaining good accuracy. Moreover, the methods can treat systematically any type of boundary condition and are mass conservative. However, all ELLAMs developed so far suffer from non-physical oscillations and are usually implemented on structured grids. In this paper, we propose a finite volume ELLAM which incorporates a novel correction step rendering the method monotone while maintaining conservation of mass. The method has been implemented on fully unstructured meshes in two space dimensions. Numerical results demonstrate the applicability of the method for problems with highly non-uniform flow fields arising from heterogeneous porous media.     

Joint EUV/Radio Observations of a Solar Filament

In this paper we compare simultaneous extreme ultraviolet (EUV) line intensity and microwave observations of a filament on the disk. The EUV line intensities were observed by the CDS and SUMER instruments on board SOHO and the radio data by the Very Large Array and the Nobeyama radioheliograph. The main results of this study are the following: (1) The Lyman continuum absorption is responsible for the lower intensity observed above the filament in the EUV lines formed in the transition region (TR) at short wavelengths. In the TR lines at long wavelengths the filament is not visible. This indicates that the proper emission of the TR at the filament top is negligible. (2) The lower intensity of coronal lines and at radio wave lengths is due to the lack of coronal emission: the radio data supply the height of the prominence, while EUV coronal lines supply the missing hot matter emission measure (EM). (3) Our observations support a prominence model of cool threads embedded in the hot coronal plasma, with a sheath-like TR around them. From the missing EM we deduce the TR thickness and from the neutral hydrogen column density, derived from the Lyman continuum and Hei absorption, we estimate the hydrogen density in the cool threads.     

Synthetic observations using POLARIS: an application to simulations of massive prestellar cores

Astrophysics and Space Science - Dynamical evolution of galaxies is a complex process, especially the centers. Gravitationally coupled gas and stellar discs have been observed to coexist in the...     

Impact of climate change on hydrological conditions of Rhine and Upper Danube rivers based on the results of regional climate and hydrological models

The main objective of the Effects of Climate Change On the Inland waterway Networks (ECCONET) EU FP7 project was to assess the effect of climate change on the inland waterway transport network with special emphasis on the Rhine and Upper Danube catchments. The assessment was based on consolidation and analysis of earlier and existing research work as well as application of existing climate change and hydrological modelling tools. A key premise at the planning stage of the project had been that all impact studies conducted within ECCONET should be comparable with each other. This can be guaranteed by the common meteorological and hydrological basis. The climate model simulations, which are the most physics- and process-oriented tools for projecting the future climate evolution, include several uncertainties. In addition, uncertainties exist in the hydrological model simulations. In ECCONET, an effort was made to quantify the uncertainty range by using “representative projections” that represent both the lower and upper signals of hydrological low-flow parameters for 2021–2050 over the Rhine catchment. Their evaluation indicated that the finally chosen two regional climate model simulations could be applied also for the Upper Danube catchments as representative projections. The raw climate model outputs have been corrected to the observation data set through application of the linear scaling and the delta-change method. The first impact studies carried out after validation of the hydrological models resulted in discharge scenarios used as input to the economic models in ECCONET.     

Determination of proper motions for AC stars: First results

Proper motions of the stars of the Astrographic Catalogue are being derived, using the Hubble Space Telecope Guide Star Catalogue as second epoch. Results on the San Fernando and Cordoba AC zones are presented. Identification with GSC stars (i.e. determination of proper motion) was successful for 97 percent of all AC stars. Comparison of the proper motions thus derived with those of Preliminary PPM South shows that the accuracy is about 0.8 to 0.9 arcsec per century. Thus we can derive proper motions for about 4 million stars, with an accuracy higher than that of the SAO Catalogue.