New results on the ages of star clusters in region B of M82     

Iraklis S. Konstantopoulos  Nate Bastian  Linda J. Smith  Gelys Trancho  Mark S. Westmoquette  Jay S. Gallagher III 《Astrophysics and Space Science》2009,324(2-4):343-346

The post-starburst region B in M82 and its massive star cluster component have been the focus of multiple studies, with reports that there is a large population of coeval clusters of age ～1 Gyr, which were created with a Gaussian initial mass distribution. This is in disagreement with other studies of young star clusters, which invariably find a featureless power-law mass distribution. Here, we present Gemini-North optical spectra of seven star clusters in M82-B and show that their ages are all between 10 and 300 Myr (a factor of 3–100 younger than previous photometric results) and that their extinctions range between near zero and 4 mag (A _V ). Using new HST ACS-HRC U-band observations we age date an additional ～30 clusters whose ages/extinctions agree well with those determined from spectroscopy. Completeness tests show that the reported ‘turn-over’ in the luminosity/mass distributions is most likely an artefact, due to the resolved nature of the clusters. We also show that the radial velocities of the clusters are inconsistent with them belonging to a bound region.  相似文献   

Hierarchical star formation in M33: properties of the star-forming regions     

Nate Bastian  Barbara Ercolano  Mark Gieles 《Astrophysics and Space Science》2009,324(2-4):293-297

Star formation within galaxies occurs on multiple scales, from spiral structure, to OB associations, to individual star clusters, and often as substructure within these clusters. This multitude of scales calls for objective methods to find and classify star-forming regions, regardless of spatial size. To this end, we present an analysis of star-forming groups in the Local Group spiral galaxy M33, based on a new implementation of the Minimum Spanning Tree (MST) method. Unlike previous studies, which limited themselves to a single spatial scale, we study star-forming structures from the effective resolution limit (～20 pc) to kpc scales. Once the groups have been identified, we study their properties, such as their size and luminosity distributions, and compare these with studies of young star clusters and giant molecular clouds (GMCs). We find evidence for a continuum of star-forming group sizes, which extends into the star cluster spatial-scale regime. We do not find a characteristic scale for OB associations, unlike that found in previous studies, and we suggest that the appearance of such a scale was caused by spatial resolution and selection effects. The luminosity function of the groups is found to be well represented by a power law with an index of ?2, as has also been found for the luminosity and mass functions of young star clusters, as well as for the mass function of GMCs. Additionally, the groups follow a similar mass-radius relation as GMCs. The size distribution of the groups is best described by a log-normal distribution, the peak of which is controlled by the spatial scale probed and the minimum number of sources used to define a group. We show that within a hierarchical distribution, if a scale is selected to find structure, the resulting size distribution will have a log-normal distribution. We find an abrupt drop of the number of groups outside a galactic radius of ～4 kpc (although individual high-mass stars are found beyond this limit), suggesting a change in the structure of the star-forming interstellar medium, possibly reflected in the lack of GMCs beyond this radius. Finally, we find that the spatial distribution of H?ii regions, GMCs, and star-forming groups are all highly correlated.  相似文献   

Radio Wave Propagation in the Corona and the Interplanetary Medium     

T.S. Bastian 《Astrophysics and Space Science》2001,277(1-2):107-116

Radio wave propagation through an inhomogeneous, random plasma produces a variety of observable phenomena – group delay, Faraday rotation, refraction, angular broadening, spectral broadening, and scintillations in phase, amplitude, and frequency. These may be exploited to constrain the mean and fluctuating properties of the medium through a variety of remote sensing techniques. In the case of the solar corona and the solar wind, the mean density, magnetic field, solar wind speed, and the spatial spectrum of the density fluctuation scan all be constrained in regions that are inaccessible to in situmeasurements. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

An early TYCHO astrometric catalogue     

E. Høg  G. Bässgen  U. Bastian  D. Egret  J. L. Halbwachs 《Astrophysics and Space Science》1991,177(1-2):109-114

The TICR catalogue is arevision of TIC, theTYCHO Input Catalogue, and will contain positions derived from the first 12 months of TYCHO observations. Its properties, its role in TYCHO data analysis, and other possible uses are discussed. Since TICR will list about 500 000 stars with a positional precision of 0."15 (rms) and since it should become available early, it could be published two years after launch, during the mission. Preferably, proper motions should be included based on TICR and older ground-based first epoch positions, thus obtaining a precision about 0.003 /year for the stars in the northern sky, but decreasing to 0.010 "/year in some parts of the southern celestial hemisphere.  相似文献   

Formation of sequences of cemented layers and hardpans within sulfide-bearing mine tailings (mine district Freiberg,Germany)     

Torsten Graupner  Andrea Kassahun  Dieter Rammlmair  Jeannet A. Meima  Dagmar Kock  Markus Furche  Adrian Fiege  Axel Schippers  Frank Melcher 《Applied Geochemistry》2007

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The early expansion of cluster cores     

N. Bastian  M. Gieles  S. P. Goodwin  G. Trancho  L. J. Smith  I. Konstantopoulos  Yu. Efremov 《Monthly notices of the Royal Astronomical Society》2008,389(1):223-230

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.  相似文献   

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

Thimo Neubauer  Peter Bastian 《Advances in water resources》2005,28(12):1292-1309

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.  相似文献   

Joint EUV/Radio Observations of a Solar Filament     

Chiuderi Drago  F.  Alissandrakis  C.E.  Bastian  T.  Bocchialini  K.  Harrison  R.A. 《Solar physics》2001,199(1):115-132

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.  相似文献   

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

Zamponi  Joaquin  Giannetti  Andrea  Bovino  Stefano  Sabatini  Giovanni  Schleicher  Dominik R. G.  K&#;rtgen  Bastian  Reissl  Stefan  Wolf  Sebastian 《Astrophysics and Space Science》2022,367(6):1-9

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...  相似文献   

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

Gabriella Szépszó  Imke Lingemann  Bastian Klein  Mária Kovács 《Natural Hazards》2014,72(1):241-262

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.  相似文献