First VLTI observations of Mira stars

We present first observations of Mira stars obtained at the Very Large Telescope Interferometer (VLTI) at 2 microns, equipped with the VINCI instrument, using different baselines with both the test siderostats and the 8.2 m unit telescopes. These observations, collected in the course of the VLTI commissining program, have targeted so far about sixty cool giant stars. In this talk, we list and describe the measurements obtained for 14 Mira stars, many of them representing first-time determinations of the visibility. In particular, we devote special attention to a study of Mira itself, for which very accurate determinations of the visibility at several baseline lenghts and orientations could be obtained. We find that a two-component model is more consistent with the data than a single stellar disc. Further observations are needed for a better understanding of this source. The VLTI will constitute an ideal instrument for such studies in the future. In particular it will permit detailed investigations of southern AGB stars, such as accurate measurements of surface structureparameters (diameters, diameter variations, asymmetries, center-to-limbvariations, special features like hot spots) and of circumstellar envelopes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation of ω Centauri from an ancient nucleated dwarf galaxy in the young Galactic disc

We first present a self-consistent dynamical model in which ω Cen is formed from an ancient nucleated dwarf galaxy merging with the first generation of the Galactic thin disc in a retrograde manner with respect to the Galactic rotation. Our numerical simulations demonstrate that during merging between the Galaxy and the ω Cen host dwarf with   M _B≃−14 mag  and its nucleus mass of  10⁷ M⊙  , the outer stellar envelope of the dwarf is nearly completely stripped, whereas the central nucleus can survive from the tidal stripping because of its compactness. The developed naked nucleus has a very bound retrograde orbit around the young Galactic disc, as observed for ω Cen, with apocentre and pericentre distances of ∼8 and ∼1 kpc, respectively. The Galactic tidal force can induce radial inflow of gas to the centre of the dwarf and consequently triggers moderately strong nuclear starbursts in a repetitive manner. This result implies that efficient nuclear chemical enrichment resulting from the later starbursts can be closely associated with the origin of the observed relatively young and metal-rich stars in ω Cen. Dynamical heating by the ω Cen host can transform the young thin disc into the thick disc during merging.     

WEKU – a GIS-Supported stochastic model of groundwater residence times in upper aquifers for the supraregional groundwater management

 The supraregional GIS-supported stochastical model, WEKU, for the determination of groundwater residence times in the upper aquifers of large groundwater provinces is presented. Using a two-dimensional analytical model of groundwater flow, groundwater residence times are determined within two extreme cases. In the first case, maximal groundwater residence times are calculated, representing the part of groundwater, that is drained by the main surface water of a groundwater catchment area. In the second case, minimal groundwater residence times for drainage into the nearest surface water are determined. Using explicit distribution functions of the input parameters, mean values as well as potential ranges of variations of the groundwater residence times are derived. The WEKU model has been used for the determination of groundwater residence times throughout Germany. The model results – mean values and deviations of the groundwater velocity and the maximal and minimal groundwater residence times in the upper aquifers – are presented by general maps and discussed in detail. It is shown that the groundwater residence times in the upper aquifer vary regionally, differentiated between less than 1 year and more than 2000 years. Using this information, the time scales can be specified, until measures to remediate polluted groundwater resources may lead to a substantial groundwater quality improvement in the different groundwater provinces of Germany. With respect to its supraregional scale of application, the WEKU model may serve as a useful tool for the supraregional groundwater management on a state, federal or international level. Received: 15 August 1995 · Accepted: 15 October 1995