A brown dwarf desert for intermediate mass stars in Sco OB2?

We present JHK observations of 22 intermediate-mass stars in Sco OB2, obtained with VLT/NACO. The survey was performed to determine the status of (sub)stellar candidate companions of A and late-B members. The distinction between companions and background stars is by a comparison with isochrones and statistical arguments. We are sensitive to companions in the separation range 0.1''-11'' (13-1430 AU) and K<17. We detect 62 secondaries of which 18 are physical companions (3 new), 11 candidates, and 33 background stars. The companion masses are in the range 0.03<M<1.19 Msun, with mass ratios 0.06<q<0.55. We include in our sample a subset of 9 targets with multi-color ADONIS observations from Kouwenhoven et al. (2005). In the ADONIS survey secondaries with K12 as background stars. Our multi-color analysis demonstrates that the simple K=12 criterion correctly classifies the secondaries in ~80% of the cases. We reanalyse the total ADONIS/NACO sample and conclude that of the 176 secondaries, 25 are physical companions, 55 are candidates, and 96 are background stars. Although we are sensitive and complete to brown dwarfs as faint as K=14 in the separation range 130-520 AU, we detect only one, giving a brown dwarf companion fraction of 0.5% (M>30 MJ). However, the number of brown dwarfs is consistent with an extrapolation of the stellar companion mass distribution. This indicates that the physical mechanism for the formation of brown dwarfs around intermediate mass stars is similar to that of stellar companions, and that the embryo ejection mechanism does not need to be invoked in order to explain the small number of brown dwarf companions among these stars.Comment: 29 pages, 9 figures, accepted by A&

Requirements modelling and formal analysis using graph operations

The increasing complexity of enterprise systems requires a more advanced analysis of the representation of services expected than is currently possible. Consequently, the specification stage, which could be facilitated by formal verification, becomes very important to the system life-cycle. This paper presents a formal modelling approach, which may be used in order to better represent the reality of the system and to verify the awaited or existing system’s properties, taking into account the environmental characteristics. For that, we firstly propose a formalization process based upon properties specification, and secondly we use Conceptual Graphs operations to develop reasoning mechanisms of verifying requirements statements. The graphic visualization of these reasoning enables us to correctly capture the system specifications by making it easier to determine if desired properties hold. It is applied to the field of Enterprise modelling

The primordial binary population II: Recovering the binary population for intermediate mass stars in Sco OB2

We characterize the binary population in the young and nearby OB association Scorpius OB2 using available observations of visual, spectroscopic, and astrometric binaries with intermediate-mass primaries. We take into account observational biases by comparing the observations with simulated observations of model associations. The available data indicate a large binary fraction (> 70% with 3sigma confidence), with a large probability that all intermediate mass stars in Sco OB2 are part of a binary system. The binary systems have a mass ratio distribution of the form f(q) ~ q^-0.4. Sco OB2 has a semi-major axis distribution of the form f(log a) ~ constant (Opik's law), in the range 5-5e6 Rsun. The log-normal period distribution of Duquennoy & Mayor results in too few spectroscopic binaries, even if the model binary fraction is 100%. Sco OB2 is a young association with a low stellar density; its current population is expected to be very similar to the primordial population. The fact that practically all stars in Sco OB2 are part of a binary (or multiple) system demonstrates that multiplicity is a fundamental factor in the star formation process, at least for intermediate mass stars.Comment: 36 pages, 11 figures, accepted by A&

Comparative Modelling of the Spectra of Cool Giants

Our ability to extract information from the spectra of stars depends on reliable models of stellar atmospheres and appropriate techniques for spectral synthesis. Various model codes and strategies for the analysis of stellar spectra are available today. We aim to compare the results of deriving stellar parameters using different atmosphere models and different analysis strategies. The focus is set on high-resolution spectroscopy of cool giant stars. Spectra representing four cool giant stars were made available to various groups and individuals working in the area of spectral synthesis, asking them to derive stellar parameters from the data provided. The results were discussed at a workshop in Vienna in 2010. Most of the major codes currently used in the astronomical community for analyses of stellar spectra were included in this experiment. We present the results from the different groups, as well as an additional experiment comparing the synthetic spectra produced by various codes for a given set of stellar parameters. Similarities and differences of the results are discussed. Several valid approaches to analyze a given spectrum of a star result in quite a wide range of solutions. The main causes for the differences in parameters derived by different groups seem to lie in the physical input data and in the details of the analysis method. This clearly shows how far from a definitive abundance analysis we still are.Comment: accepted for publication in A&A. This version includes also the online tables. Reference spectra will later be available via the CD

GTP and Ca2+ Modulate the Inositol 1,4,5-Trisphosphate-Dependent Ca2+ Release in Streptolysin O-Permeabilized Bovine Adrenal Chromaffin Cells

The inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release was studied using streptolysin O-permeabilized bovine adrenal chromaffin cells. The IP3-induced Ca2+ release was followed by Ca2+ reuptake into intracellular compartments. The IP3-induced Ca2+ release diminished after sequential applications of the same amount of IP3. Addition of 20 μM GTP fully restored the sensitivity to IP3. Guanosine 5'-O-(3-thio)triphosphate (GTPγS) could not replace GTP but prevented the action of GTP. The effects of GTP and GTPγS were reversible. Neither GTP nor GTPγS induced release of Ca2+ in the absence of IP3. The amount of Ca2+ whose release was induced by IP3 depended on the free Ca2+ concentration of the medium. At 0.3 μM free Ca2+, a half-maximal Ca2+ release was elicited with ∼0.1 μM IP3. At 1 μM free Ca2+, no Ca2+ release was observed with 0.1 μM IP3; at this Ca2+ concentration, higher concentrations of IP3 (0.25 μM) were required to evoke Ca2+ release. At 8 μM free Ca2+, even 0.25 μM IP3 failed to induce release of Ca2+ from the store. The IP3-induced Ca2+ release at constant low (0.2 μM) free Ca2+ concentrations correlated directly with the amount of stored Ca2+. Depending on the filling state of the intracellular compartment, 1 mol of IP3 induced release of between 5 and 30 mol of Ca2+

The Primordial Binary Population - I: A near-infrared adaptive optics search for close visual companions to A star members of Scorpius OB2

We present the results of a near-infrared adaptive optics survey with the aim to detect close companions to Hipparcos members in the three subgroups of the nearby OB association Sco OB2: Upper Scorpius (US), Upper Centaurus Lupus (UCL) and Lower Centaurus Crux (LCC). We have targeted 199 A-type and late B-type stars in the Ks band, and a subset also in the J and H band. We find 151 stellar components other than the target stars. A brightness criterion is used to separate these components into 77 background stars and 74 candidate physical companion stars. Out of these 74 candidate companions, 41 have not been reported before (14 in US; 13 in UCL; 14 in LCC). Companion star masses range from 0.1 to 3 Msun. The mass ratio distribution follows f(q) = q^-0.33, which excludes random pairing. No close (rho < 3.75'') companion stars or background stars are found in the magnitude range 12 < Ks < 14. The lack of stars with these properties cannot be explained by low-number statistics, and may imply a lower limit on the companion mass of ~ 0.1 Msun. Close stellar components with Ks > 14 are observed. If these components are very low-mass companion stars, a gap in the companion mass distribution might be present. The small number of close low-mass companion stars could support the embryo-ejection formation scenario for brown dwarfs. Our findings are compared with and complementary to visual, spectroscopic, and astrometric data on binarity in Sco OB2. We find an overall companion star fraction of 0.52 in this association. This paper is the first step toward our goal to derive the primordial binary population in Sco OB2.Comment: 27 pages, to accepted by A&

A progressive refinement approach for the visualisation of implicit surfaces

Visualising implicit surfaces with the ray casting method is a slow procedure. The design cycle of a new implicit surface is, therefore, fraught with long latency times as a user must wait for the surface to be rendered before being able to decide what changes should be introduced in the next iteration. In this paper, we present an attempt at reducing the design cycle of an implicit surface modeler by introducing a progressive refinement rendering approach to the visualisation of implicit surfaces. This progressive refinement renderer provides a quick previewing facility. It first displays a low quality estimate of what the final rendering is going to be and, as the computation progresses, increases the quality of this estimate at a steady rate. The progressive refinement algorithm is based on the adaptive subdivision of the viewing frustrum into smaller cells. An estimate for the variation of the implicit function inside each cell is obtained with an affine arithmetic range estimation technique. Overall, we show that our progressive refinement approach not only provides the user with visual feedback as the rendering advances but is also capable of completing the image faster than a conventional implicit surface rendering algorithm based on ray casting

The Gaia-ESO Survey: the most metal-poor stars in the Galactic bulge

We present the first results of the EMBLA survey (Extremely Metal-poor BuLge stars with AAOmega), aimed at finding metal-poor stars in the Milky Way bulge, where the oldest stars should now preferentially reside. EMBLA utilises SkyMapper photometry to pre-select metal-poor candidates, which are subsequently confirmed using AAOmega spectroscopy. We describe the discovery and analysis of four bulge giants with -2.72<=[Fe/H]<=-2.48, the lowest metallicity bulge stars studied with high-resolution spectroscopy to date. Using FLAMES/UVES spectra through the Gaia-ESO Survey we have derived abundances of twelve elements. Given the uncertainties, we find a chemical similarity between these bulge stars and halo stars of the same metallicity, although the abundance scatter may be larger, with some of the stars showing unusual [{\alpha}/Fe] ratios.Comment: 7 pages, 5 figures. Accepted for publication by MNRA