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Hansen C.J.,Landessternwarte | Bergemann M.,Max Planck Institute for Astrophysics | Cescutti G.,Leibniz Institute for Astrophysics Potsdam | Francois P.,University Paris Diderot | And 3 more authors.
Astronomy and Astrophysics | Year: 2013

Context. Strontium has proven itself to be one of the most important neutron-capture elements in the study of metal-poor stars. Thanks to the strong absorption lines of Sr, they can be detected even in the most metal-poor stars and also in low-resolution spectra. However, we still cannot explain the large star-to-star abundance scatter we derive for metal-poor stars. Aims. Here we compare Galactic chemical evolution (GCE) predictions with improved abundances for Sr i and Sr ii, including updated atomic data, to evaluate possible explanations for the large star-to-star scatter at low metallicities. Methods. We have derived abundances under both local thermodynamic equilibrium (LTE) and non-LTE (NLTE) for stars spanning a large interval of metallicities, as well as a broad range of other stellar parameters. Gravities and metallicities are also determined in NLTE. We employed MARCS stellar atmospheres and MOOG for the LTE spectrum synthesis, while MAFAGS and DETAIL were used to derive the NLTE abundances. We verified the consistency of the two methods in LTE. Results. We confirm that the ionisation equilibrium between Sr i and Sr ii is satisfied under NLTE but not LTE, where the difference between neutral and ionised Sr is on average ~0.3 dex. We show that the NLTE corrections are of increasing importance as the metallicity decreases. For the stars with [Fe/H] >-3, the Sr i NLTE correction is ~0.35/0.55 dex in dwarfs/giants, while the Sr ii NLTE correction is <±0.05 dex. Conclusions. On the basis of the large NLTE corrections to Sr i, Sr i should not be applied as a chemical tracer under LTE, while it is a good tracer under NLTE. Sr ii, on the other hand, is a good tracer under both LTE and NLTE (down to [Fe/H] ~-3), and LTE is a safe assumption for this majority species (if the NLTE corrections are not available). However, the Sr abundance from Sr ii lines depends on determining an accurate surface gravity, which can be obtained from the NLTE spectroscopy of Fe lines or from parallax measurements. We could not explain the star-to-star scatter (which remains under both LTE and NLTE) by the use of the Galactic chemical evolution model, since Sr yields to date have been too uncertain to draw firm conclusions. At least two nucleosynthetic production sites seem necessary to account for this large scatter. © 2013 ESO.

Nicklas H.E.,University of Gottingen | Anwand H.,University of Gottingen | Fleischmann A.,University of Gottingen | Kohler C.,University of Gottingen | And 2 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2012

The foundation of the MUSE instrument with its high multiplexing factor of twenty-four spectrographs is formed through its central main structure that accommodates all instrumental subsystems and links them with the telescope. Due to instrument's dimension and complexity, the requirements on structural performance are demanding. How its performance was tested and optimized through reverse engineering is addressed. Intimately mated with this central structure is an optical relay system that splits the single telescopic field into twenty-four subfields. Each of those is individually directed along three dimensions across the structure through a folding and imaging setup of an optical relay system that at the end feeds one of the twenty-four spectrographs. This opto-mechanical relay system was tested when mounted onto the main structure. The results obtained so far are given here. © 2012 SPIE.

News Article | December 21, 2016

DFG to fund 12 projects for the development of new technologies through new call/ New major research instrument for X-raying reinforced concrete components approved The Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) is funding new instrumentation for various fields of knowledge-driven research through two calls. The Joint Committee of the largest research funding organisation and central self-governing organisation of the research community in Germany took the relevant funding decisions at its December session in Bonn. The first decision relates to the call for "New Instrumentation for Research", issued for the first time, which is designed to enable the development of new instrumentation technologies for research questions which cannot be answered with currently available equipment. The newly developed instrumentation should be made available to as many researchers as possible for the purposes of basic research. After the very strong response to the call issued in January 2016 - which attracted a total of 79 proposals - the DFG Joint Committee has now decided to fund twelve projects for an initial three years with a total funding volume of €8 million. The projects relate to natural sciences (including geosciences), life sciences, medical technology, and engineering sciences and are based at the following universities and non-university research institutions: Aalen University; Charité - Universitätsmedizin Berlin (university hospital); University of Bonn; Technical University of Darmstadt; Erlangen University Hospital, University Medical Center Freiburg and Radiological Institute, German Cancer Research Centre, Heidelberg; Leibniz Institute for Applied Geophysics, Hanover, and Leibniz Institute of Photonic Technology, Jena; Heidelberg University; University of Jena; University of Cologne, Leibniz Institute for Astrophysics, Potsdam, and Landessternwarte Königstuhl (state observatory), Heidelberg; Technical University of Munich, University of Greifswald and Max Planck Institute of Plasma Physics, Greifswald; University Hospital Münster; University of Würzburg and University of Mainz. The second decision relates to a major instrumentation initiative. Through this initiative, the DFG will make €8 million available for the construction and commissioning of a new type of system at the Technical University of Kaiserslautern that will be able to X-ray components made of reinforced concrete and other materials using computed tomography (CT). The system will use X-rays of 9 mega-electronvolts, much more powerful than medical X-ray systems, allowing it to probe reinforced concrete components up to 30 centimetres in diameter and 6 metres in length. It will even be possible to X-ray components while they are experiencing stress or destruction; the three-dimensional images of these processes will provide researchers with valuable information. The research carried out with the new equipment is intended to provide information about the durability and properties of established construction materials and also facilitate the development of improved materials and composites. For 20 percent of its usage time, the new X-ray system will also be available to other scientific working groups in Germany. For "New Instrumentation for Research": Dr Achim Tieftrunk, Scientific Instrumentation and Information Technology Division, tel. +49 228 885-2816, For the major instrumentation initiative: Dr Michael Royeck, Scientific Instrumentation and Information Technology Division, tel. +49 228 885-2976,

Hansen T.,Landessternwarte | Hansen C.J.,Copenhagen University | Christlieb N.,Landessternwarte | Beers T.C.,University of Notre Dame | And 8 more authors.
Astrophysical Journal | Year: 2015

We present a high-resolution elemental-abundance analysis for a sample of 23 very metal-poor ([Fe/H] < -2.0) stars, 12 of which are extremely metal-poor ([Fe/H] < -3.0), and 4 of which are ultra-metal-poor ([Fe/H] < -4.0). These stars were targeted to explore differences in the abundance ratios for elements that constrain the possible astrophysical sites of element production, including Li, C, N, O, the α-elements, the iron-peak elements, and a number of neutron-capture elements. This sample substantially increases the number of known carbon-enhanced metal-poor (CEMP) and nitrogen-enhanced metal-poor (NEMP) stars - our program stars include eight that are considered "normal" metal-poor stars, six CEMP-no stars, five CEMP-s stars, two CEMP-r stars, and two CEMP-r/s stars. One of the CEMP-r stars and one of the CEMP-r/s stars are possible NEMP stars. We detect lithium for three of the six CEMP-no stars, all of which are Li.

Hansen T.,Landessternwarte | Hansen C.J.,Landessternwarte | Christlieb N.,Landessternwarte | Yong D.,Australian National University | And 8 more authors.
Astrophysical Journal | Year: 2014

We present an elemental abundance analysis for four newly discovered ultra metal-poor stars from the Hamburg/ESO survey, with [Fe/H] ≤ -4. Based on high-resolution, high signal-to-noise spectra, we derive abundances for 17 elements in the range from Li to Ba. Three of the four stars exhibit moderate to large overabundances of carbon, but have no enhancements in their neutron-capture elements. The most metal-poor star in the sample, HE 0233-0343 ([Fe/H] = -4.68), is a subgiant with a carbon enhancement of [C/Fe] = +3.5, slightly above the carbon-enhancement plateau suggested by Spite et al. No carbon is detected in the spectrum of the fourth star, but the quality of its spectrum only allows for the determination of an upper limit on the carbon abundance ratio of [C/Fe] < +1.7. We detect lithium in the spectra of two of the carbon-enhanced stars, including HE 0233-0343. Both stars with Li detections are Li-depleted, with respect to the Li plateau for metal-poor dwarfs found by Spite & Spite. This suggests that whatever site(s) produced C either do not completely destroy lithium, or that Li has been astrated by early-generation stars and mixed with primordial Li in the gas that formed the stars observed at present. The derived abundances for the α elements and iron-peak elements of the four stars are similar to those found in previous large samples of extremely and ultra metal-poor stars. Finally, a large spread is found in the abundances of Sr and Ba for these stars, possibly influenced by enrichment from fast rotating stars in the early universe. © 2014. The American Astronomical Society. All rights reserved..

Bergemann M.,Max Planck Institute for Astrophysics | Hansen C.J.,Landessternwarte | Bautista M.,Western Michigan University | Ruchti G.,Max Planck Institute for Astrophysics
Astronomy and Astrophysics | Year: 2012

We investigate the statistical equilibrium of neutral and singly-ionized strontium in late-type stellar atmospheres. Particular attention is given to the completeness of the model atom, which includes new energy levels, transition probabilities, photoionization and electron-impact excitation cross-sections computed with the R-matrix method. The NLTE model is applied to the analysis of Sr I and Sr II lines in the spectra of the Sun, Procyon, Arcturus, and HD 122563, showing a significant improvement in the ionization balance compared to LTE line formation calculations, which predict abundance discrepancies of up to 0.5 dex. The solar Sr abundance is log ε = 2.93 ± 0.04 dex, in agreement with the meteorites. We present the grid of NLTE abundance corrections for Sr I and Sr II lines that covers a wide range of stellar parameters. © ESO, 2012.

Wollert M.,Max Planck Institute for Astronomy | Brandner W.,Max Planck Institute for Astronomy | Reffert S.,Landessternwarte | Schlieder J.E.,Max Planck Institute for Astronomy | And 3 more authors.
Astronomy and Astrophysics | Year: 2014

The K-type binary star HD 102077 was proposed as a candidate member of the TW Hydrae association (TWA) which is a young (5-15 Myr) moving group in close proximity (~50 pc) to the solar system. The aim of this work is to verify this hypothesis by different means. We first combine diffraction-limited observations from the ESO NTT 3.5 m telescope in SDSS-i′ and SDSS-z′ passbands (three epochs) and ESO 3.6 m telescope in H-band (one epoch) with literature data to obtain a new, amended orbit fit of the visual binary. We then estimate the spectral types of both components from the i′ - z′ colours and reanalyse the Hipparcos parallax and proper motion taking the orbital motion into account. Moreover, we use two high-resolution spectra of HD 102077 obtained with the fibre-fed optical echelle spectrograph FEROS at the MPG/ESO 2.2 m telescope to determine the radial velocity and the lithium equivalent width (EW) of the system. Finally, we use all the information to discuss the kinematic properties of HD 102077 and to estimate the age of the system. The orbital elements of the HD 102077 trajectory are well constrained and we derive a total system mass of 2.6 ± 0.8 M⊙ and a semi-major axis of 14.9 ± 1.6 AU. From the i′ - z′ colours we infer an integrated spectral type of K2V, and individual spectral types of K0 ± 1 and K5 ± 1 for primary and secondary, respectively. The radial velocity corrected for the orbital motion of the system is 17.6 ± 2 km s -1. Even though the parallax determination from the Hipparcos data is not influenced by the orbital motion, the proper motion changes to μα * cosδ = -137.84 ± 1.26 mas yr -1 and μδ = -33.53 ± 1.45 mas yr -1. With the resultant space motion, the probability of HD 102077 being a member of TWA is less than 1%. Furthermore, the lithium equivalent width of 200 ± 4 mÅ is consistent with an age between 30 Myr and 120 Myr and thus older than the predicted age of TWA. The comparison of HD 102077's temperature and luminosity to isochrones supports this result. In conclusion, HD 102077's age, galactic space motion, and position do not fit TWA or any other young moving group. © ESO, 2014.

Appenzeller I.,Landessternwarte | Bertout C.,CNRS Paris Institute of Astrophysics
Astronomy and Astrophysics | Year: 2013

Context. Because of the presence of rotation and accretion disks, classical T Tauri stars have symmetry planes that are normally inclined relative to the plane of the sky. The inclination angles affect the observed spectral properties of these objects. Aims. We study the influence of the inclination angles on classical T Tauri star spectra in an empirical manner. Methods. Published inclination angles, derived from the stellar photospheric rotation or from spatially resolved circumstellar disk observations, are compared with various observed spectral properties, and correlations are established and investigated. Results. Inclinations derived from the stellar rotation are found to be much less accurate than the published disk inclinations, and no significant correlations between spectral properties and inclinations based on rotation data could be detected. In contrast, significant correlations are found between the disk inclination angles and the apparent velocities observed for the forbidden emission lines and the wind absorption features of permitted lines. These data support the assumption of cone-like polar winds with opening angles smaller than ≈45. Other spectral features show weaker or no inclination dependence. Using these results, the true (deprojected) flow velocities of the polar winds are derived for the investigated sample of T Tauri stars. Deprojected wind-ejection velocities appear to differ by a factor of two among the stars in our sample, which spans a range of mass-loss rates from 10-10 M/yr to 3 × 10-7 M/yr. © 2013 ESO.

Hansen C.J.,Landessternwarte | Andersen A.C.,Copenhagen University | Christlieb N.,Landessternwarte
Astronomy and Astrophysics | Year: 2014

This work presents a large consistent study of molybdenum (Mo) and ruthenium (Ru) abundances in the Milky Way. These two elements are important nucleosynthetic diagnostics. In our sample of 71 Galactic metal-poor field stars, we detect Ru and/or Mo in 52 of these (59 including upper limits). The sample consists of high-resolution, high signal-to-noise spectra covering both dwarfs and giants from [Fe/H] = -0.63 down to -3.16. Thus we provide information on the behaviour of Mo I and Ru I at higher and lower metallicity than is currently known. In this sample we find a wide spread in the Mo and Ru abundances, which is typical of heavy elements. We confirm earlier findings of Mo enhanced stars around [Fe/H] = -1.5 and add ~15 stars both dwarfs and giants with normal (<0.3 dex) Mo and Ru abundances, as well as more than 15 Mo and Ru enhanced (>0.5 dex) stars to the currently known stellar sample. This indicates that several formation processes, in addition to high entropy winds, can be responsible for the formation of elements like Mo and Ru. We trace the formation processes by comparing Mo and Ru to elements (Sr, Zr, Pd, Ag, Ba, and Eu) with known formation processes. Based on how tight the two elements correlate with each other, we are able to distinguish if they share a common formation process and how important this contribution is to the element abundance. We find clear indications of contributions from several different formation processes, namely the p-process, and the slow (s-), and rapid (r-) neutron-capture processes. From these correlations we find that Mo is a highly convolved element that receives contributions from both the s-process and the p-process and less from the main and weak r-processes, whereas Ru is mainly formed by the weak r-process as is silver. We also compare our absolute elemental stellar abundances to relative isotopic abundances of presolar grains extracted from meteorites. Their isotopic abundances can be directly linked to the formation process (e.g. r-only isotopes) providing a unique comparison between observationally derived abundances and the nuclear formation process. The comparison to abundances in presolar grains shows that the r-/s-process ratios from the presolar grains match the total elemental chemical composition derived from metal-poor halo stars with [Fe/H] around -1.5 to -1.1 dex. This indicates that both grains and stars around and above [Fe/H] = -1.5 are equally (well) mixed and therefore do not support a heterogeneous presolar nebula. An inhomogeneous interstellar medium (ISM) should only be expected at lower metallicities. Our data, combined with the abundance ratios of presolar grains, could indicate that the AGB yields are less efficiently mixed into stars than into presolar grains. Finally, we detect traces of s-process material at [Fe/H] = -1.5, indicating that this process is at work at this and probably at even lower metallicity. © ESO, 2014.

Hansen C.J.,Landessternwarte
AIP Conference Proceedings | Year: 2012

Studying a range of old metal-poor stars provides information over cosmological timescales of our Galaxy. Such studies are indicative of the pristine gases and evolution of the Milky Way. Deriving stellar parameters and abundances from high-resolution observations of stars at various stellar evolution stages (including old dwarfs and RR Lyrae), allows us to use these abundances as tracers of an even earlier progenitor population. Here, we carry out a detailed abundance study of mainly heavy elements (Z > 38), i.e. neutron-capture elements, which we at low metallicities ([Fe/H] < -2.5) take as pure supernova type II products. A comparison of the derived abundances to type II supernova yields of heavy elements provides knowledge of the old stellar generations as well as properties of neutron-capture formation sites. © 2012 American Institute of Physics.

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