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Heidelberg, Germany

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.

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.

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.

Osborne J.P.,University of Leicester | Page K.L.,University of Leicester | Beardmore A.P.,University of Leicester | Bode M.F.,Liverpool John Moores University | And 12 more authors.
Astrophysical Journal | Year: 2011

Swift X-ray observations of the ∼60 day supersoft phase of the recurrent nova RS Ophiuchi (RS Oph) 2006 show the progress of nuclear burning on the white dwarf (WD) in exquisite detail. First seen 26 days after the optical outburst, this phase started with extreme variability likely due to variable absorption, although intrinsic WD variations are not excluded. About 32 days later, a steady decline in count rate set in. NLTE model atmosphere spectral fits during the supersoft phase show that the effective temperature of the WD increases from ∼65 eV to ∼90 eV during the extreme variability phase, falling slowly after about day 60 and more rapidly after day 80. The bolometric luminosity is seen to be approximately constant and close to Eddington from day 45 up to day 60, the subsequent decline possibly signaling the end of extensive nuclear burning. Before the decline, a multiply-periodic ∼35 s modulation of the soft X-rays was present and may be the signature of a nuclear fusion driven instability. Our measurements are consistent with a WD mass near the Chandrasekhar limit; combined with a deduced accumulation of mass transferred from its binary companion, this leads us to suggest that RS Oph is a strong candidate for a future supernova explosion. The main uncertainty now is whether the WD is the CO type necessary for a Type Ia supernova. This may be confirmed by detailed abundance analyses of spectroscopic data from the outbursts. © 2011. The American Astronomical Society. All rights reserved. Printedin the U.S.A.

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.

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