Florian Kerber

Sulphur and zinc abundances in Galactic halo stars revisited

Aims. Based on a new set of sulphur abundances in very metal-poor stars and an improved analysis of previous data, we aim at resolving current discrepancies on the trend of S/Fe vs. Fe/H and thereby gain better insight into the nucleosynthesis of sulphur. The trends of Zn/Fe and S/Zn will also be studied. Methods. High resolution VLT/UVES spectra of 40 main-sequence stars with −3.3 < [Fe/H] < −1.0 are used to derive S abundances from the weak λ8694. 6S i line and the stronger λλ9212.9, 9237. 5p air of Si lines. For one star, the S abundance is also derived from the S i triplet at 1.046 µm recently observed with the VLT infrared echelle spectrograph CRIRES. Fe and Zn abundances are derived from lines in the blue part of the UVES spectra, and effective temperatures are obtained from the profile of the Hβ line. Results. Comparison of sulphur abundances from the weak and strong S i lines provides important constraints on non-LTE effects. The high sulphur abundances reported by others for some metal-poor stars are not confirmed; instead, when taking non-LTE corrections into account, the Galactic halo stars distribute around a plateau at [S/Fe] ∼ +0.2 dex with a scatter of 0.07 dex only. [Zn/Fe] is close to zero for metallicities in the range −2.0 < [Fe/H] < −1.0 but increases to a level of [Zn/Fe] ∼ +0. 1t o+0.2 dex in the range −2.7 < [Fe/H] < −2.0. At still lower metallicities [Zn/Fe] rises steeply to a value of [Zn/Fe] ∼ +0. 5d ex at [Fe/H] = −3.2. Conclusions. The trend of S/Fe vs. Fe/H corresponds to the trends of Mg/Fe, Si/Fe, and Ca/Fe and indicates that sulphur in Galactic halo stars has been made by α-capture processes in massive SNe. The observed scatter in S/Fe is much smaller than predicted from current stochastic models of the chemical evolution of the early Galaxy, suggesting that either the models or the calculated yields of massive SNe should be revised. We also examine the behaviour of S/Zn and find that departures from the solar ratio are significantly reduced at all metallicities if non-LTE corrections to the abundances of these two elements are adopted. This effect, if confirmed, would reduce the usefulness of the S/Zn ratio as a diagnostic of past star-formation activity, but would bring closer together the values measured in damped Lyman-alpha systems and in Galactic stars.

The X-shooter pipeline

The X-shooter data reduction pipeline, as part of the ESO-VLT Data Flow System, provides recipes for Paranal Science Operations, and for Data Product and Quality Control Operations at Garching headquarters. At Paranal, it is used for the quick-look data evaluation. The pipeline recipes can be executed either with EsoRex at the command line level or through the Gasgano graphical user interface. The recipes are implemented with the ESO Common Pipeline Library (CPL). X-shooter is the first of the second generation of VLT instruments. It makes possible to collect in one shot the full spectrum of the target from 300 to 2500 nm, subdivided in three arms optimised for UVB, VIS and NIR ranges, with an efficiency between 15% and 35% including the telescope and the atmosphere, and a spectral resolution varying between 3000 and 17,000. It allows observations in stare, offset modes, using the slit or an IFU, and observing sequences nodding the target along the slit. Data reduction can be performed either with a classical approach, by determining the spectral format via 2D-polynomial transformations, or with the help of a dedicated instrument physical model to gain insight on the instrument and allowing a constrained solution that depends on a few parameters with a physical meaning. In the present paper we describe the steps of data reduction necessary to fully reduce science observations in the different modes with examples on typical data calibrations and observations sequences.

Molecfit: A general tool for telluric absorption correction. . II. Quantitative evaluation on ESO-VLT/X-Shooterspectra

Context. Absorption by molecules in the Earth’s atmosphere strongly affects ground-based astronomical observations. The resulting absorption line strength and shape depend on the highly variable physical state of the atmosphere, i.e. pressure, temperature, and mixing ratio of the different molecules involved. Usually, supplementary observations of so-called telluric standard stars (TSS) are needed to correct for this effect, which is expensive in terms of telescope time. We have developed the software package molecfit to provide synthetic transmission spectra based on parameters obtained by fitting narrow ranges of the observed spectra of scientific objects. These spectra are calculated by means of the radiative transfer code LBLRTM and an atmospheric model. In this way, the telluric absorption correction for suitable objects can be performed without any additional calibration observations of TSS. Aims. We evaluate the quality of the telluric absorption correction using molecfit with a set of archival ESO-VLT/X-Shooter visible and near-infrared spectra. Methods. Thanks to the wavelength coverage from the U to the K band, X-Shooter is well suited to investigate the quality of the telluric absorption correction with respect to the observing conditions, the instrumental set-up, input parameters of the code, the signal-to-noise of the input spectrum, and the atmospheric profiles. These investigations are based on two figures of merit, Ioff and Ires, that describe the systematic offsets and the remaining small-scale residuals of the corrections. We also compare the quality of the telluric absorption correction achieved with molecfit to the classical method based on a telluric standard star. Results. The evaluation of the telluric correction with molecfit shows a convincing removal of atmospheric absorption features. The comparison with the classical method reveals that molecfit performs better because it is not prone to the bad continuum reconstruction, noise, and intrinsic spectral features introduced by the telluric standard star. Conclusions. Fitted synthetic transmission spectra are an excellent alternative to the correction based on telluric standard stars. Moreover, molecfit offers wide flexibility for adaption to various instruments and observing sites.

Galactic Planetary Nebulae and their central stars - I. An accurate and homogeneous set of coordinates

We have used the 2nd generation of the Guide Star Catalogue (GSC-II) as a reference astrometric catalogue to compile the positions of 1086 Galactic Planetary Nebulae (PNe) listed in the Strasbourg ESO Catalogue (SEC), its supplement and the version 2000 of the Catalogue of Planetary Nebulae. This constitutes about 75% of all known PNe. For these PNe, the ones with a known central star (CS) or with a small diameter, we have derived coordinates with an absolute accuracy of0: 00 35 in each coordinate, which is the intrinsic astrometric precision of the GSC-II. For another 226, mostly extended, objects without a GSC-II counterpart we give coordinates based on the second epoch Digital Sky Survey (DSS-II). While these coordinates may have systematic osets relative to the GSC-II of up to 5 arcsecs, our new coordinates usually represent a significant improvement over the previous catalogue values for these large objects. This is the first truly homogeneous compilation of PNe positions over the whole sky and the most accurate one available so far.

High-resolution FUSE and HST ultraviolet spectroscopy of the white dwarf central star of Sh 2-216

Context. We perform a comprehensive spectral analysis of LS V +46°21  in order to compare its photospheric properties to theoretical predictions from stellar evolution theory as well as from diffusion calculations. Aims. LS V +46°21 is the DAO-type central star of the planetary nebula Sh 2-216. High-resolution, high- S/N ultraviolet observations obtained with FUSE and STIS aboard the HST as well as the optical spectrum have been analyzed in order to determine the photospheric parameters and the spectroscopic distance. Methods. We performed a detailed spectral analysis of the ultraviolet and optical spectrum by means of state-of-the-art NLTE model-atmosphere techniques. Results. From the N IV – N V, O IV – O VI, Si IV – Si V, and Fe V – Fe VII ionization equilibria, we determined an effective temperature of

EPICS: direct imaging of exoplanets with the E-ELT

(95\pm 2)\,{\rm kK}

V605 Aquilae: The Older Twin of Sakurai’s Object*

with high precision. The surface gravity is

The 2007 ESO Instrument Calibration Workshop

\log g~=~6.9\pm 0.2

Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage

. An unexplained discrepancy appears between the spectroscopic distance

A dense disk of dust around the born-again Sakurai's object

d = 224^{+46}_{-58}\,{\rm pc}