S. Van Eck

HERMES: a high-resolution fibre-fed spectrograph for the Mercator telescope

The HERMES high-resolution spectrograph project aims at exploiting the specific potential of small but flexible telescopes in observational astrophysics. The optimised optical design of the spectrograph is based on the well-proven concept of white-pupil beam folding for high-resolution spectroscopy. In this contribution we present the complete project, including the spectrograph design and procurement details, the telescope adaptor and calibration unit, the detector system, as well as the optimised data-reduction pipeline. We present a detailed performance analysis to show that the spectrograph performs as specified both in optical quality and in total efficiency. With a spectral resolution of 85 000 (63 000 for the low-resolution fibre), a spectral coverage from 377 to 900 nm in a single exposure and a peak efficiency of 28%, HERMES proves to be an ideal instrument for building up time series of high-quality data of variable (stellar) phenomena.

Gaia FGK benchmark stars: Metallicity

Context. To calibrate automatic pipelines that determine atmospheric parameters of stars, one needs a sample of stars, or “benchmark stars”, with well-defined parameters to be used as a reference. Aims. We provide detailed documentation of the iron abundance determination of the 34 FGK-type benchmark stars that are selected to be the pillars for calibration of the one billion Gaia stars. They cover a wide range of temperatures, surface gravities, and metallicities. Methods. Up to seven different methods were used to analyze an observed spectral library of high resolutions and high signal-to-noise ratios. The metallicity was determined by assuming a value of effective temperature and surface gravity obtained from fundamental relations; that is, these parameters were known a priori and independently from the spectra. Results. We present a set of metallicity values obtained in a homogeneous way for our sample of benchmark stars. In addition to this value, we provide detailed documentation of the associated uncertainties. Finally, we report a value of the metallicity of the cool giant ψ Phe for the first time.

MESS (Mass-loss of Evolved StarS), a Herschel key program

MESS (Mass-loss of Evolved StarS) is a guaranteed time key program that uses the PACS and SPIRE instruments on board the Herschel space observatory to observe a representative sample of evolved stars, that include asymptotic giant branch (AGB) and post-AGB stars, planetary nebulae and red supergiants, as well as luminous blue variables, Wolf-Rayet stars and supernova remnants. In total, of order 150 objects are observed in imaging and about 50 objects inspectroscopy. This paper describes the target selection and target list, and the observing strategy. Key science projects are described, and illustrated using results obtained during Herschel’s science demonstration phase. Aperture photometry is given for the 70 AGB and post-AGB stars observed up to October 17, 2010, which constitutes the largest single uniform database of far-IR and sub-mm fluxes for late-type stars.

CS 30322--023: An Ultra Metal-Poor TP-AGB Star?

Context. The remarkable properties of CS 30322-023 became apparent during the course of a high-resolution spectroscopic study of a sample of 23 carbon-enhanced, metal-poor (CEMP) stars. Aims. This sample is studied in order to gain a better understanding of s- and r-process nucleosynthesis at low metallicity, an d to investigate the role of duplicity. Methods. High-resolution UVES spectra have been obtained, and abundances are derived using 1-D, plane-parallel OSMARCS models under the LTE hypothesis. The derived atmospheric parameters and observed abundances are compared to evolutionary tracks and nucleosynthesis predictions to infer the evolutionary status of CS 30322-023. Results. CS 30322-023 is remarkable in having the lowest surface gravity (log g≤ −0.3) among the metal-poor stars studied to date. As a result of its rather low temperature (4100 K), abundances could be derived for 35 chemical elements; the abundance pattern of CS 30322-023 is one of the most well-specified of all known extremely metal -poor stars. With [Fe/H]=−3.5, CS 30322-023 is the most metal-poor star to exhibit a clear s-process signature, and the most metal-poor “lead star” known. The available evidence indicates that CS 30322-023 is presently a thermally-pulsing asymptotic giant branch (TP-AGB) star, with no strong indication of binarity thus far (although a s ignal of period 192 d is clearly present in the radial-velocity data, this is like ly due to pulsation of the stellar envelope). Low-mass TP-AGB stars are not expected to be exceedingly rare in a magnitude-limited sample such as the HK survey, because their high luminosities make it possible to sample them over a very large volume. The strong N overabundance and the low 12 C/ 13 C ratio (4) in this star is typical of the operation of the CN cycle. Coupled with a Na overabundance and the absence of a strong C overabundance, this pattern seems to imply that hot-bottom burning operated in this star, which should then have a mass of at least 2 M⊙. However, the luminosity associated with this mass would put the star at a distance of about 50 kpc, in the outskirts of the galactic ha lo, where no recent star formation is expected to have taken place. We explore alternative scenarios in which the observed abundance pattern results from some mixing mechanism yet to be identified oc curring in a single low-metallicity 0.8 M⊙ AGB star, or from pollution by matter from an intermediate-mass AGB companion which has undergone hot-bottom burning. We stress, however, that our abundances may be subject to uncertainties due to NLTE or 3D granulation effects which were not taken into consideration.

CH in stellar atmospheres: an extensive linelist

The advent of high-resolution spectrographs and detailed stellar atmosphere modelling has strengthened the need for accurate molecular data. Carbon-enhanced metal-poor (CEMP) stars spectra are interesting objects with which to study transitions from the CH molecule. We combine programs for spectral analysis of molecules and stellar-radiative transfer codes to build an extensive CH linelist, including predissociation broadening as well as newly identified levels. We show examples of strong predissociation CH lines in CEMP stars, and we stress the important role played by the CH features in the Bond-Neff feature depressing the spectra of barium stars by as much as 0.2 mag in the λ = 3000-5500 A range. Because of the extreme thermodynamic conditions prevailing in stellar atmospheres (compared to the laboratory), molecular transitions with high energy levels can be observed. Stellar spectra can thus be used to constrain and improve molecular data.

Chemically tagging the Hyades stream: does it partly originate from the Hyades cluster?

The Hyades stream has long been thought to be a dispersed vestige of the Hyades cluster. However, recent analyses of the parallax distribution, of the mass function, and of the actionspace distribution of stream stars have shown it to be rather composed of orbits trapped at a resonance of a density disturbance. This resonant scenario should leave a clearly different signature in the element abundances of stream stars than the dispersed cluster scenario, since the Hyades cluster is chemically homogeneous. Here, we study the metallicity as well as the element abundances of Li, Na, Mg, Fe, Zr, Ba, La, Ce, Nd and Eu for a random sample of stars belonging to the Hyades stream, and compare them with those of stars from the Hyades cluster. From this analysis: (i) we independently confirm that the Hyades stream cannot be solely composed of stars originating in the Hyades cluster; (ii) we show that some stars (namely 2/21) from the Hyades stream nevertheless have abundances compatible with an origin in the cluster; (iii) we emphasize that the use of Li as a chemical tag of the cluster origin of mainsequence stars is very efficient in the range 5500 K ≤ T eff ≤ 6200 K, since the Li sequence in the Hyades cluster is very tight, while at the same time spanning a large abundance range; (iv) we show that, while this evaporated population has a metallicity excess of ∼0.2 dex with respect to the local thin-disc population, identical to that of the Hyades cluster, the remainder of the Hyades stream population has still a metallicity excess of ∼0.06–0.15 dex, consistent with an origin in the inner Galaxy and (v) we show that the Hyades stream can be interpreted as an inner 4:1 resonance of the spiral pattern: this then also reproduces an orbital family compatible with the Sirius stream, and places the origin of the Hyades stream up to 1 kpc inwards from the solar radius, which might explain the observed metallicity excess of the stream population.

Gaia FGK benchmark stars: abundances of α and iron-peak elements

Context. In the current era of large spectroscopic surveys of the Milky Way, reference stars for calibrating astrophysical parameters and chemical abundances are of paramount importance. Aims. We determine elemental abundances of Mg, Si, Ca, Sc, Ti, V, Cr, Mn, Co, and Ni for our predefined set of Gaia FGK benchmark stars. Methods. By analysing high-resolution spectra with a high signal-to-noise ratio taken from several archive datasets, we combined results of eight different methods to determine abundances on a line-by-line basis. We performed a detailed homogeneous analysis of the systematic uncertainties, such as differential versus absolute abundance analysis. We also assessed errors that are due to non-local thermal equilibrium and the stellar parameters in our final abundances. Results. Our results are provided by listing final abundances and the different sources of uncertainties, as well as line-by-line and method-by-method abundances. Conclusions. The atmospheric parameters of the Gaia FGK benchmark stars are already being widely used for calibration of several pipelines that are applied to different surveys. With the added reference abundances of ten elements, this set is very suitable for calibrating the chemical abundances obtained by these pipelines.

The Gaia-ESO survey : Processing FLAMES-UVES spectra

The Gaia-ESO Survey is a large public spectroscopic survey that aims to derive radial velocities and fundamental parameters of about 10(5) Milky Way stars in the field and in clusters. Observations are carried out with the multi-object optical spectrograph FLAMES, using simultaneously the medium-resolution (R similar to 20 000) GIRAFFE spectrograph and the high-resolution (R similar to 47 000) UVES spectrograph. In this paper we describe the methods and the software used for the data reduction, the derivation of the radial velocities, and the quality control of the FLAMES-UVES spectra. Data reduction has been performed using a workflow specifically developed for this project. This workflow runs the ESO public pipeline optimizing the data reduction for the Gaia-ESO Survey, automatically performs sky subtraction, barycentric correction and normalisation, and calculates radial velocities and a first guess of the rotational velocities. The quality control is performed using the output parameters from the ESO pipeline, by a visual inspection of the spectra and by the analysis of the signal-to-noise ratio of the spectra. Using the observations of the first 18 months, specifically targets observed multiple times at different epochs, stars observed with both GIRAFFE and UVES, and observations of radial velocity standards, we estimated the precision and the accuracy of the radial velocities. The statistical error on the radial velocities is sigma similar to 0.4 km s(-1) and is mainly due to uncertainties in the zero point of the wavelength calibration. However, we found a systematic bias with respect to the GIRAFFE spectra (similar to 0.9 km s(-1)) and to the radial velocities of the standard stars (similar to 0.5 km s(-1)) retrieved from the literature. This bias will be corrected in the future data releases, when a common zero point for all the set-ups and instruments used for the survey is be established.

Mass ratio from Doppler beaming and Rømer delay versus ellipsoidal modulation in the Kepler data of KOI-74

We present a light curve analysis and radial velocity study of KOI-74, an eclipsing A star\(+\) white dwarf binary with a 5.2 day orbit. Aside from new spectroscopy covering the orbit of the system, we used 212 days of publicly available Kepler observations and present the first complete light curve fitting to these data, modelling the eclipses and transits, ellipsoidal modulation, reflection, and Doppler beaming. Markov Chain Monte Carlo simulations are used to determine the system parameters and uncertainty estimates. Our results are in agreement with earlier studies, except that we find an inclination of \(87.0\pm 0.4^\circ \), which is significantly lower than the previously published value. The altered inclination leads to different values for the relative radii of the two stars and therefore also the mass ratio deduced from the ellipsoidal modulations seen in this system. We find that the mass ratio derived from the radial velocity amplitude (\(q=0.104\pm 0.004\)) disagrees with that derived from the ellipsoidal modulation (\(q=0.052\pm 0.004\) assuming corotation). This mismatch was found before, but with our smaller inclination, the discrepancy is even larger than previously reported. Accounting for the rapid rotation of the A-star, instead of assuming corotation with the binary orbit, is found to increase the discrepancy even further by lowering the mass ratio to \(q=0.047\pm 0.004\). These results indicate that one has to be extremely careful in using the amplitude of an ellipsoidal modulation signal in a close binary to determine the mass ratio, when a proof of corotation is not firmly established. The same problem could arise whenever an ellipsoidal modulation amplitude is used to derive the mass of a planet orbiting a host star that is not in corotation with the planet’s orbit. The radial velocities that can be inferred from the detected Doppler beaming in the light curve are found to be in agreement with our spectroscopic radial velocity determination. We also report the first measurement of Romer delay in a light curve of a compact binary. This delay amounts to \(-56\pm 17\) s and is consistent with the mass ratio derived from the radial velocity amplitude. The firm establishment of this mass ratio at \(q=0.104\pm 0.004\) leaves little doubt that the companion of KOI-74 is a low mass white dwarf.

A CORAVEL radial-velocity monitoring of giant Ba and S stars: Spectroscopic orbits and intrinsic variations. I. ?

With the aim of deriving the binary frequency among Ba and S stars, 56 new spectroscopic orbits (46 and 10, respectively) have been derived for these chemically- peculiar red giants monitored with the CORAVEL spec- trometers. These orbits are presented in this paper (38 or- bits) and in a companion paper (Udry et al. 1998, Paper II; 18 orbits). The results for 12 additional long-period binary stars (6 and 6, respectively), for which only minimum pe- riods (generally exceeding 10 y) can be derived, are also presented here (10) and in Paper II (2). The global analy- sis of this material, with a few supplementary orbits from the literature, is presented in Jorissen et al. (1998). For the subsample of Mira S, SC and (Tc-poor) C stars showing intrinsic radial-velocity variations due to at- mospheric phenomena, orbital solutions (when available) have been retained if the velocity and photometric pe- riods are dierent (3 stars). However, it is emphasized that these orbit determinations are still tentative. Three stars have been found with radial-velocity variations syn- chronous with the light variations. Pseudo-orbital solu- tions have been derived for those stars. In the case of RZ Peg, a line-doubling phenomenon is observed near maxi- mum light, and probably reflects the shock wave propa- gating through the photosphere.