Close double degenerate binaries are one of the favoured progenitor channels for type Ia supernovae, but it is unclear how many suitable systems there are in the Galaxy. We report results of a large ...radial velocity survey for double degenerate (DD) binaries using the UVES spectrograph at the ESO VLT (ESO SN Ia Progenitor surveY – SPY). Exposures taken at different epochs are checked for radial velocity shifts indicating close binary systems. We observed 689 targets classified as DA white dwarfs (displaying hydrogen-rich atmospheres), of which 46 were found to possess a cool companion. We measured radial velocities (RV) of the remaining 643 DA white dwarfs. We managed to secure observations at two or more epochs for 625 targets, supplemented by eleven objects meeting our selection criteria from literature. The data reduction and analysis methods applied to the survey data are described in detail. The sample contains 39 double degenerate binaries, only four of which were previously known. Twenty are double-lined systems, in which features from both components are visible, the other 19 are single-lined binaries. We provide absolute RVs transformed to the heliocentric system suitable for kinematic studies. Our sample is large enough to sub-divide by mass: 16 out of 44 low mass targets (≤0.45 M⊙) are detected as DDs, while just 23 of the remaining 567 targets with multiple spectra and mass > 0.45 M⊙ are double. The detected fraction amongst the low mass objects (36.4 ± 7.3%) is significantly higher than for the higher-mass, carbon-oxygen core dominated part of the sample (3.9 ± 0.8%), but it is much lower than expected from the detection efficiency for companion masses of 0.05 M⊙ or higher and a 100% binary fraction. This suggests either companion stars of with a mass below 0.05 M⊙ or some of the low mass white dwarfs are single.
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Data from complex modern astronomical instruments often consist of a large number of different science and calibration files, and their reduction requires a variety of software tools. The execution ...chain of the tools represents a complex workflow that needs to be tuned and supervised, often by individual researchers that are not necessarily experts for any specific instrument. The efficiency of data reduction can be improved by using automatic workflows to organise data and execute a sequence of data reduction steps. To realize such efficiency gains, we designed a system that allows intuitive representation, execution and modification of the data reduction workflow, and has facilities for inspection and interaction with the data. Reflex includes novel concepts to increase the efficiency of astronomical data processing. While Reflex is a specific implementation of astronomical scientific workflows within the Kepler workflow engine, the overall design choices and methods can also be applied to other environments for running automated science workflows.
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Context. While the near-infrared wavelength regime is becoming more and more important for astrophysics there is a marked lack of spectrophotometric standard star data that would allow the flux ...calibration of such data. Furthermore, flux calibrating medium- to high-resolution échelle spectroscopy data is challenging even in the optical wavelength range, because the available flux standard data are often too coarsely sampled. Aims. We will provide standard star reference data that allow users to derive response curves from 300 nm to 2500 nm for spectroscopic data of medium to high resolution, including those taken with échelle spectrographs. In addition we describe a method to correct for moderate telluric absorption without the need of observing telluric standard stars. Methods. As reference data for the flux standard stars we use theoretical spectra derived from stellar model atmospheres. We verify that they provide an appropriate description of the observed standard star spectra by checking for residuals in line cores and line overlap regions in the ratios of observed (X-shooter) spectra to model spectra. The finally selected model spectra are then corrected for remaining mismatches and photometrically calibrated using independent observations. The correction of telluric absorption is performed with the help of telluric model spectra. Results. We provide new, finely sampled reference spectra without telluric absorption for six southern flux standard stars that allow the users to flux calibrate their data from 300 nm to 2500 nm, and a method to correct for telluric absorption using atmospheric models.
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An MK (Morgan-Keenan)-like system of spectral classification for hot subdwarfs is presented. We find that a three-dimensional spectral type, consisting of a “spectral” class, a “luminosity” class, ...and a “helium” class, is necessary to classify the sdO (subdwarf O) and sdB (subdwarf B) stars. In addition, the helium-strong stars appear to form two parallel spectral sequences: one showing strong lines of CII, CIII, or CIV, and the other with these same lines weak or absent. We also give a preliminary calibration of the new spectral types in terms of effective temperature, surface gravity, and surface helium-to-hydrogen abundance ratio, and show the relation between the new spectral types and the natural groups defined by the Palomar-Green (PG) survey.
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Context. In the framework of the Virtual Observatory (VO), the German Astrophysical VO (GAVO) developed the registered service TheoSSA (Theoretical Stellar Spectra Access). It provides easy access to ...stellar spectral energy distributions (SEDs) and is intended to ingest SEDs calculated by any model-atmosphere code, generally for all effective temperatures, surface gravities, and elemental compositions. We will establish a database of SEDs of flux standards that are easily accessible via TheoSSA's web interface. Aims. The OB-type subdwarf Feige 110 is a standard star for flux calibration. State-of-the-art non-local thermodynamic equilibrium stellar-atmosphere models that consider opacities of species up to trans-iron elements will be used to provide a reliable synthetic spectrum to compare with observations. Methods. In case of Feige 110, we demonstrate that the model reproduces not only its overall continuum shape from the far-ultraviolet (FUV) to the optical wavelength range but also the numerous metal lines exhibited in its FUV spectrum. Results. We present a state-of-the-art spectral analysis of Feige 110. We determined , logg = 6.00 + or - 0.20, and the abundances of He, N, P, S, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, and Ge. Ti, V, Mn, Co, Zn, and Ge were identified for the first time in this star. Upper abundance limits were derived for C, O, Si, Ca, and Sc. Conclusions. The TheoSSA database of theoretical SEDs of stellar flux standards guarantees that the flux calibration of astronomical data and cross-calibration between different instruments can be based on models and SEDs calculated with state-of-the-art
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Images taken with modern detectors require calibration via flat fielding to obtain the same flux scale across the whole image. One method for obtaining the best possible flat-fielding accuracy is to ...derive a photometric model from dithered stellar observations. A large variety of effects have been taken into account in such modelling. Recently, Moehler et al. discovered systematic variations in available flat frames for the European Southern Observatory's FOcal Reducer and low-dispersion Spectrographs (FORS) instrument that change with the orientation of the projected image on the sky. The effect on photometry is large compared to other systematic effects that have already been taken into account. In this paper, we present a correction method for this effect: a generalization of the fitting procedure of Bramich & Freudling to include a polynomial representation of rotating flat-fields. We then applied the method to the specific case of FORS2 photometric observations of a series of standard star fields, and provide parametrized solutions that can be applied by the users. We found polynomial coefficients to describe the static and rotating large-scale systematic flat-field variations across the FORS2 field of view. Applying these coefficients to FORS2 data, the systematic changes in the flux scale across FORS2 images can be improved by ∼1 to ∼2 per cent of the total flux. This represents a significant improvement in the era of large-scale surveys, which require homogeneous photometry at the 1 per cent level or better.
Aims. The present study was conducted to determine the optical extinction curve for Cerro Paranal under typical clear-sky observing conditions, with the purpose of providing the community with a ...function to be used to correct the observed spectra, with an accuracy of 0.01 mag airmass-1. Additionally, this work was meant to analyze the variability of the various components, to derive the main atmospheric parameters, and to set a term of reference for future studies, especially in view of the construction of the Extremely Large Telescope on the nearby Cerro Armazones. Methods. The extinction curve of Paranal was obtained through low-resolution spectroscopy of 8 spectrophotometric standard stars observed with FORS1 mounted at the 8.2 m Very Large Telescope, covering a spectral range 3300–8000 Å. A total of 600 spectra were collected on more than 40 nights distributed over six months, from October 2008 to March 2009. The average extinction curve was derived using a global fit algorithm, which allowed us to simultaneously combine all the available data. The main atmospheric parameters were retrieved using the LBLRTM radiative transfer code, which was also utilised to study the impact of variability of the main molecular bands of O2, O3, and H2O, and to estimate their column densities. Results. In general, the extinction curve of Paranal appears to conform to those derived for other astronomical sites in the Atacama desert, like La Silla and Cerro Tololo. However, a systematic deficit with respect to the extinction curve derived for Cerro Tololo before the El Chichón eruption is detected below 4000 Å. We attribute this downturn to a non standard aerosol composition, probably revealing the presence of volcanic pollutants above the Atacama desert. An analysis of all spectroscopic extinction curves obtained since 1974 shows that the aerosol composition has been evolving during the last 35 years. The persistence of traces of non meteorologic haze suggests the effect of volcanic eruptions, like those of El Chichón and Pinatubo, lasts several decades. The usage of the standard CTIO and La Silla extinction curves implemented in IRAF and MIDAS produce systematic over/under-estimates of the absolute flux.
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Aims. We analyze the reddening, surface helium abundance and spectroscopic mass of 115 blue horizontal branch (HB) and blue hook (BH) stars in ω Centauri, spanning the cluster HB from the blue edge ...of the instability strip (Teff = 8000 K) to BH objects with Teff ≈ 50 000 K. Methods. The temperatures, gravities, and surface helium abundances were measured on low-resolution spectra fitting the Balmer and helium lines with a grid of synthetic spectra. From these parameters, the mass and reddening were estimated. Results. The mean cluster reddening is E(B − V) = 0.115 ± 0.004, in good agreement with previous estimates, but we evidence a pattern of differential reddening in the cluster area. The stars in the western half are more reddened than in the southwest quadrant by 0.03−0.04 mag. We find that the helium abundances measured on low-resolution spectra are systematically higher by 0.20−0.25 dex than the measurements based on higher resolution. No difference in surface helium abundance is detected between HB stars in ω Centauri and in three comparison clusters, and the stars in the range 11 500−20 000 K follow a trend with temperature, which probably reflects a variable efficiency of the diffusion processes. There is mild evidence that two families of extreme HB (EHB) cluster stars (Teff ≥ 20 000 K) could exist, as observed in the field, with ~15% of the objects being helium depleted by a factor of ten with respect to the main population. The distribution of helium abundance above 30 000 K is bimodal, but we detect a fraction of He-poor objects lower than previous investigations. The observations are consistent with these being stars evolving off the HB. Their spatial distribution is not uniform across the cluster, but this asymmetric distribution is only marginally significative. We also find that EHB stars with anomalously high spectroscopic mass could be present in ω Centauri, as previously found in other clusters. The derived temperature-color relation reveals that the HB stars hotter than ~11 000 K are fainter than the expectations of the canonical models in the U band, while no anomaly is detected in B and V. This behavior, not observed in NGC 6752, is a new peculiarity of ω Centauri HB stars. More investigation is needed to reach a full comprehension of this complex observational picture.
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We found that the script to determine the masses of the stars contains two errors. This script and a related one have been used to determine masses of globular cluster stars and distances to field ...stars in 12 papers published between 1990 and 2014. While the numerical values need to be revised none of the conclusions are affected. We provide the updated numerical values and figures for all 12 publications here. In addition we describe the effects on those refereed publications that used the distances to the field stars.
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We use high-resolution spectra obtained with the ESO Very Large Telescope to measure surface abundance patterns of 43 blue straggler stars (BSSs) in 47 Tuc. We discovered that a subpopulation of BSSs ...shows a significant depletion of carbon and oxygen with respect to the dominant population. This evidence would suggest the presence of CNO burning products on the BSS surface coming from a deeply peeled parent star, as expected in the case of a mass transfer process. This is the first detection of a chemical signature clearly pointing to a specific BSS formation process in a globular cluster.
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