Search for vertical stratification of metals in atmospheres of blue horizontal-branch stars

The observed abundance peculiarities of many chemical species relative to the expected cluster metallicity in blue horizontal-branch (BHB) stars presumably appear as a result of atomic diffusion in the photosphere. The slow rotation (typically $v\sin{i}<$ 10 km s$^{-1}$) of BHB stars with effective temperatures $T_{\rm eff}>$ 11,500 K supports this idea since the diffusion mechanism is only effective in a stable stellar atmosphere. In this work we search for observational evidence of vertical chemical stratification in the atmospheres of six hot BHB stars: B84, B267 and B279 in M15 and WF2-2541, WF4-3085 and WF4-3485 in M13. We undertake an abundance stratification analysis of the stellar atmospheres of the aforementioned stars, based on acquired Keck HIRES spectra. We have found from our numerical simulations that three stars (B267, B279 and WF2-2541) show clear signatures of the vertical stratification of iron whose abundance increases toward the lower atmosphere, while the other two stars (B84 and WF4-3485) do not. For WF4-3085 the iron stratification results are inconclusive. B267 also shows a signature of titanium stratification. Our estimates for radial velocity, $v\sin{i}$ and overall iron, titanium and phosphorus abundances agree with previously published data for these stars after taking the measurement errors into account. The results support the hypothesis regarding the efficiency of atomic diffusion in the stellar atmospheres of BHB stars with $T_{\rm eff}>$ 11,500 K.Comment: 8 pages, 12 figures, accepted for publication in Astronomy and Astrophysic

CCD Photometry of the Classic Second Parameter Globular Clusters M3 and M13

We present high-precision V, B-V color-magnitude diagrams (CMDs) for the classic second parameter globular clusters M3 and M13 from wide-field deep CCD photometry. The data for the two clusters were obtained during the same photometric nights with the same instrument, allowing us to determine accurate relative ages. Based on a differential comparison of the CMDs using the Delta (B-V) method, an age difference of 1.7 +/- 0.7 Gyr is obtained between these two clusters. We compare this result with our updated horizontal-branch (HB) population models, which confirm that the observed age difference can produce the difference in HB morphology between the clusters. This provides further evidence that age is the dominant second parameter that influences HB morphology.Comment: 27 pages, 12 figures, Accepted for publication in A

An Overview of the Rotational Behavior of Metal--Poor Stars

The present paper describes the behavior of the rotational velocity in metal--poor stars ([Fe/H]<-0.5 dex) in different evolutionary stages, based on Vsini values from the literature. Our sample is comprised of stars in the field and some Galactic globular clusters, including stars on the main sequence, the red giant branch (RGB), and the horizontal branch (HB). The metal--poor stars are, mainly, slow rotators, and their Vsini distribution along the HR diagram is quite homogeneous. Nevertheless, a few moderate to high values of Vsini are found in stars located on the main sequence and on the HB. We show that the overall distribution of Vsini values is basically independent of metallicity for the stars in our sample. In particular, the fast-rotating main sequence stars in our sample present similar rotation rates as their metal-rich counterparts, suggesting that some of them may actually be fairly young, in spite of their low metallicity, or else that at least some of them would be better classified as blue straggler stars. We do not find significant evidence of evolution in Vsini values as a function of position on the RGB; in particular, we do not confirm previous suggestions that stars close to the RGB tip rotate faster than their less evolved counterparts. While the presence of fast rotators among moderately cool blue HB stars has been suggested to be due to angular momentum transport from a stellar core that has retained significant angular momentum during its prior evolution, we find that any such transport mechanisms must likely operate very fast as the star arrives on the zero-age HB (ZAHB), since we do not find a link between evolution off the ZAHB and Vsini values. We present an extensive tabulation of all quantities discussed in this paper, including rotation velocities, temperatures, gravitieComment: 22 pages, 10 figure

The virtual observatory service TheoSSA: Establishing a database of synthetic stellar flux standards. II. NLTE spectral analysis of the OB-type subdwarf Feige 110

In the framework of the Virtual Observatory (VO), the German Astrophysical Virtual Observatory (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 temperature, surface gravities, and elemental compositions. We will establish a database of SEDs of flux standards that are easily accessible via TheoSSA's web interface. The OB-type subdwarf Feige 110 is a standard star for flux calibration. State-of-the-art non-local thermodynamic equilibrium (NLTE) 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. 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. We present a state-of-the-art spectral analysis of Feige 110. We determined $T_\mathrm{eff} = 47\,250 \pm 2000\,\mathrm{K}$, $\log g = 6.00 \pm 0.20$ 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. 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 model-atmosphere codes.Comment: 19 pages, 7 figure

Automated data reduction workflows for astronomy

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 the 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. The European Southern Observatory (ESO) has developed Reflex, an environment to automate data reduction workflows. Reflex is implemented as a package of customized components for the Kepler workflow engine. Kepler provides the graphical user interface to create an executable flowchart-like representation of the data reduction process. Key features of Reflex are a rule-based data organiser, infrastructure to re-use results, thorough book-keeping, data progeny tracking, interactive user interfaces, and a novel concept to exploit information created during data organisation for the workflow execution. 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.Comment: 12 pages, 7 figure

The ESO supernovae type Ia progenitor survey (SPY) : The radial velocities of 644 DA white dwarfs

Accepted for publication in A&A. 59 pages.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 (displaying hydrogen-rich atmospheres), of which 46 turned out 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. 20 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 Msun are double. Although 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%), it is lower than the detection efficiency based upon companion star masses >= 0.05 Msun. This suggests either companion stars of mass <0.05 Msun, or that some of the low mass white dwarfs are single.Peer reviewe

CCD Photometry of Faint Variable Stars in the Globular Cluster NGC 6752

We present the results of a photometric survey for variable stars in the field of the nearby globular cluster NGC 6752. The cluster was monitored in 1996 and 1997 for a total of 54 hours with 3 different CCD cameras mounted on the 1.0-m Swope telescope. Eleven new variables were identified: 3 SX Phe stars, 7 contact binaries and 1 candidate detached eclipsing binary. All 3 SX Phe variables are likely members of the cluster while only 1 out of the 7 contact binaries is a potential cluster member. As a by-product of our survey we obtained UBV photometry for a large sample of stars in the cluster field. Two stars with U-B \approx -1.0 and V=19.3 and V=20.6 were identified. They lie along the extended horizontal branch of the cluster, and are likely to be faint sdB stars from NGC 6752.Comment: 18 pages, LaTex, 9 figures (Fig. 1 not available), accepted for publication in the Astronomical Journa

Hot Horizontal-Branch Stars: The Ubiquitous Nature of the "Jump" in Stromgren u, Low Gravities, and the Role of Radiative Levitation of Metals

A "jump" in the BHB distribution in the V, u-y CMD was recently detected in the GC M13. It is morphologically best characterized as a discontinuity in u, u-y, with stars in the range 11,500<Teff(K)<20,000 deviating systematically from (in the sense of appearing brighter and/or hotter than) canonical ZAHBs. We present u, y photometry of 14 GCs obtained with 3 different telescopes (Danish, NOT, HST) and demonstrate that the u-jump is present in every GC whose HB extends beyond 11,500K, irrespective of [Fe/H], mixing history on the RGB, and other GC parameters. We suggest that the u-jump is a ubiquitous feature, intrinsic to all HB stars hotter than 11,500K. We draw a parallel between the ubiquitous nature of the u-jump and the problem of low measured gravities among BHB stars. We note that the "logg-jump" occurs over the same temperature range as the u-jump, and that it occurs in every metal-poor GC for which gravities have been determined--irrespective of [Fe/H], mixing history on the RGB, or any other GC parameters. Furthermore, the u-jump and the logg-jump are connected on a star-by-star basis. The two are likely different manifestations of the same physical phenomenon. We present a framework which may simultaneously account for the u-jump and the logg-jump. Reviewing spectroscopic data for several field BHB stars, as well as two BHB stars in the GC NGC 6752, we find evidence that radiative levitation of heavy elements takes place at Teff>11,500 K, dramatically enhancing their abundances in the atmospheres of BHB stars in the "critical" temperature region. Model atmospheres taking diffusion effects into account are badly needed, and will likely lead to better overall agreement between canonical evolutionary theory and observations for BHB stars.Comment: ApJ, Main Journal, accepted. Contains several changes and update

Single and Composite Hot Subdwarf Stars in the Light of 2MASS Photometry

Utilizing the Two Micron All Sky Survey (2MASS) Second Incremental Data Release Catalog, we have retrieved near-IR magnitudes for several hundred hot subdwarfs (sdO and sdB stars) drawn from the "Catalogue of Spectroscopically Identified Hot Subdwarfs" (Kilkenny, Heber, & Drilling 1988, 1992). This sample size greatly exceeds that of previous studies of hot subdwarfs. Examining 2MASS photometry alone or in combination with visual photometry (Johnson BV or Stromgren uvby) available in the literature, we show that it is possible to identify hot subdwarf stars that exhibit atypically red IR colors that can be attributed to the presence of an unresolved late type companion. Utilizing this large sample, we attempt for the first time to define an approximately volume limited sample of hot subdwarfs. We discuss the considerations, biases, and difficulties in defining such a sample. We find that, of the hot subdwarfs in Kilkenny et al., about 40% in a magnitude limited sample have colors that are consistent with the presence of an unresolved late type companion. Binary stars are over-represented in a magnitude limited sample. In an approximately volume limited sample the fraction of composite-color binaries is about 30%.Comment: to appear in Sept 2003 AJ, 41 pages total, 12 figures, 2 tables are truncated (full tables to appear in electronic journal or available by request

Making FORS2 fit for exoplanet observations (again)

For about three years, it was known that precision spectrophotometry with FORS2 suffered from systematic errors that made quantitative observations of planetary transits impossible. We identified the Longitudinal Atmospheric Dispersion Compensator (LADC) as the most likely culprit, and therefore engaged in a project to exchange the LADC prisms with the uncoated ones from FORS1. This led to a significant improvement in the depth of FORS2 zero points, a reduction in the systematic noise, and should make FORS2 again competitive for transmission spectroscopy of exoplanets.Comment: To appear in the March issue of the ESO Messenge