374 research outputs found
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
Discovery of Carbon/Oxygen depleted Blue Straggler Stars in 47 Tucanae: the chemical signature of a mass-transfer formation process
We use high-resolution spectra obtained with the ESO Very Large Telescope to
measure surface abundance patterns of 43 Blue Stragglers stars (BSS) in 47 Tuc.
We discovered that a sub-population of BSS 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 mass-transfer process.
This is the first detection of a chemical signature clearly pointing to a
specific BSS formation process in a globular cluster.Comment: Published on 2006, August 10, in ApJ 647, L5
Chemical Abundances and Rotation Velocities of Blue Horizontal-Branch Stars in Six Globular Clusters
High-resolution spectroscopic measurements of blue horizontal-branch stars in
six metal-poor globular clusters -- M3, M13, M15, M68, M92, and NGC 288 --
reveal remarkable variations in photospheric composition and rotation velocity
as a function of a star's position along the horizontal branch. For the cooler
stars (Teff < 11200 K), the derived abundances are in good agreement with the
canonical cluster metallicities, and we find a wide range of v sin i rotation
velocities, some as high as 40 km/s. In the hotter stars, however, most metal
species are strongly enhanced, by as much as 3 dex, relative to the expected
cluster metallicity, while helium is depleted by 2 dex or more. In addition,
the hot stars all rotate slowly, with v sin i < 8 km/s. The anomalous
abundances appear to be due to atomic diffusion mechanisms -- gravitational
settling of helium, and radiative levitation of metals -- in the non-convective
atmospheres of these hot stars. We discuss the influence of these photospheric
metal enhancements on the stars' photometric properties, and explore possible
explanations for the observed distribution of rotation velocities.Comment: 77 pages, 27 figures, accepted for November 2003 publication in ApJ
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
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
Rotation Velocities of Red and Blue Field Horizontal-Branch Stars
We present measurements of the projected stellar rotation velocities (v sin
i) of a sample of 45 candidate field horizontal-branch (HB) stars spanning a
wide range of effective temperature, from red HB stars with Teff ~ 5000 K to
blue HB stars with Teff of 17000 K. Among the cooler blue HB stars (Teff =
7500-11500 K), we confirm prior studies showing that although a majority of
stars rotate at v sin i < 15 km/s, there exists a subset of "fast rotators"
with v sin i as high as 30-35 km/s. All but one of the red HB stars in our
sample have v sin i < 10 km/s, and no analogous rotation bimodality is evident.
We also identify a narrow-lined hot star (Teff ~ 16000 K) with enhanced
photospheric metal abundances and helium depletion, similar to the abundance
patterns found among hot BHB stars in globular clusters, and four other stars
that may also belong in this category. We discuss details of the spectral line
fitting procedure that we use to deduce v sin i, and explore how measurements
of field HB star rotation may shed light on the issue of HB star rotation in
globular clusters.Comment: 47 pages, 9 figures, 5 tables, accepted for publication in November
2003 ApJ
The Promiscuous Nature of Stars in Clusters
The recent availability of special purpose computers designed for calculating
gravitational interactions of N-bodies at extremely high speed has provided the
means to model globular clusters on a star-by-star basis for the first time. By
endeavouring to make the N-body codes that operate on these machines as
realistic as possible, the addition of stellar evolution being one example,
much is being learnt about the interaction between the star cluster itself and
the stars it contains. A fascinating aspect of this research is the ability to
follow the orbits of individual stars in detail and to document the formation
of observed exotic systems. This has revealed that many stars within a star
cluster lead wildly promiscuous lives, interacting, often intimately and in
rapid succession, with a variety of neighbours.Comment: 15 pages, 1 figure, to appear in the Astrophysical Journa
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