257 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
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 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
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
Hot subdwarfs in the Galactic Bulge
Recent observations and theories suggest that extreme horizontal branch (EHB)
stars and their progeny should be the cause of the UV excess seen in the
spectra of many elliptical galaxies. Since the Galactic Bulge is the closest
representation of an old, metal-rich spheroid in which we are able to study the
EHB scenario in detail, we obtained spectra of bulge EHB star candidates and we
confirm their status as hot evolved stars. It is the first time that such stars
are unambiguously identified in the Galactic Bulge.Comment: 10 pages, 3 figures, Astrophysical Journal Letter, accepte
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
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
HST Observations of New Horizontal Branch Structures in the Globular Cluster omega Centauri
The globular cluster omega Centauri contains the largest known population of
very hot horizontal branch (HB) stars. We have used the Hubble Space Telescope
to obtain a far-UV/optical color-magnitude diagram of three fields in omega
Cen. We find that over 30% of the HB objects are ``extreme'' HB or hot post-HB
stars. The hot HB stars are not concentrated toward the cluster center, which
argues against a dynamical origin for them. A wide gap in the color
distribution of the hot HB stars appears to correspond to gaps found earlier in
several other clusters. This suggests a common mechanism, probably related to
giant branch mass loss. The diagram contains a significant population of hot
sub-HB stars, which we interpret as the ``blue-hook'' objects predicted by
D'Cruz et al. (1996a). These are produced by late He-flashes in stars which
have undergone unusually large giant branch mass loss. omega Cen has a
well-known spread of metal abundance, and our observations are consistent with
a giant branch mass loss efficiency which increases with metallicity.Comment: Submitted to ApJ, 12 pages, including 3 figures, also available at
http://www.physics.usyd.edu.au/~noella/research.htm
A Catalog of Spectroscopically Confirmed White Dwarfs from the Sloan Digital Sky Survey Data Release 4
We present a catalog of 9316 spectroscopically confirmed white dwarfs from
the Sloan Digital Sky Survey Data Release 4. We have selected the stars through
photometric cuts and spectroscopic modeling, backed up by a set of visual
inspections. Roughly 6000 of the stars are new discoveries, roughly doubling
the number of spectroscopically confirmed white dwarfs. We analyze the stars by
performing temperature and surface gravity fits to grids of pure hydrogen and
helium atmospheres. Among the rare outliers are a set of presumed helium-core
DA white dwarfs with estimated masses below 0.3 Msun, including two candidates
that may be the lowest masses yet found. We also present a list of 928 hot
subdwarfs.Comment: Accepted by the Astrophysical Journal Supplements, 25 pages, 24
figures, LaTeX. The electronic catalog, as well as diagnostic figures and
links to the spectra, is available at http://das.sdss.org/wdcat/dr4
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
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