75 research outputs found
Quantitative chemical tagging, stellar ages and the chemo-dynamical evolution of the Galactic disc
The early science results from the new generation of high-resolution stellar
spectroscopic surveys, such as GALAH and the Gaia-ESO survey, will represent
major milestones in the quest to chemically tag the Galaxy. Yet this technique
to reconstruct dispersed coeval stellar groups has remained largely untested
until recently. We build on previous work that developed an empirical chemical
tagging probability function, which describes the likelihood that two field
stars are conatal, that is, they were formed in the same cluster environment.
In this work we perform the first ever blind chemical tagging experiment, i.e.,
tagging stars with no known or otherwise discernable associations, on a sample
of 714 disc field stars with a number of high quality high resolution
homogeneous metal abundance measurements. We present evidence that chemical
tagging of field stars does identify coeval groups of stars, yet these groups
may not represent distinct formation sites, e.g. as in dissolved open clusters,
as previously thought. Our results point to several important conclusions,
among them that group finding will be limited strictly to chemical abundance
space, e.g. stellar ages, kinematics, colors, temperature and surface gravity
do not enhance the detectability of groups. We also demonstrate that in
addition to its role in probing the chemical enrichment and kinematic history
of the Galactic disc, chemical tagging represents a powerful new stellar age
determination technique.Comment: 12 pages, 9 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society (MNRAS
TGCat, The Chandra Transmission Grating Catalog and Archive: Systems, Desgin and Accessibility
The recently released Chandra Transmission Grating Catalog and Archive, TGCat, presents a fully dynamic on-line catalog allowing users to browse and categorize Chandra gratings observations quickly and easily, generate custom plots of resulting response corrected spectra on-line without the need for special software and to download analysis ready products from multiple observations in one convenient operation. TGCat has been registered as a VO resource with the NVO providing direct access to the catalogs interface. The catalog is supported by a back-end designed to automatically fetch newly public data, process, archive and catalog them, At the same time utilizing an advanced queue system integrated into the archive's MySQL database allowing large processing projects to take advantage of an unlimited number of CPUs across a network for rapid completion. A unique feature of the catalog is that all of the high level functions used to retrieve inputs from the Chandra archive and to generate the final data products are available to the user in an ISIS written library with detailed documentation. Here we present a structural overview of the Systems, Design, and Accessibility features of the catalog and archive.United States. National Aeronautics and Space Administration (Chandra X-ray Center (CXC) NASA contract NAS8-03060)United States. National Aeronautics and Space Administration (Smithsonian Astrophysical Observatory (SAO) contract SV3-73016 for the Chandra X-Ray Center and Science Instruments
The velocity ellipsoid in the Galactic disc using Gaia DR1
The stellar velocity ellipsoid of the solar neighbour (d < 200 pc) is re-examined using intermediate-old mono-abundance stellar groups with high-quality chemistry data together with parallaxes and proper motions from Gaia DR1. We find the average velocity dispersion values for the three space velocity components for the thin and thick discs of (σU, σV, σW)thin = (33 ± 4, 28 ± 2, 23 ± 2) and (σU, σV, σW)thick = (57 ± 6, 38 ± 5, 37 ± 4) km s-1, respectively. The mean values of the ratio between the semi-axes of the velocity ellipsoid for the thin disc are found to be σV/σU = 0.70 ± 0.13 and σW/σU is 0.64 ± 0.08, while for the thick disc σV/σU = 0.67 ± 0.11 and σW/σU is 0.66 ± 0.11. Inputting these dispersions into the linear Strömberg relation for the thin disc groups, we find the Sun's velocity with respect to the Local Standard of Rest in Galactic rotation to be V⊙ = 13.9 ± 3.4 kms-1. A relation is found between the vertex deviation and the chemical abundances for the thin disc, ranging from -5 to +40° as iron abundance increases. For the thick disc we find a vertex deviation of luv ~- 15°. The tilt angle (luw) in the U-W plane for the thin disc groups ranges from -10 to +15°, but there is no evident relation between luw and the mean abundances. However, we find a weak relation for luw as a function of iron abundances and a-elements for most of the groups in the thick disc, where the tilt angle decreases from -5 to -20° when [Fe/H] decreases and [α/Fe] increases. The velocity anisotropy parameter is independent of the chemical group abundances and its value is nearly constant for both discs (β ~ 0.5), suggesting that the combined disc is dynamically relaxed.BA and SRM acknowledge support from National
Science Foundation grant AST-1616636
X-ray Flares of EV Lac: Statistics, Spectra, Diagnostics
We study the spectral and temporal behavior of X-ray flares from the active
M-dwarf EV Lac in 200 ks of exposure with the Chandra/HETGS. We derive flare
parameters by fitting an empirical function which characterizes the amplitude,
shape, and scale. The flares range from very short (<1 ks) to long (10 ks)
duration events with a range of shapes and amplitudes for all durations. We
extract spectra for composite flares to study their mean evolution and to
compare flares of different lengths. Evolution of spectral features in the
density-temperature plane shows probable sustained heating. The short flares
are significantly hotter than the longer flares. We determined an upper limit
to the Fe K fluorescent flux, the best fit value being close to what is
expected for compact loops.Comment: 9 pages; 9 figures; latex/emulateapj style; Submitted to The
Astrophysical Journa
The Chandra Source Catalog
The Chandra Source Catalog (CSC) is a general purpose virtual X-ray
astrophysics facility that provides access to a carefully selected set of
generally useful quantities for individual X-ray sources, and is designed to
satisfy the needs of a broad-based group of scientists, including those who may
be less familiar with astronomical data analysis in the X-ray regime. The first
release of the CSC includes information about 94,676 distinct X-ray sources
detected in a subset of public ACIS imaging observations from roughly the first
eight years of the Chandra mission. This release of the catalog includes point
and compact sources with observed spatial extents <~ 30''. The catalog (1)
provides access to the best estimates of the X-ray source properties for
detected sources, with good scientific fidelity, and directly supports
scientific analysis using the individual source data; (2) facilitates analysis
of a wide range of statistical properties for classes of X-ray sources; and (3)
provides efficient access to calibrated observational data and ancillary data
products for individual X-ray sources, so that users can perform detailed
further analysis using existing tools. The catalog includes real X-ray sources
detected with flux estimates that are at least 3 times their estimated 1 sigma
uncertainties in at least one energy band, while maintaining the number of
spurious sources at a level of <~ 1 false source per field for a 100 ks
observation. For each detected source, the CSC provides commonly tabulated
quantities, including source position, extent, multi-band fluxes, hardness
ratios, and variability statistics, derived from the observations in which the
source is detected. In addition to these traditional catalog elements, for each
X-ray source the CSC includes an extensive set of file-based data products that
can be manipulated interactively.Comment: To appear in The Astrophysical Journal Supplement Series, 53 pages,
27 figure
Statistical Characterization of the Chandra Source Catalog
The first release of the Chandra Source Catalog (CSC) contains ~95,000 X-ray
sources in a total area of ~0.75% of the entire sky, using data from ~3,900
separate ACIS observations of a multitude of different types of X-ray sources.
In order to maximize the scientific benefit of such a large, heterogeneous
data-set, careful characterization of the statistical properties of the
catalog, i.e., completeness, sensitivity, false source rate, and accuracy of
source properties, is required. Characterization efforts of other, large
Chandra catalogs, such as the ChaMP Point Source Catalog (Kim et al. 2007) or
the 2 Mega-second Deep Field Surveys (Alexander et al. 2003), while
informative, cannot serve this purpose, since the CSC analysis procedures are
significantly different and the range of allowable data is much less
restrictive. We describe here the characterization process for the CSC. This
process includes both a comparison of real CSC results with those of other,
deeper Chandra catalogs of the same targets and extensive simulations of
blank-sky and point source populations.Comment: To be published in the Astrophysical Journal Supplement Series (Fig.
52 replaced with a version which astro-ph can convert to PDF without issues.
Discovery of astrometric accelerations by dark companions in the globular cluster ω Centauri
We present results from the search for astrometric accelerations of stars in ω Centauri using 13 years of regularly-scheduled Hubble Space Telescope WFC3/UVIS calibration observations in the cluster core. The high-precision astrometry of∼160000 sources was searched for significant deviations from linear proper motion. This led to the discovery of four cluster members and one foreground field star with compelling acceleration patterns. We interpret them as the result of the gravitational pull by an invisible companion and determined preliminary Keplerian orbit parameters, including the companion’s mass. For the cluster members our analysis suggests periods ranging from 8.8 to 19+ years and dark companions in the mass range of ∼0.7to∼1.4M⊙. At least one companion could exceed the upper mass-boundary of white dwarfs and can be classified as a neutron-star candidate
The Arecibo Legacy Fast ALFA Survey: III. HI Source Catalog of the Northern Virgo Cluster Region
We present the first installment of HI sources extracted from the Arecibo
Legacy Fast ALFA (ALFALFA) extragalactic survey, initiated in 2005. Sources
have been extracted from 3-D spectral data cubes and then examined
interactively to yield global HI parameters. A total of 730 HI detections are
catalogued within the solid angle 11h44m < R.A.(J2000) < 14h00m and +12deg <
Dec.(J2000) < +16deg, and redshift range -1600 \kms < cz < 18000 \kms. In
comparison, the HI Parkes All-Sky Survey (HIPASS) detected 40 HI signals in the
same region. Optical counterparts are assigned via examination of digital
optical imaging databases. ALFALFA HI detections are reported for three
distinct classes of signals: (a) detections, typically with S/N > 6.5; (b) high
velocity clouds in the Milky Way or its periphery; and (c) signals of lower S/N
(to ~ 4.5) which coincide spatially with an optical object of known similar
redshift. Although this region of the sky has been heavily surveyed by previous
targeted observations based on optical flux-- or size-- limited samples, 69% of
the extracted sources are newly reported HI detections. The resultant
positional accuracy of HI sources is 20" (median). The median redshift of the
sample is ~7000 \kms and its distribution reflects the known local large scale
structure including the Virgo cluster. Several extended HI features are found
in the vicinity of the Virgo cluster. A small percentage (6%) of HI detections
have no identifiable optical counterpart, more than half of which are high
velocity clouds in the Milky Way vicinity; the remaining 17 objects do not
appear connected to or associated with any known galaxy.Comment: Astronomical Journal, in pres
A new design for friction stir spot joining of Al alloys and carbon fibre reinforced composites
Friction stir spot welding (FSSW) has been recently developed to join dissimilar materials. However, the traditional requirement for a rotating tool consists of a pin and shoulder in FSSW leads to a complex joining process and unpredictable defects. In this study, a new static-shoulder design in FSSW was proposed and developed to join Al alloys to carbon fiber-reinforced polymer (CFRP) composites. The main joining parameters, including pin rotational speed, pin feed rate and pin plunge depth, were varied to investigate their effects on the joining temperature, materials interaction and the strength of joints. The pin rotational speed had the largest influence on the joining temperature. Lap shear tensile testing was conducted to evaluate the performance of the joints. The joints exhibited the ultimate lap shear force from 230 to 260 N. A brittle fracture occurred with the displacement-at-fracture load of 0.35-0.41 mm. Cross-sectional images revealed the creation of undulations on the surface of Al alloys in the joining zone. The undulations created a macro-mechanical interlocking bonding between the materials, which determined the performance of the joints. For a flat pin, by increasing the plunge depth from 1.25 to 1.30 mm, the undulation size increased from 0.21 to 0.26 mm, which can enhance the macro-mechanical interlocking bonding between Al alloys and CFRP and accordingly increased the ultimate shear force of the joints from 230 to 241 N. Use of a fluted pin significantly influenced the flow of the plasticized Al alloy which created pronounced undulations and large Al alloy spikes of 0.46 mm. These features seemed to establish an efficient macro-mechanical interlocking bonding, which resulted in a noticeable improvement in the performance of the joint. For a plunge depth of 1.30 mm, the ultimate shear force increased to 261 N using the fluted pin
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