1,489 research outputs found
Repeatable method of thermal stress fracture test of brittle materials
Method heats specimens slowly and with sufficient control so that the critical temperature gradient in the specimens cannot occur before temperature equilibrium is reached
New stellar encounters discovered in the second Gaia data release
Passing stars may play an important role in the evolution of our solar
system. We search for close stellar encounters to the Sun among all 7.2 million
stars in Gaia-DR2 that have six-dimensional phase space data. We characterize
encounters by integrating their orbits through a Galactic potential and
propagating the correlated uncertainties via a Monte Carlo resampling. After
filtering to remove spurious data, we find 694 stars that have median (over
uncertainties) closest encounter distances within 5 pc, all occurring within 15
Myr from now. 26 of these have at least a 50% chance of coming closer than 1 pc
(and 7 within 0.5 pc), all but one of which are newly discovered here. We
further confirm some and refute several other previously-identified encounters,
confirming suspicions about their data. The closest encounter in the sample is
Gl 710, which has a 95% probability of coming closer than 0.08 pc (17 000 AU).
Taking mass estimates from Gaia astrometry and multiband photometry for
essentially all encounters, we find that Gl 710 also has the largest impulse on
the Oort cloud. Using a Galaxy model, we compute the completeness of the
Gaia-DR2 encountering sample as a function of perihelion time and distance.
Only 15% of encounters within 5 pc occurring within +/- 5 Myr of now have been
identified, mostly due to the lack of radial velocities for faint and/or cool
stars. Accounting for the incompleteness, we infer the present rate of
encounters within 1 pc to be 19.7 +/- 2.2 per Myr, a quantity expected to scale
quadratically with the encounter distance out to at least several pc.
Spuriously large parallaxes in our sample from imperfect filtering would tend
to inflate both the number of encounters found and this inferred rate. The
magnitude of this effect is hard to quantify.Comment: 12 pages. Accepted to A&A. Added to this version: section 3.2 and
Fig. 8 (CMD) with discussion of astrometric quality metrics; full versions of
tables 2 and 3 as ancillary dat
Deconvolution with Shapelets
We seek to find a shapelet-based scheme for deconvolving galaxy images from
the PSF which leads to unbiased shear measurements. Based on the analytic
formulation of convolution in shapelet space, we construct a procedure to
recover the unconvolved shapelet coefficients under the assumption that the PSF
is perfectly known. Using specific simulations, we test this approach and
compare it to other published approaches. We show that convolution in shapelet
space leads to a shapelet model of order
with and being the maximum orders of the intrinsic
galaxy and the PSF models, respectively. Deconvolution is hence a
transformation which maps a certain number of convolved coefficients onto a
generally smaller number of deconvolved coefficients. By inferring the latter
number from data, we construct the maximum-likelihood solution for this
transformation and obtain unbiased shear estimates with a remarkable amount of
noise reduction compared to established approaches. This finding is
particularly valid for complicated PSF models and low images, which
renders our approach suitable for typical weak-lensing conditions.Comment: 9 pages, 9 figures, submitted to A&
Non-local Thermodynamic Equilibrium Stellar Spectroscopy with 1D and ă3Dă Models. II. Chemical Properties of the Galactic Metal-poor Disk and the Halo
From exploratory studies and theoretical expectations it is known that simplifying approximations in spectroscopic analysis (local thermodynamic equilibrium (LTE), 1D) lead to systematic biases of stellar parameters and abundances. These biases depend strongly on surface gravity, temperature and, in particular, for LTE versus non-LTE (NLTE), on metallicity of the stars. Here we analyze the [Mg/Fe] and [Fe/H] plane of a sample of 326 stars, comparing LTE and NLTE results obtained using 1D hydrostatic models and averaged models. We show that compared to the NLTE benchmark, the other three methods display increasing biases toward lower metallicities, resulting in false trends of [Mg/Fe] against [Fe/H], which have profound implications for interpretations by chemical evolution models. In our best NLTE model, the halo and disk stars show a clearer behavior in the [Mg/Fe]â[Fe/H] plane, from the knee in abundance space down to the lowest metallicities. Our sample has a large fraction of thick disk stars and this population extends down to at least [Fe/H] ~ â1.6 dex, further than previously proven. The thick disk stars display a constant [Mg/Fe] â 0.3 dex, with a small intrinsic dispersion in [Mg/Fe] that suggests that a fast SN Ia channel is not relevant for the disk formation. The halo stars reach higher [Mg/Fe] ratios and display a net trend of [Mg/Fe] at low metallicities, paired with a large dispersion in [Mg/Fe]. These indicate the diverse origin of halo stars from accreted low-mass systems to stochastic/inhomogeneous chemical evolution in the Galactic halo
Time resolved fission in metal clusters
We explore from a theoretical point of view pump and probe (P&P) analysis for
fission of metal clusters where probe pulses are generalized to allow for
scanning various frequencies. We show that it is possible to measure the time
the system needs to develop to scission. This is achieved by a proper choice of
both delay and frequency of the probe pulse. A more detailed analysis even
allows to access the various intermediate stages of the fission process.Comment: 4 pages, 4 figure
Informal ActionâAdjudicationâRule Making: Some Recent Developments in Federal Administrative Law
Direct energy consumption of ICT hardware is only âhalf the story.â In order to get the âwhole story,â energy consumption during the entire life cycle has to be taken into account. This chapter is a first step toward a more comprehensive picture, showing the âgrey energyâ (i.e., the overall energy requirements) as well as the releases (into air, water, and soil) during the entire life cycle of exemplary ICT hardware devices by applying the life cycle assessment method. The examples calculated show that a focus on direct energy consumption alone fails to take account of relevant parts of the total energy consumption of ICT hardware as well as the relevance of the production phase. As a general tendency, the production phase is more and more important the smaller (and the more energy-efficient) the devices are. When in use, a tablet computer is much more energy-efficient than a desktop computer system with its various components, so its production phase has a much greater relative importance. Accordingly, the impacts due to data transfer when using Internet services are also increasingly relevant the smaller the end-user device is, reaching up to more than 90Â % of the overall impact when using a tablet computer.QC 20140825</p
Soft clustering analysis of galaxy morphologies: A worked example with SDSS
Context: The huge and still rapidly growing amount of galaxies in modern sky
surveys raises the need of an automated and objective classification method.
Unsupervised learning algorithms are of particular interest, since they
discover classes automatically. Aims: We briefly discuss the pitfalls of
oversimplified classification methods and outline an alternative approach
called "clustering analysis". Methods: We categorise different classification
methods according to their capabilities. Based on this categorisation, we
present a probabilistic classification algorithm that automatically detects the
optimal classes preferred by the data. We explore the reliability of this
algorithm in systematic tests. Using a small sample of bright galaxies from the
SDSS, we demonstrate the performance of this algorithm in practice. We are able
to disentangle the problems of classification and parametrisation of galaxy
morphologies in this case. Results: We give physical arguments that a
probabilistic classification scheme is necessary. The algorithm we present
produces reasonable morphological classes and object-to-class assignments
without any prior assumptions. Conclusions: There are sophisticated automated
classification algorithms that meet all necessary requirements, but a lot of
work is still needed on the interpretation of the results.Comment: 18 pages, 19 figures, 2 tables, submitted to A
Evidence of a dynamically evolving Galactic warp
In a cosmological setting, the disc of a galaxy is expected to continuously
experience gravitational torques and perturbations from a variety of sources,
which can cause the disc to wobble, flare and warp. Specifically, the study of
galactic warps and their dynamical nature can potentially reveal key
information on the formation history of galaxies and the mass distribution of
their halos. Our Milky Way presents a unique case study for galactic warps,
thanks to the detailed knowledge of its stellar distribution and kinematics.
Using a simple model of how the warp's orientation is changing with time, we
here measure the precession rate of the Milky Way's warp using 12 million giant
stars from Gaia Data Release 2, finding that it is precessing at km/s/kpc in the direction of Galactic rotation,
about one third the angular velocity at the Sun's position in the Galaxy. The
direction and magnitude of the warp's precession rate favour the scenario that
the warp is the results of a recent or ongoing encounter with a satellite
galaxy, rather than the relic of the ancient assembly history of the Galaxy.Comment: Published in Nature Astronomy. Final accepted version here:
https://www.nature.com/articles/s41550-020-1017-3 and full-text access via
SharedIt link: https://rdcu.be/b2ph
The bulk of the black hole growth since z ~ 1 occurs in a secular universe: no major merger-AGN connection
What is the relevance of major mergers and interactions as triggering mechanisms for active galactic nuclei (AGNs)
activity? To answer this long-standing question, we analyze 140 XMM-Newton-selected AGN host galaxies and
a matched control sample of 1264 inactive galaxies over z ~ 0.3â1.0 and M_â < 10^(11.7) M_â with high-resolution
Hubble Space Telescope/Advanced Camera for Surveys imaging from the COSMOS field. The visual analysis of
their morphologies by 10 independent human classifiers yields a measure of the fraction of distorted morphologies
in the AGN and control samples, i.e., quantifying the signature of recent mergers which might potentially be
responsible for fueling/triggering the AGN. We find that (1) the vast majority (>85%) of the AGN host galaxies
do not show strong distortions and (2) there is no significant difference in the distortion fractions between active
and inactive galaxies. Our findings provide the best direct evidence that, since z ~ 1, the bulk of black hole (BH)
accretion has not been triggered by major galaxy mergers, therefore arguing that the alternative mechanisms, i.e.,
internal secular processes and minor interactions, are the leading triggers for the episodes of major BH growth.We
also exclude an alternative interpretation of our results: a substantial time lag between merging and the observability
of the AGN phase could wash out the most significant merging signatures, explaining the lack of enhancement
of strong distortions on the AGN hosts. We show that this alternative scenario is unlikely due to (1) recent major
mergers being ruled out for the majority of sources due to the high fraction of disk-hosted AGNs, (2) the lack of
a significant X-ray signal in merging inactive galaxies as a signature of a potential buried AGN, and (3) the low
levels of soft X-ray obscuration for AGNs hosted by interacting galaxies, in contrast to model predictions
Anthelmintic activity and cytotoxic effects of compounds isolated from the fruits of Ozoroa insignis del. (Anacardiaceae)
Ozoroa insignis Del. is an ethnobotanical plant widely used in traditional medicine for various ailments, including schistosomiasis, tapeworm, and hookworm infections. From the so far not investigated fruits of Ozoroa insignis, the anthelmintic principles could be isolated through bioassay-guided isolation using Caenorhabditis elegans and identified by NMR spectroscopic analysis and mass spectrometric studies. Isolated 6-[8(Z)-pentadecenyl] anacardic (1), 6-[10(Z)-heptadecenyl] anacardic acid (2), and 3-[7(Z)-pentadecenyl] phenol (3) were evaluated against the 5 parasitic organisms Schistosoma mansoni (adult and newly transformed schistosomula), Strongyloides ratti, Heligmosomoides polygyrus, Necator americanus, and Ancylostoma ceylanicum, which mainly infect humans and other mammals. Compounds 1-3 showed good activity against Schistosoma mansoni, with compound 1 showing the best activity against newly transformed schistosomula with 50% activity at 1microM. The isolated compounds were also evaluated for their cytotoxic properties against PC-3 (human prostate adenocarcinoma) and HT-29 (human colorectal adenocarcinoma) cell lines, whereby compounds 2 and 3 showed antiproliferative activity in both cancer cell lines, while compound 1 exhibited antiproliferative activity only on PC-3 cells. With an IC50 value of 43.2 microM, compound 3 was found to be the most active of the 3 investigated compounds
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