6,514 research outputs found
An Attempt to Detect the Galactic Bulge at 12 microns with IRAS
Surface brightness maps at 12 microns, derived from observations with the
Infrared Astronomical Satellite (IRAS), are used to estimate the integrated
flux at this wavelength from the Galactic bulge as a function of galactic
latitude along the minor axis. A simple model was used to remove Galactic disk
emission (e.g. unresolved stars and dust) from the IRAS measurements. The
resulting estimates are compared with predictions for the 12 micron bulge
surface brightness based on observations of complete samples of optically
identified M giants in several minor axis bulge fields. No evidence is found
for any significant component of 12m emission in the bulge other than that
expected from the optically identified M star sample plus normal, lower
luminosity stars. Known large amplitude variables and point sources from the
IRAS catalogue contribute only a small fraction to the total 12 micron flux.Comment: Accepted for publication in ApJ; 13 pages of text including tables in
MS WORD97 generated postscript; 3 figures in postscript by Sigma Plo
OH-selected AGB and post-AGB objects I.Infrared and maser properties
Using 766 compact objects from a survey of the galactic Plane in the 1612-MHz
OH line, new light is cast on the infrared properties of evolved stars on the
TP-AGB and beyond. The usual mid-infrared selection criteria, based on IRAS
colours, largely fail to distinguish early post-AGB stages. A two-colour
diagram from narrower-band MSX flux densities, with bimodal distributions,
provides a better tool to do the latter. Four mutually consistent selection
criteria for OH-masing red PPNe are given, as well as two for early post-AGB
masers and one for all post--AGB masers, including the earliest ones. All these
criteria miss a group of blue, high-outflow post-AGB sources with 60-mum
excess; these will be discussed in detail in Paper II. The majority of post-AGB
sources show regular double-peaked spectra in the OH 1612-MHz line, with fairly
low outflow velocities, although the fractions of single peaks and irregular
spectra may vary with age and mass. The OH flux density shows a fairly regular
relation with the stellar flux and the envelope optical depth, with the maser
efficiency increasing with IRAS colour R21. The OH flux density is linearly
correlated with the 60-mum flux density.Comment: 16 pages, LaTex, 22 figures, AJ (accepted
Global interlaboratory assessments of perfluoroalkyl substances under the Stockholm Convention on persistent organic pollutants
The Global Monitoring Plan (GMP) according to article 16 of the Stockholm Convention on Persistent Organic Pollutants (POPs) requires that POP laboratories must be capable – at any time – to analyse samples for POPs within a variation of ±25%. Based on this target error of 25%, a statistical model using z-scores was applied to assess the performance of analytical laboratories for POPs and a number of matrices. Since the second round of these ‘Bi-ennial Global Interlaboratory Assessment on Persistent Organic Pollutants (POPs)’, carried out in 2012/2013, perfluoroalkyl substances (PFASs) have been included into the proficiency tests. The third round was carried out in 2016/2017. The test materials included test solutions of PFASs analytical standards, the abiotic matrices sediment, air (extract) and water and the biotic matrices fish, human milk and human plasma. The number of laboratories submitting results for PFASs remained quite stable (IL2 = 27 laboratories; IL3 = 29), but there was broader geographic distribution observed in IL3: in addition to the laboratories from Asia and the Western Europe/other groups, two laboratories from Africa participated, two from Central-Eastern Europe and one from the Latin American/Caribbean region. Considering that PFASs were introduced for the first time in round 2, the results were good to reasonable compared to those of a number of other POPs included in the same study. However, it shall also be mentioned that for some matrices and PFASs, the number of laboratories submitting results was too small and the results too scattered to derive a consensus value. This was especially true for the PFOS precursor compounds and the air matrix. Also, laboratories struggle with the analysis of the branched PFOS isomers. These interlaboratory assessments on PFASs gave promising results and demonstrated the importance of proficiency tests in an international environment to generate trust in laboratory results. The need to participate regularly in such intercomparison assessments is highlighted. The results show the current level of PFAS analysis, which varies by laboratory and by matrix rather than per geographic region
The footprint of cometary dust analogues: II. Morphology as a tracer of tensile strength and application to dust collection by the Rosetta spacecraft
The structure of cometary dust is a tracer of growth processes in the
formation of planetesimals. Instrumentation on board the Rosetta mission to
comet 67P/Churyumov- Gerasimenko captured dust particles and analysed them in
situ. However, these deposits are a product of a collision within the
instrument. We conducted laboratory experiments with cometary dust analogues,
simulating the collection process by Rosetta instruments (specifically COSIMA,
MIDAS). In Paper I we reported that velocity is a key driver in determining the
appearance of deposits. Here in Paper II we use materials with different
monomer sizes, and study the effect of tensile strength on the appearance of
deposits. We find that mass transfer efficiency increases from 1 up to
10% with increasing monomer diameter from 0.3 m to 1.5 m (i.e.
tensile strength decreasing from 12 to 3 kPa), and velocities
increasing from 0.5 to 6 m/s. Also, the relative abundance of small fragments
after impact is higher for material with higher tensile strength. The
degeneracy between the effects of velocity and material strength may be lifted
by performing a closer study of the deposits. This experimental method makes it
possible to estimate the mass transfer efficiency in the COSIMA instrument.
Extrapolating these results implies that more than half of the dust collected
during the Rosetta mission has not been imaged. We analysed two COSIMA targets
containing deposits from single collisions. The collision that occurred closest
to perihelion passage led to more small fragments on the target.Comment: 13 pages, 11 figures, accepted for publication in MNRA
HST Snapshot Survey of Post-AGB Objects
The results from a Hubble Space Telescope (HST) snapshot survey of post-AGB
objects are shown. The aim of the survey is to complement existing HST images
of PPN and to connect various types of nebulosities with physical and chemical
properties of their central stars. Nebulosities are detected in 15 of 33
sources. Images and photometric and geometric measurements are presented. For
sources with nebulosities we see a morphological bifurcation into two groups,
DUPLEX and SOLE, as previous studies have found. We find further support to the
previous results suggesting that this dichotomy is caused by a difference in
optical thickness of the dust shell. The remaining 18 sources are classified as
stellar post-AGB objects, because our observations indicate a lack of
nebulosity. We show that some stellar sources may in fact be DUPLEX or SOLE
based on their infrared colors. The cause of the differences among the groups
are investigated. We discuss some evidence suggesting that high progenitor-mass
AGB stars tend to become DUPLEX post-AGB objects. Intermediate progenitor-mass
AGB stars tend to be SOLE post-AGB objects. Most of the stellar sources
probably have low mass progenitors and do not seem to develop nebulosities
during the post-AGB phase and therefore do not become planetary nebulae.Comment: 21 pages, 11 figure
Superheating and solid-liquid phase coexistence in nanoparticles with non-melting surfaces
We present a phenomenological model of melting in nanoparticles with facets
that are only partially wet by their liquid phase. We show that in this model,
as the solid nanoparticle seeks to avoid coexistence with the liquid, the
microcanonical melting temperature can exceed the bulk melting point, and that
the onset of coexistence is a first-order transition. We show that these
results are consistent with molecular dynamics simulations of aluminum
nanoparticles which remain solid above the bulk melting temperature.Comment: 8 pages, 5 figure
Ultrafast core-loss spectroscopy in four-dimensional electron microscopy
We demonstrate ultrafast core-electron energy-loss spectroscopy in four-dimensional electron microscopy as an element-specific probe of nanoscale dynamics. We apply it to the study of photoexcited graphite with femtosecond and nanosecond resolutions. The transient core-loss spectra, in combination with ab initio molecular dynamics simulations, reveal the elongation of the carbon-carbon bonds, even though the overall behavior is a contraction of the crystal lattice. A prompt energy-gap shrinkage is observed on the picosecond time scale, which is caused by local bond length elongation and the direct renormalization of band energies due to temperature-dependent electron–phonon interactions
Low-bitrate video coding with third order geometric transformations
This paper describes low-bitrate video compression based upon the characterization of the new frame as a set of geometric transformations of objects of the previous frame. Objects with motion are detected and the motion is estimated. The estimated motion (motion field) is used to obtain the parameters for the geometric transformations. The pertinent geometric transformations are rotation, translation, zooming and isotropic and anisotropic distortion. The motivation for choosing this set of third-order transformations is that we have at our disposal special ASICS for real-time video processing. We only want to transform moving objects and therefore the boundaries of the moving objects must be known. The boundaries of the objects are represented by closed contours
Basal melt rates beneath Whillans Ice Stream, West Antarctica
This is the published version, also available here: http://dx.doi.org/10.3189/002214310793146241.Basal water lubricates and enables the fast flow of the West Antarctic ice streams which exist under low gravitational driving stress. Identification of sources and rates of basal meltwater production can provide insight into the dynamics of ice streams and the subglacial hydrology, which remain insufficiently described by glaciological theory. Combining measurements and analytic modeling, we identify two regions where basal meltwater is produced beneath Whillans Ice Stream, West Antarctica. Downstream of the onset of shear crevasses, strong basal melt (20–50 mm a−1) is concentrated beneath the relatively narrow shear margins. Farther upstream, melt rates are consistently 3–7 mm a−1 across the width of the ice stream. We show that the transition in melt-rate patterns is coincident with the onset of shear margin crevassing and streaming flow and related to the development of significant lateral shear resistance, which reorganizes the resistive stress regime and induces a concentration of basal resistance adjacent to the shear margin. Finally, we discuss how downstream freeze-on in the ice-stream center coupled with melt beneath the shear margin might result in a slowing but widening ice stream
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