514 research outputs found
Infrared Spectra of Meteoritic SiC Grains
We present here the first infrared spectra of meteoritic SiC grains. The
mid-infrared transmission spectra of meteoritic SiC grains isolated from the
Murchison meteorite were measured in the wavelength range 2.5--16.5 micron, in
order to make available the optical properties of presolar SiC grains. These
grains are most likely stellar condensates with an origin predominately in
carbon stars. Measurements were performed on two different extractions of
presolar SiC from the Murchison meteorite. The two samples show very different
spectral appearance due to different grain size distributions. The spectral
feature of the smaller meteoritic SiC grains is a relatively broad absorption
band found between the longitudinal and transverse lattice vibration modes
around 11.3 micron, supporting the current interpretation about the presence of
SiC grains in carbon stars. In contrast to this, the spectral feature of the
large (> 5 micron) grains has an extinction minimum around 10 micron. The
obtained spectra are compared with commercially available SiC grains and the
differences are discussed. This comparison shows that the crystal structure
(e.g., beta-SiC versus alpha-SiC) of SiC grains plays a minor role on the
optical signature of SiC grains compared to e.g. grain size.Comment: 7 pages, 6 figures. To appear in A&
Concerning Order and Disorder in the Ensemble of Cu-O Chain Fragments in Oxygen Deficient Planes of Y-Ba-Cu-O
In connection with numerous X-ray and neutron investigations of some high
temperature superconductors (YBaCuO and related compounds) a
non-trivial part of the structure factor, coming from partly disordered
Cu-O--O-Cu chain fragments, situated within basal planes, CuO, can
be a subject of theoretical interest. Closely connected to such a diffusive
part of the structure factor are the correlation lengths, which are also
available in neutron and X-ray diffraction studies and depend on a degree of
oxygen disorder in a basal plane. The quantitative measure of such a disorder
can be associated with temperature of a sample anneal, , at which oxygen
in a basal plane remains frozen-in high temperature equilibrium after a fast
quench of a sample to room or lower temperature. The structure factor evolution
with is vizualized in figures after the numerical calculations. The
theoretical approach employed in the paper has been developed for the
orthorhombic state of YBCO.Comment: Revtex, 27 pages, 14 PostScript figures upon request, ITP/GU/94/0
Disks around the Young Stars TW Hya and HD 100546 Imaged at 3.4 Millimeters with the Australia Telescope Compact Array
We present observations of the young stars TW Hya and HD 100546 made with the
Australia Telescope Compact Array at 89 GHz with resolution and
mJy continuum sensitivity. Compact thermal dust continuum emission is
detected from disks surrounding both stars. HD 100546 also shows hints of
extended emission, presumably a residual protostellar envelope, which is also
visible in scattered light at optical wavelengths. For TW Hya, HCO J=1--0
line emission from the circumstellar disk is detected and spatially resolved.
The observed size and intensity are in good agreement with model calculations
based on an irradiated disk with substantial depletions derived previously from
single dish observations of higher-J HCO transitions.Comment: 19 pages including 6 figures; accepted for publication in Ap
Similarity of slow stripe fluctations between Sr-doped cuprates and oxygen-doped nickelates
Stripe fluctuations in La2NiO4.17 have been studied by 139La NMR using the
field and temperature dependence of the linewidth and relaxation rates. In the
formation process of the stripes the NMR line intensity is maximal below 230K,
starts to diminish around 140K, disappears around 50K and recovers at 4K. These
results are shown to be consistent with, but completely complementary to
neutron measurements, and to be generic for oxygen doped nickelates and
underdoped cuprates.Comment: 4 pages including 4 figure
Mid-infrared spectroscopy of carbon stars in the Small Magellanic Cloud
We have observed a sample of 36 objects in the Small Magellanic Cloud (SMC)
with the Infrared Spectrometer on the Spitzer Space Telescope. Nineteen of
these sources are carbon stars. An examination of the near- and mid-infrared
photometry shows that the carbon-rich and oxygen-rich dust sources follow two
easily separated sequences. A comparison of the spectra of the 19 carbon stars
in the SMC to spectra from the Infrared Space Observatory (ISO) of carbon stars
in the Galaxy reveals significant differences. The absorption bands at 7.5 um
and 13.7 um due to C2H2 are stronger in the SMC sample, and the SiC dust
emission feature at 11.3 um is weaker. Our measurements of the MgS dust
emission feature at 26-30 um are less conclusive, but this feature appears to
be weaker in the SMC sample as well. All of these results are consistent with
the lower metallicity in the SMC. The lower abundance of SiC grains in the SMC
may result in less efficient carbon-rich dust production, which could explain
the excess C2H2 gas seen in the spectra. The sources in the SMC with the
strongest SiC dust emission tend to have redder infrared colors than the other
sources in the sample, which implies more amorphous carbon, and they also tend
to show stronger MgS dust emission. The weakest SiC emission features tend to
be shifted to the blue; these spectra may arise from low-density shells with
large SiC grains.Comment: Accepted by ApJ 21 March, 2006, 13 pages (emulateapJ), 12 figure
Imaging Oxygen Distribution in Marine Sediments. The Importance of Bioturbation and Sediment Heterogeneity
The influence of sediment oxygen heterogeneity, due to bioturbation, on diffusive oxygen flux was investigated. Laboratory experiments were carried out with 3 macrobenthic species presenting different bioturbation behaviour patterns:the polychaetes Nereis diversicolor and Nereis virens, both constructing ventilated galleries in the sediment column, and the gastropod Cyclope neritea, a burrowing species which does not build any structure. Oxygen two-dimensional distribution in sediments was quantified by means of the optical planar optode technique. Diffusive oxygen fluxes (mean and integrated) and a variability index were calculated on the captured oxygen images. All species increased sediment oxygen heterogeneity compared to the controls without animals. This was particularly noticeable with the polychaetes because of the construction of more or less complex burrows. Integrated diffusive oxygen flux increased with oxygen heterogeneity due to the production of interface available for solute exchanges between overlying water and sediments. This work shows that sediment heterogeneity is an important feature of the control of oxygen exchanges at the sediment–water interface
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
AhR transcriptional activity in serum of Inuits across Greenlandic districts
<p>Abstract</p> <p>Background</p> <p>Human exposure to lipophilic persistent organic pollutants (POPs) including polychlorinated dibenzo-<it>p</it>-dioxins/furans (PCDDs/PCDFs), polychlorinated biphenyls (PCBs) and organochlorine pesticide is ubiquitous. The individual is exposed to a complex mixture of POPs being life-long beginning during critical developmental windows. Exposure to POPs elicits a number of species- and tissue-specific toxic responses, many of which involve the aryl hydrocarbon receptor (AhR). The aim of this study was to compare the actual level of integrated AhR transcriptional activity in the lipophilic serum fraction containing the actual POP mixture among Inuits from different districts in Greenland, and to evaluate whether the AhR transactivity is correlated to the bio-accumulated POPs and/or lifestyle factors.</p> <p>Methods</p> <p>The study included 357 serum samples from the Greenlandic districts: Nuuk and Sisimiut (South West Coast), Qaanaaq (North Coast) and Tasiilaq (East Coast). The bio-accumulated serum POPs were extracted by ethanol: hexane and clean-up on Florisil columns. Effects of the serum extract on the AhR transactivity was determined using the Hepa 1.12cR mouse hepatoma cell line carrying an AhR-luciferase reporter gene, and the data was evaluated for possible association to the serum levels of 14 PCB congeners, 10 organochlorine pesticide residues and/or lifestyle factors.</p> <p>Results</p> <p>In total 85% of the Inuit samples elicited agonistic AhR transactivity in a district dependent pattern. The median level of the AhR-TCDD equivalent (AhR-TEQ) of the separate genders was similar in the different districts. For the combined data the order of the median AhR-TEQ was Tasiilaq > Nuuk ≥ Sisimiut > Qaanaaq possibly being related to the different composition of POPs. In overall, the AhR transactivity was inversely correlated to the levels of sum POPs, age and/or intake of marine food.</p> <p>Conclusion</p> <p>i) We observed that the proportion of dioxin like (DL) compounds in the POP mixture was the dominating factor affecting the level of serum AhR transcriptional activity even at very high level of non DL-PCBs; ii) The inverse association between the integrated serum AhR transactivity and sum of POPs might be explained by the higher level of compounds antagonizing the AhR function probably due to selective POP bioaccumulation in the food chain.</p
Xenohormone transactivities are inversely associated to serum POPs in Inuit
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