966 research outputs found
Photochemistry of the PAH pyrene in water ice: the case for ion-mediated solid-state astrochemistry
Context. Icy dust grains play an important role in the formation of complex
inter- and circumstellar molecules. Observational studies show that polycyclic
aromatic hydrocarbons (PAHs) are abundantly present in the ISM in the gas
phase. It is likely that these non-volatile species freeze out onto dust grains
as well and participate in the astrochemical solid-state network, but
experimental PAH ice studies are largely lacking. Methods. Near UV/VIS
spectroscopy is used to track the in situ VUV driven photochemistry of pyrene
containing ices at temperatures ranging from 10 to 125 K. Results. The main
photoproducts of VUV photolyzed pyrene ices are spectroscopically identified
and their band positions are listed for two host ices, \water and CO. Pyrene
ionisation is found to be most efficient in \water ices at low temperatures.
The reaction products, triplet pyrene and the 1-hydro-1-pyrenyl radical are
most efficiently formed in higher temperature water ices and in low temperature
CO ice. Formation routes and band strength information of the identified
species are discussed. Additionally, the oscillator strengths of Py, Py^+ and
PyH are derived and a quantitative kinetic analysis is performed by fitting a
chemical reaction network to the experimental data. Conclusions. Pyrene is
efficiently ionised in water ice at temperatures below 50 K. Hydrogenation
reactions dominate the chemistry in low temperature CO ice with trace amounts
of water. The results are put in an astrophysical context by determining the
importance of PAH ionisation in a molecular cloud. The photoprocessing of a
sample PAH in ice described in this manuscript indicates that PAH
photoprocessing in the solid state should also be taken into account in
astrochemical models.Comment: 11 pages, 8 figures, accepted for publication in A&
Electrical activity of carbon-hydrogen centers in Si
The electrical activity of Cs-H defects in Si has been investigated in a combined modeling and experimental study. High-resolution Laplace capacitance spectroscopy with the uniaxial stress technique has been used to measure the stress-energy tensor and the results are compared with theoretical modeling. At low temperatures, implanted H is trapped as a negative-U center with a donor level in the upper half of the gap. However, at higher temperatures, H migrates closer to the carbon impurity and the donor level falls, crossing the gap. At the same time, an acceptor level is introduced into the upper gap making the defect a positive-U center
The ancient heritage of water ice in the solar system
Identifying the source of Earth's water is central to understanding the
origins of life-fostering environments and to assessing the prevalence of such
environments in space. Water throughout the solar system exhibits
deuterium-to-hydrogen enrichments, a fossil relic of low-temperature,
ion-derived chemistry within either (i) the parent molecular cloud or (ii) the
solar nebula protoplanetary disk. Utilizing a comprehensive treatment of disk
ionization, we find that ion-driven deuterium pathways are inefficient,
curtailing the disk's deuterated water formation and its viability as the sole
source for the solar system's water. This finding implies that if the solar
system's formation was typical, abundant interstellar ices are available to all
nascent planetary systems.Comment: 33 pages, 7 figures including main text and supplementary materials.
Published in Scienc
Unlocking CO Depletion in Protoplanetary Disks II. Primordial C/H Predictions Inside the CO Snowline
CO is thought to be the main reservoir of volatile carbon in protoplanetary
disks, and thus the primary initial source of carbon in the atmospheres of
forming giant planets. However, recent observations of protoplanetary disks
point towards low volatile carbon abundances in many systems, including at
radii interior to the CO snowline. One potential explanation is that gas phase
carbon is chemically reprocessed into less volatile species, which are frozen
on dust grain surfaces as ice. This mechanism has the potential to change the
primordial C/H ratio in the gas. However, current observations primarily probe
the upper layers of the disk. It is not clear if the low volatile carbon
abundances extend to the midplane, where planets form. We have run a grid of
198 chemical models, exploring how the chemical reprocessing of CO depends on
disk mass, dust grain size distribution, temperature, cosmic ray and X-ray
ionization rate, and initial water abundance. Building on our previous work
focusing on the warm molecular layer, here we analyze the results for our grid
of models in the disk midplane at 12 au. We find that either an ISM level
cosmic ray ionization rate or the presence of UV photons due to a low dust
surface density are needed to chemically reduce the midplane CO gas abundance
by at least an order of magnitude within 1 Myr. In the majority of our models
CO does not undergo substantial reprocessing by in situ chemistry and there is
little change in the gas phase C/H and C/O ratios over the lifetime of the
typical disk. However, in the small sub-set of disks where the disk midplane is
subject to a source of ionization or photolysis, the gas phase C/O ratio
increases by up to nearly 9 orders of magnitude due to conversion of CO into
volatile hydrocarbons.Comment: Accepted for publication in ApJ, 15 pages, 10 figures, 3 table
Exploring the Origins of Deuterium Enrichments in Solar Nebular Organics
Deuterium-to-hydrogen (D/H) enrichments in molecular species provide clues
about their original formation environment. The organic materials in primitive
solar system bodies have generally higher D/H ratios and show greater D/H
variation when compared to D/H in solar system water. We propose this
difference arises at least in part due to 1) the availability of additional
chemical fractionation pathways for organics beyond that for water, and 2) the
higher volatility of key carbon reservoirs compared to oxygen. We test this
hypothesis using detailed disk models, including a sophisticated, new disk
ionization treatment with a low cosmic ray ionization rate, and find that disk
chemistry leads to higher deuterium enrichment in organics compared to water,
helped especially by fractionation via the precursors CHD/CH. We
also find that the D/H ratio in individual species varies significantly
depending on their particular formation pathways. For example, from
AU, CH can reach , while D/H in CHOH
remains locally unaltered. Finally, while the global organic D/H in our models
can reproduce intermediately elevated D/H in the bulk hydrocarbon reservoir,
our models are unable to reproduce the most deuterium-enriched organic
materials in the solar system, and thus our model requires some inheritance
from the cold interstellar medium from which the Sun formed.Comment: 11 pages, 7 figures, accepted for publication in Ap
Lymph node tissue kallikrein-related peptidase 6 mRNA: a progression marker for colorectal cancer
BACKGROUND: A most important characteristic feature for poor prognosis in colorectal cancer (CRC) is the presence of lymph node metastasis. Determination of carcinoembryonic antigen (CEA) mRNA levels in lymph nodes has proven powerful for quantification of disseminated tumour cells. Here, we investigate the utility of human tissue kallikrein-related peptidase 6 (KLK6) mRNA as a progression biomarker to complement CEA mRNA, for improved selection of patients in need of adjuvant therapy and intensified follow-up after surgery. METHODS: Lymph nodes of pTNM stage I-IV CRC-(166 patients/503 lymph nodes) and control (23/108) patients were collected at surgery and analysed by quantitative RT-PCR. RESULTS: Lymph node KLK6 positivity was an indicator of poor outcome (hazard ratio 3.7). Risk of recurrence and cancer death increased with KLK6 lymph node levels. Patients with KLK6 lymph node levels above the 90th percentile had a hazard ratio of 6.5 and 76 months shorter average survival time compared to patients with KLK6 negative nodes. The KLK6 positivity in lymph nodes with few tumour cells, that is, low CEA mRNA levels, also indicated poor prognosis (hazard ratio 2.8). CONCLUSION: In CRC patients, lymph node KLK6 positivity indicated presence of aggressive tumour cells associated with poor prognosis and high risk of tumour recurrence. British Journal of Cancer (2012) 107, 150-157. doi: 10.1038/bjc.2012.220 www.bjcancer.com Published online 14 June 2012 (C) 2012 Cancer Research U
Nominated procurement and the indirect control of nominated sub-suppliers: evidence from the Sri Lankan apparel supply chain
This article describes and discusses nominated procurement as a means through which buyers select sub-suppliers to achieve sustainability compliance upstream in emerging economies' supply chains. Hence, it critically examines the ways buyers articulate nominated procurement and the unfolding supply chain consequences. Based on in-depth interviews and fieldwork in the Sri Lankan apparel supply chain, the findings indicate that buyers accomplish sustainability compliance among their sub-suppliers while prioritizing their own business agenda. In doing so, however, buyers perpetuate “suboptimal compliance” of raw material suppliers and “sandwiching” of direct suppliers as harmful consequences on the supply chain. These consequences link theoretically with commercial, geographical, compliance and extended-compliance pressure. This article contributes to the advancement of the Sustainable Supply Chain Management literature by theorizing about nominated procurement, direct and indirect pressure, and pointing to the supply chain consequences beyond achievements in sustainability compliance
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