1,072 research outputs found
Photo-desorption of H2O:CO:NH3 circumstellar ice analogs: Gas-phase enrichment
We study the photo-desorption occurring in HO:CO:NH ice mixtures
irradiated with monochromatic (550 and 900 eV) and broad band (250--1250 eV)
soft X-rays generated at the National Synchrotron Radiation Research Center
(Hsinchu, Taiwan). We detect many masses photo-desorbing, from atomic hydrogen
(m/z = 1) to complex species with m/z = 69 (e.g., CHNO, CHO,
CHN), supporting the enrichment of the gas phase.
At low number of absorbed photons, substrate-mediated exciton-promoted
desorption dominates the photo-desorption yield inducing the release of weakly
bound (to the surface of the ice) species; as the number of weakly bound
species declines, the photo-desorption yield decrease about one order of
magnitude, until porosity effects, reducing the surface/volume ratio, produce a
further drop of the yield.
We derive an upper limit to the CO photo-desorption yield, that in our
experiments varies from 1.4 to 0.007 molecule photon in the range ~absorbed photons cm. We apply these findings to a
protoplanetary disk model irradiated by a central T~Tauri star
Accretion and photodesorption of CO ice as a function of the incident angle of deposition
Non-thermal desorption of inter- and circum-stellar ice mantles on dust
grains, in particular ultraviolet photon-induced desorption, has gained
importance in recent years. These processes may account for the observed gas
phase abundances of molecules like CO toward cold interstellar clouds. Ice
mantle growth results from gas molecules impinging on the dust from all
directions and incidence angles. Nevertheless, the effect of the incident angle
for deposition on ice photo-desorption rate has not been studied. This work
explores the impact on the accretion and photodesorption rates of the incidence
angle of CO gas molecules with the cold surface during deposition of a CO ice
layer. Infrared spectroscopy monitored CO ice upon deposition at different
angles, ultraviolet-irradiation, and subsequent warm-up. Vacuum-ultraviolet
spectroscopy and a Ni-mesh measured the emission of the ultraviolet lamp.
Molecules ejected from the ice to the gas during irradiation or warm-up were
characterized by a quadrupole mass spectrometer. The photodesorption rate of CO
ice deposited at 11 K and different incident angles was rather stable between 0
and 45. A maximum in the CO photodesorption rate appeared around
70-incidence deposition angle. The same deposition angle leads to the
maximum surface area of water ice. Although this study of the surface area
could not be performed for CO ice, the similar angle dependence in the
photodesorption and the ice surface area suggests that they are closely
related. Further evidence for a dependence of CO ice morphology on deposition
angle is provided by thermal desorption of CO ice experiments
SOFT X-RAY IRRADIATION OF PURE CARBON MONOXIDE INTERSTELLAR ICE ANALOGUES
There is an increasing evidence for the existence of large organic molecules
in the interstellar and circumstellar medium. Very few among such species are
readily formed in conventional gas-phase chemistry under typical conditions of
interstellar clouds. Attention has therefore focused on interstellar ices as a potential
source of these relatively complex species. Laboratory experiments show
that irradiation of interstellar ice analogues by fast particles or ultraviolet radiation
can induce significant chemical complexity. However, stars are sources of
intense X-rays at almost every stage of their formation and evolution. Such radiation
may thus provide chemical changes in regions where ultraviolet radiation
is severely inhibited.
After H2O, CO is often the most abundant component of icy grain mantles in
dense interstellar clouds and circumstellar disks. In this work we present irradiation
of a pure carbon monoxide ice using a soft X-ray spectrum peaked at 0.3 keV.
Analysis of irradiated samples shows formation of CO2, C2O, C3O2, C3, C4O and
CO3/C5. Comparison of X-rays and ultraviolet irradiation experiments, of the
same energy dose, show that X-rays are more efficient than ultraviolet radiation
in producing new species. With the exception of CO2, X-ray photolysis induces
formation of a larger number of products with higher abundances, e.g., C3O2
column density is about one order of magnitude higher in the X-ray experiment.
To our knowledge this is the first report on X-ray photolysis of CO ices. The
present results show that X-ray irradiation represents an efficient photo-chemical
way to convert simple ices to more complex species
Aplicación de técnicas y examen sistemático de métodos de los servicios y operaciones realizadas por una microempresa de transporte nacional e internacional
In the transportation industry there is a great competitive level between large transportation companies at national and international level with wide coverage, great benefits and direct access, however there are gaps in the system leaving aside a large market that needs these services, obtaining large delays, which will be presented in this writing, this market are those people who require deliveries, shipments or search for packages to and from abroad and even advice on how effective can be these shipments, mostly small businesses cannot acquire the services of transportation companies with large coverage. Based on this, the case study was applied in a microenterprise SA, a company specialized in logistics and transportation at national and international level, both export and import of goods, also has a complementary service of customs consultancy, where we could analyze in detail the step by step process that is performed in this entity noting certain factors where the process is being delayed. This allowed us to establish the processes to be followed.
This project focuses on the search for improvements in the delivery and shipment of packages nationally and internationally in order to provide the customer with a good experience in the service provided. The respective analysis was carried out showing two major bottlenecks such as order taking and parcel collection, based on this we used tools such as: the application of time study, productivity calculation and critical analysis. Each of these processes were exposed in detail, but we limited the information only with the direct relation with the subject of the study and the time that is handled to work, so that a clear understanding of the processes to follow to achieve the established objective was obtained obtaining an improvement plan for the company.En la industria del transporte se evidencia una gran nivel competitivo entre grandes empresas de transporte a nivel nacional e internacional con amplias coberturas, grandes beneficios y accesos directos, sin embargo hay vacíos en el sistema dejando aparte un amplio mercado que necesita de estos servicios, obteniendo grandes retrasos, en los cuales se presentaran en este escrito, dicho mercado son aquellas personas que requieren realizar entregas, envíos o búsqueda de paquetes desde y hacia el exterior e incluso asesoramientos de que tan efectivos pueden ser estos envíos, en su gran mayoría los pequeños emprendimientos no pueden adquirir los servicios de empresas de transporte de gran cobertura. Basado a esto el caso de estudio fue aplicado en una microempresa de transporte, una compañía especializada en logística y transporte a nivel nacional e internacional, tanto de exportación como de importación de mercancías, también cuenta con servicio complementario de asesoría aduanera, donde pudimos analizar detalladamente el paso a paso del proceso que se realiza en esta entidad notando ciertos factores donde el proceso se está retrasando. Así se permitido establecer los procesos a seguir.
Este proyecto se centra en la búsqueda de mejoras de entrega y envíos de paquetes a nivel nacional e internacional para así brindarle al cliente una buena experiencia en el servicio prestado. Se realizó el respectivo análisis demostrando dos grandes cuellos de botella como son toma de pedido y recogida de paquetería, basándonos a esto utilizamos las herramientas tales como: la aplicación de estudio de tiempo, cálculo de productividad y realización de análisis crítico. Cada uno de estos procesos fueron expuestos de manera detallada, pero limitamos la información únicamente con la relación directa con el tema del estudio y el tiempo que se maneja para trabajar, de forma que se obtuviera una clara comprensión de los procesos a seguir para alcanzar el objetivo establecido obteniendo un plan de mejora para la empresa
 
Gas phase Elemental abundances in Molecular cloudS (GEMS) : II. On the quest for the sulphur reservoir in molecular clouds: the H2S case
Context. Sulphur is one of the most abundant elements in the Universe. Surprisingly, sulphuretted molecules are not as abundant as expected in the interstellar medium and the identity of the main sulphur reservoir is still an open question.Aims. Our goal is to investigate the H2S chemistry in dark clouds, as this stable molecule is a potential sulphur reservoir.Methods. Using millimeter observations of CS, SO, H2S, and their isotopologues, we determine the physical conditions and H2S abundances along the cores TMC 1-C, TMC 1-CP, and Barnard 1b. The gas-grain model NAUTILUS is used to model the sulphur chemistry and explore the impact of photo-desorption and chemical desorption on the H2S abundance.Results. Our modeling shows that chemical desorption is the main source of gas-phase H2S in dark cores. The measured H2S abundance can only be fitted if we assume that the chemical desorption rate decreases by more than a factor of 10 when n(H) > 2 x 10(4). This change in the desorption rate is consistent with the formation of thick H2O and CO ice mantles on grain surfaces. The observed SO and H2S abundances are in good agreement with our predictions adopting an undepleted value of the sulphur abundance. However, the CS abundance is overestimated by a factor of 5-10. Along the three cores, atomic S is predicted to be the main sulphur reservoir.Conclusions. The gaseous H2S abundance is well reproduced, assuming undepleted sulphur abundance and chemical desorption as the main source of H2S. The behavior of the observed H2S abundance suggests a changing desorption efficiency, which would probe the snowline in these cold cores. Our model, however, highly overestimates the observed gas-phase CS abundance. Given the uncertainty in the sulphur chemistry, we can only conclude that our data are consistent with a cosmic elemental S abundance with an uncertainty of a factor of 10.Peer reviewe
A BAX/BAK and Cyclophilin D-Independent Intrinsic Apoptosis Pathway
Most intrinsic death signals converge into the activation of pro-apoptotic BCL-2 family members BAX and BAK at the mitochondria, resulting in the release of cytochrome c and apoptosome activation. Chronic endoplasmic reticulum (ER) stress leads to apoptosis through the upregulation of a subset of pro-apoptotic BH3-only proteins, activating BAX and BAK at the mitochondria. Here we provide evidence indicating that the full resistance of BAX and BAK double deficient (DKO) cells to ER stress is reverted by stimulation in combination with mild serum withdrawal. Cell death under these conditions was characterized by the appearance of classical apoptosis markers, caspase-9 activation, release of cytochrome c, and was inhibited by knocking down caspase-9, but insensitive to BCL-XL overexpression. Similarly, the resistance of BIM and PUMA double deficient cells to ER stress was reverted by mild serum withdrawal. Surprisingly, BAX/BAK-independent cell death did not require Cyclophilin D (CypD) expression, an important regulator of the mitochondrial permeability transition pore. Our results suggest the existence of an alternative intrinsic apoptosis pathway emerging from a cross talk between the ER and the mitochondria
Effective Rheology of Bubbles Moving in a Capillary Tube
We calculate the average volumetric flux versus pressure drop of bubbles
moving in a single capillary tube with varying diameter, finding a square-root
relation from mapping the flow equations onto that of a driven overdamped
pendulum. The calculation is based on a derivation of the equation of motion of
a bubble train from considering the capillary forces and the entropy production
associated with the viscous flow. We also calculate the configurational
probability of the positions of the bubbles.Comment: 4 pages, 1 figur
Elliptic flow of charged particles in Pb-Pb collisions at 2.76 TeV
We report the first measurement of charged particle elliptic flow in Pb-Pb
collisions at 2.76 TeV with the ALICE detector at the CERN Large Hadron
Collider. The measurement is performed in the central pseudorapidity region
(||<0.8) and transverse momentum range 0.2< < 5.0 GeV/. The
elliptic flow signal v, measured using the 4-particle correlation method,
averaged over transverse momentum and pseudorapidity is 0.087 0.002
(stat) 0.004 (syst) in the 40-50% centrality class. The differential
elliptic flow v reaches a maximum of 0.2 near = 3
GeV/. Compared to RHIC Au-Au collisions at 200 GeV, the elliptic flow
increases by about 30%. Some hydrodynamic model predictions which include
viscous corrections are in agreement with the observed increase.Comment: 10 pages, 4 captioned figures, published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/389
Gas phase Elemental abundances in Molecular cloudS (GEMS) : III. Unlocking the CS chemistry: the CS plus O reaction
Context. Carbon monosulphide (CS) is among the most abundant gas-phase S-bearing molecules in cold dark molecular clouds. It is easily observable with several transitions in the millimeter wavelength range, and has been widely used as a tracer of the gas density in the interstellar medium in our Galaxy and external galaxies. However, chemical models fail to account for the observed CS abundances when assuming the cosmic value for the elemental abundance of sulfur. Aims. The CS+O -> CO + S reaction has been proposed as a relevant CS destruction mechanism at low temperatures, and could explain the discrepancy between models and observations. Its reaction rate has been experimentally measured at temperatures of 150-400 K, but the extrapolation to lower temperatures is doubtful. Our goal is to calculate the CS+O reaction rate at temperatures Methods. We performed ab initio calculations to obtain the three lowest potential energy surfaces (PES) of the CS+O system. These PESs are used to study the reaction dynamics, using several methods (classical, quantum, and semiclassical) to eventually calculate the CS + O thermal reaction rates. In order to check the accuracy of our calculations, we compare the results of our theoretical calculations for T similar to 150-400 K with those obtained in the laboratory. Results. Our detailed theoretical study on the CS+O reaction, which is in agreement with the experimental data obtained at 150-400 K, demonstrates the reliability of our approach. After a careful analysis at lower temperatures, we find that the rate constant at 10 K is negligible, below 10(-15) cm(3) s(-1), which is consistent with the extrapolation of experimental data using the Arrhenius expression. Conclusions. We use the updated chemical network to model the sulfur chemistry in Taurus Molecular Cloud 1 (TMC 1) based on molecular abundances determined from Gas phase Elemental abundances in Molecular CloudS (GEMS) project observations. In our model, we take into account the expected decrease of the cosmic ray ionization rate, zeta(H2), along the cloud. The abundance of CS is still overestimated when assuming the cosmic value for the sulfur abundance.Peer reviewe
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