5,326 research outputs found
The chemical structure of the very young starless core L1521E
L1521E is a dense starless core in Taurus that was found to have relatively
low molecular depletion by earlier studies, thus suggesting a recent formation.
We aim to characterize the chemical structure of L1521E and compare it to the
more evolved L1544 pre-stellar core. We have obtained 2.52.5
arcminute maps toward L1521E using the IRAM-30m telescope in transitions of
various species. We derived abundances for the species and compared them to
those obtained toward L1544. We estimated CO depletion factors. Similarly to
L1544, -CH and CHOH peak at different positions. Most species
peak toward the -CH peak. The CO depletion factor derived toward the
dust peak is 4.31.6, which is about a factor of three lower
than that toward L1544. The abundances of sulfur-bearing molecules are higher
toward L1521E than toward L1544 by factors of 2-20. The abundance of
methanol is similar toward the two cores. The higher abundances of
sulfur-bearing species toward L1521E than toward L1544 suggest that significant
sulfur depletion takes place during the dynamical evolution of dense cores,
from the starless to pre-stellar stage. The CO depletion factor measured toward
L1521E suggests that CO is more depleted than previously found. Similar
CHOH abundances between L1521E and L1544 hint that methanol is forming at
specific physical conditions in Taurus, characterized by densities of a few
10 cm and (H)10 cm, when CO
starts to catastrophically freeze-out, while water can still be significantly
photodissociated, so that the surfaces of dust grains become rich in solid CO
and CHOH, as already found toward L1544. Methanol can thus provide
selective crucial information about the transition region between dense cores
and the surrounding parent cloud.Comment: Accepted for publication in A&A, abstract abridge
First sample of nitrogen isotopic ratio measurements in low-mass protostars
Context. The nitrogen isotopic ratio is considered an important diagnostic
tool of the star formation process, and is particularly important
because it is directly linked to molecular nitrogen . However, theoretical
models still lack to provide an exhaustive explanation for the observed
values.
Aims. Recent theoretical works suggest that the behaviour is
dominated by two competing reactions that destroy : dissociative
recombination and reaction with CO. When CO is depleted from the gas phase, if
recombination rate is lower with respect to the one, the
rarer isotopologue is destroyed faster. This implies that the isotopic
ratio in protostars should be lower than the one in prestellar cores, and
consistent with the elemental value of ~440. We aim to test this hypothesis,
producing the first sample of measurements in low mass
protostars.
Methods. We observe the and lowest rotational
transition towards six young stellar objects in Perseus and Taurus molecular
clouds. We model the spectra with a custom python code using a constant
approach to fit the observations. We discuss in appendix the validity
of this hypothesis. The derived column densities are used to compute the
nitrogen isotopic ratio.
Results. Our analysis yields an average of in the protostellar sample. This is consistent with the protosolar
value of 440, and significantly lower than the average value previously
obtained in a sample of prestellar objects. Conclusions. Our results are in
agreement with the hypothesis that, when CO is depleted from the gas-phase,
dissociative recombinations with free electrons destroy faster
than , leading to high isotopic ratios in prestellar cores, where CO is
frozen on dust grains.Comment: Accepted on A&A on 09 Oct 202
Vibrational Stability of NLC Linac accelerating structure
The vibration of components of the NLC linac, such as accelerating structures
and girders, is being studied both experimentally and analytically. Various
effects are being considered including structural resonances and vibration
caused by cooling water in the accelerating structure. This paper reports the
status of ongoing work.Comment: 3 pages 8 figures Presented at EPAC 2002 Paris Franc
Baseline LHC machine parameters and configuration of the 2015 proton run
This paper shows the baseline LHC machine parameters for the 2015 start-up.
Many systems have been upgraded during LS1 and in 2015 the LHC will operate at
a higher energy than before and with a tighter filling scheme. Therefore, the
2015 commissioning phase risks to be less smooth than in 2012. The proposed
starting configuration puts the focus on feasibility rather than peak
performance and includes margins for operational uncertainties. Instead, once
beam experience and a better machine knowledge has been obtained, a push in
and performance can be envisaged. In this paper, the focus is on
collimation settings and reach in ---other parameters are covered in
greater depth by other papers in these proceedings.Comment: submitted for publication in a CERN yellow report (Proceedings of the
LHC Performance Workshop - Chamonix 2014
Comparazione di metodi termovisivi per l’identificazione di aree umide su materiali dell’edilizia storica
Water content inside building materials (plaster, brick, stone) is usefull to evaluate their decay level. Passive and active termography are compared, in order to define the most reliable procedure, firstly to map the moisture diffusion and secondary to evaluate the moisture content in the surfaces. Laboratory researches carried out in the last decades and scientific literature permitted to determine that the superficial decay in porous materials is more related to the evaporative speed of the surfaces and the presence of soluble salts than to their absorption capability. Moreover, evaporative fluxes were studied at different environmental conditions and water content in order to find out a correlation between moisture content, evaporation and boundary conditions. The thermal characteristics of timber are highly different from the characteristics of porous materials such as brick and stone and mortar, particularly the thermal capacity of wood is lower. Nevertheless, because of the lower heat capacity of wood, the presence of water greatly affects the wood thermal capacity: the active procedure, guarantees the best results. Lab tests and study case permit to evaluate the sensitivity of the method
Dry granular flows: micromechanical interpretation of impacts on rigid obstacles.
The evaluation of impact forces exerted by flowing granular masses on rigid obstacles is of fundamental importance for the assessment of the associated risk and for the design of protection measures. A number of formulae are available in the literature for the maximum impact force; most of them are based on oversimplifying hypotheses about the behaviour of the granular material. For practical applications, formulations based on either hydrodynamic or elastic body models are often employed. These formulations require the use of empirical correcting factors. In order to better understand the impact mechanics, the authors have recently performed an extensive numerical campaign by using a Discrete Element approach (PFC3D code), where a dry granular mass is represented as a random distribution of rigid spherical particles. A new design formula, combining the hydrodynamic and elastic body theories, has been proposed on the base of the results obtained at the macroscopic scale. The parameters of the formula have been correlated with geometrical factors, namely front inclination and flow height.
In this paper, the same DEM model is further used in order to investigate the relationship between the evolution with time of the impact force and the micromechanics of the granular mass. In particular, information about contact forces and particle velocities will be discussed and critically compared with macroscopic results. In order to progressively introduce the complexity of the impact phenomenon, three geometrical conditions are considered: a) vertical front, confined flow; b) vertical front, free surface flow; c) inclined front, free surface flow
Nitrogen fractionation in ammonia and its insights on nitrogen chemistry
Context. Observations of in the interstellar medium are
becoming more frequent thanks to the increased telescope capabilities. However,
interpreting these data is still puzzling. In particular, measurements of in diazenylium revealed high levels of anti-fractionation in
cold cores.
Aims. Furuya & Aikawa (2018), using astrophysical simulations coupled with a
gas-grain chemical code, concluded that the N-depletion in prestellar
cores could be inherited from the initial stages, when is
selectively photodissociated and 15N atoms deplete onto the dust grain, forming
ammonia ices. We aim to test this hypothesis.
Methods. We targeted three sources (the prestellar core L1544, the
protostellar envelope IRAS4A, and the shocked region L1157-B1) with distinct
degrees of desorption or sputtering of the ammonia ices. We observed the NH3
isotopologues with the GBT, and we inferred the via a
spectral fitting of the observed inversion transitions.
Results. NH3(1,1) is detected in L1544 and IRAS4A, whilst only upper
limits are deduced in L1157-B1. The NH3 isotopic ratio is significantly lower
towards the protostar than at the centre of L1544, where it is consistent with
the elemental value. We also present the first spatially resolved map of NH3
nitrogen isotopic ratio towards L1544.
Conclusions. Our results are in agreement with the hypothesis that ammonia
ices are enriched in N, leading to a decrease of the
ratio when the ices are sublimated into the gas phase for instance due to the
temperature rise in protostellar envelopes. The ammonia
value at the centre of L1544 is a factor of 2 lower than that of N2H+,
suggesting that the dominant formation pathway is hydrogenation of N atoms on
dust grains, followed by non-thermal desorption.Comment: Accepted for publication in A&A on 29/05/2
Design of polarization-insensitive superconducting single photon detectors with high-index dielectrics
In this paper, the design of superconducting-nanowire single-photon detectors
which are insensitive to the polarization of the incident light is
investigated. By using high-refractive-index dielectrics, the index mismatch
between the nanowire and the surrounding media is reduced. This enhances the
absorption of light with electric field vector perpendicular to the nanowire
segments, which is generally hindered in this kind of detectors. Building on
this principle and focusing on NbTiN nanowire devices, we present several
easy-to-realize cavity architectures which allow high absorption efficiency (in
excess of 90%) and polarization insensitivity simultaneously. Designs based on
ultranarrow nanowires, for which the polarization sensitivity is much more
marked, are also presented. Finally, we briefly discuss the specific advantages
of this approach in the case of WSi or MoSi nanowires
Effects of genetic and agronomic factors on grain composition in oats
The effects of genetic variability and agronomic practises on the chemical compounds of the grain were evaluated in a set of oat cultivars (16 naked and four husked) grown in different experiments in the years 2004–2008. In the first experiment, carried out for two years, β-glucan and Total Dietary Fibre (TDF) from eight oat cultivars, grown under three different levels of nitrogen fertilization and two seed doses, were considered. The second experiment, involving 12 cultivars for two years, explored the variation of β-glucan solubility and further characterized six cultivars for the content of TDF and arabinoxylans. In both experiments, genotype was found to exert the largest effect on the grain composition; nitrogen levels and seed doses had positive significant effects on protein and β-glucan contents, but did not affect fibre content. Among the naked cultivars, Irina, Abel, Luna, Hendon and Expression showed a good ability to accumulate the examined grain compounds. However, in general, the highest contents of protein and β-glucan were found in the groats of husked cultivars, suggesting that specific breeding programs are a crucial step to identify the suitable naked oat genotypes to produce foods of high nutritional value
Measurements of heavy ion beam losses from collimation
The collimation efficiency for Pb ion beams in the LHC is predicted to be
lower than requirements. Nuclear fragmentation and electromagnetic dissociation
in the primary collimators create fragments with a wide range of Z/A ratios,
which are not intercepted by the secondary collimators but lost where the
dispersion has grown sufficiently large. In this article we present
measurements and simulations of loss patterns generated by a prototype LHC
collimator in the CERN SPS. Measurements were performed at two different
energies and angles of the collimator. We also compare with proton loss maps
and find a qualitative difference between Pb ions and protons, with the maximum
loss rate observed at different places in the ring. This behavior was predicted
by simulations and provides a valuable benchmark of our understanding of ion
beam losses caused by collimation.Comment: 12 pages, 20 figure
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