278 research outputs found
Deuterium enrichment of ammonia produced by surface N+H/D addition reactions at low temperature
The surface formation of NH3 and its deuterated isotopologues – NH_2D, NHD_2, and ND_3 – is investigated at low temperatures through the simultaneous addition of hydrogen and deuterium atoms to nitrogen atoms in CO-rich interstellar ice analogues. The formation of all four ammonia isotopologues is only observed up to 15 K, and drops below the detection limit for higher temperatures. Differences between hydrogenation and deuteration yields result in a clear deviation from a statistical distribution in favour of deuterium enriched species. The data analysis suggests that this is due to a higher sticking probability of D atoms to the cold surface, a property that may generally apply to molecules that are formed in low temperature surface reactions. The results found here are used to interpret ammonia–deuterium fractionation as observed in pre-protostellar cores
Solid CO_2 in low-mass young stellar objects: Comparison between Spitzer and laboratory spectra
Context. Solid interstellar CO_2 is an abundant component of ice dust mantles. Its ubiquity towards quiescent molecular clouds, as well as protostellar envelopes, has recently been confirmed by the IRS (InfraRed Spectrograph) aboard the Spitzer Space Telescope. Although it has been shown that CO_2 cannot be efficiently formed in the gas phase, the CO_2 surface formation pathway is still unclear. To date several CO_2 surface formation mechanisms induced by energetic (e.g., UV photolysis and cosmic ray irradiation) and non-energetic (e.g., cold atom addition) input have been proposed.
Aims. Our aim is to investigate the contribution of cosmic ray irradiation to the formation of CO_2 in different regions of the interstellar medium (ISM). To achieve this goal we compared quantitatively laboratory data with the CO_2 bending mode band profile observed towards several young stellar objects (YSOs) and a field star by the Spitzer Space Telescope.
Methods. All the experiments presented here were performed at the Laboratory for Experimental Astrophysics in Catania (Italy). The interstellar relevant samples were all irradiated with fast ions (30−200 keV) and subsequently annealed in a stainless steel high vacuum chamber (P < 10^(-7) mbar). Chemical and structural modifications of the ice samples were monitored by means of infrared spectroscopy. Laboratory spectra were then used to fit some thirty observational spectra.
Results. A qualitative analysis shows that a good fit can be obtained with a minimum of two components. The choice of the laboratory components is based on the chemical-physical condition of each source. A quantitative analysis of the sources with known visual extinction (A_V) and methanol abundances highlights that the solid carbon dioxide can be efficiently and abundantly formed after ion irradiation of interstellar ices in all the selected YSOs in a time compatible with cloud lifetimes (3 × 10^7 years). Only in the case of field stars can the expected CO_2 column density formed upon energetic input not explain the observed abundances. This result, to be confirmed along the line of sight to different quiescent clouds, gives an indirect indication that CO_2 can also be formed in an early cloud stage through surface reactions induced by non-energetic mechanisms. In a later stage, when ices are exposed to higher UV and cosmic ray doses, the CO_2 total abundance is strongly affected by energetic formation mechanisms.
Conclusions. Our results indicate that energetic processing of icy grain mantles significantly contribute to the formation of solid phase interstellar CO_2
SURFRESIDE2: An ultrahigh vacuum system for the investigation of surface reaction routes of interstellar interest
Article / Letter to editorSterrewach
Water formation at low temperatures by surface O2 hydrogenation I: characterization of ice penetration
Water is the main component of interstellar ice mantles, is abundant in the
solar system and is a crucial ingredient for life. The formation of this
molecule in the interstellar medium cannot be explained by gas-phase chemistry
only and its surface hydrogenation formation routes at low temperatures (O, O2,
O3 channels) are still unclear and most likely incomplete. In a previous paper
we discussed an unexpected zeroth-order H2O production behavior in O2 ice
hydrogenation experiments compared to the first-order H2CO and CH3OH production
behavior found in former studies on hydrogenation of CO ice. In this paper we
experimentally investigate in detail how the structure of O2 ice leads to this
rare behavior in reaction order and production yield. In our experiments H
atoms are added to a thick O2 ice under fully controlled conditions, while the
changes are followed by means of reflection absorption infrared spectroscopy
(RAIRS). The H-atom penetration mechanism is systematically studied by varying
the temperature, thickness and structure of the O2 ice. We conclude that the
competition between reaction and diffusion of the H atoms into the O2 ice
explains the unexpected H2O and H2O2 formation behavior. In addition, we show
that the proposed O2 hydrogenation scheme is incomplete, suggesting that
additional surface reactions should be considered. Indeed, the detection of
newly formed O3 in the ice upon H-atom exposure proves that the O2 channel is
not an isolated route. Furthermore, the addition of H2 molecules is found not
to have a measurable effect on the O2 reaction channel.Comment: 1 page, 1 figur
Water formation at low temperatures by surface O2 hydrogenation II: the reaction network
Water is abundantly present in the Universe. It is the main component of
interstellar ice mantles and a key ingredient for life. Water in space is
mainly formed through surface reactions. Three formation routes have been
proposed in the past: hydrogenation of surface O, O2, and O3. In a previous
paper [Ioppolo et al., Astrophys. J., 2008, 686, 1474] we discussed an
unexpected non-standard zeroth-order H2O2 production behaviour in O2
hydrogenation experiments, which suggests that the proposed reaction network is
not complete, and that the reaction channels are probably more interconnected
than previously thought. In this paper we aim to derive the full reaction
scheme for O2 surface hydrogenation and to constrain the rates of the
individual reactions. This is achieved through simultaneous H-atom and O2
deposition under ultra-high vacuum conditions for astronomically relevant
temperatures. Different H/O2 ratios are used to trace different stages in the
hydrogenation network. The chemical changes in the forming ice are followed by
means of reflection absorption infrared spectroscopy (RAIRS). New reaction
paths are revealed as compared to previous experiments. Several reaction steps
prove to be much more efficient (H + O2) or less efficient (H + OH and H2 + OH)
than originally thought. These are the main conclusions of this work and the
extended network concluded here will have profound implications for models that
describe the formation of water in space.Comment: 1 page, 1 figur
H-atom addition and abstraction reactions in mixed CO, H2CO and CH3OH ices: an extended view on complex organic molecule formation
Complex organic molecules (COMs) have been observed not only in the hot cores
surrounding low- and high- mass protostars, but also in cold dark clouds.
Therefore, it is interesting to understand how such species can be formed
without the presence of embedded energy sources. We present new laboratory
experiments on the low-temperature solid state formation of three complex
molecules: methyl formate (HC(O)OCH3), glycolaldehyde (HC(O)CH2OH) and ethylene
glycol (H2C(OH)CH2OH), through recombination of free radicals formed via H-atom
addition and abstraction reactions at different stages in the CO-H2CO-CH3OH
hydrogenation network at 15 K. The experiments extend previous CO hydrogenation
studies and aim at resembling the physical&chemical conditions typical of the
CO freeze-out stage in dark molecular clouds, when H2CO and CH3OH form by
recombination of accreting CO molecules and H-atoms on ice grains. We confirm
that H2CO, once formed through CO hydrogenation, not only yields CH3OH through
ongoing H-atom addition reactions, but is also subject to H-atom-induced
abstraction reactions, yielding CO again. In a similar way, H2CO is also formed
in abstraction reactions involving CH3OH. The dominant methanol H-atom
abstraction product is expected to be CH2OH, while H-atom additions to H2CO
should at least partially proceed through CH3O intermediate radicals. The
occurrence of H-atom abstraction reactions in ice mantles leads to more
reactive intermediates (HCO, CH3O and CH2OH) than previously thought, when
assuming sequential H-atom addition reactions only. This enhances the
probability to form COMs through radical-radical recombination without the need
of UV photolysis or cosmic rays as external triggers.Comment: 20 pages, 8 figure
Relevance of the H_2 + O reaction pathway for the surface formation of interstellar water. Combined experimental and modeling study
The formation of interstellar water is commonly accepted to occur on the surfaces of icy dust grains in dark molecular clouds at low temperatures (10–20 K), involving hydrogenation reactions of oxygen allotropes. As a result of the large abundances of molecular hydrogen and atomic oxygen in these regions, the reaction H_2 + O has been proposed to contribute significantly to the formation of water as well. However, gas-phase experiments and calculations, as well as solid-phase experimental work contradict this hypothesis. Here, we use precisely executed temperature-programmed desorption (TPD) experiments in an ultra-high vacuum setup combined with kinetic Monte Carlo simulations to establish an upper limit of the water production starting from H_2 and O. These reactants were brought together in a matrix of CO_2 in a series of (control) experiments at different temperatures and with different isotopological compositions. The water detected with the quadrupole mass spectrometer upon TPD was found to originate mainly from contamination in the chamber itself. However, if water is produced in small quantities on the surface through H_2 + O, this can only be explained by a combined classical and tunneled reaction mechanism. An absolutely conservative upper limit for the reaction rate was derived with a microscopic kinetic Monte Carlo model that converts the upper limit into the highest possible reaction rate. Incorporating this rate into simulation runs for astrochemically relevant parameters shows that the upper limit to the contribution of the reaction H_2 + O in OH, and hence water formation, is 11% in dense interstellar clouds. Our combined experimental and theoretical results indicate, however, that this contribution is most likely much lower
Competitiveness and the Logistics Performance Index: The ANOVA method application for Africa, Asia, and the EU regions
Abstract This paper analyses the impact of strategic sub-components of the Global Competitiveness Index (GCI) on the Logistics Performance Index (LPI). As a hypothesis, it is assumed that there is a relationship between the LPI and selected factors in GCI, which were grouped into three clusters: infrastructure, human factor, and institutions. The purpose is to investigate which of those groups has the most significant impact on the LPI - an interactive comparative analysis tool created by the World Bank that addresses logistics issues in a broad context against world regions' development or countries' economies. For this purpose, the LPI was used as the dependent variable, while a linear regression model measured some GCI components' influence. The study was conducted for Africa, Asia, and the EU, employing the ANOVA method. The paper finds the three clusters are related to higher efficiency. While the new method shows these clusters are essential for improving the logistics performance index, an extensive range of factors might affect logistics sector performance in both geography and stage of development. In Europe, human factor is far more critical for progressively improving the LPI, while necessary infrastructure remains crucial in Asia. All three factors are central to Africa's logistics development
Hydrogenation reactions in interstellar CO ice analogues
Hydrogenation reactions of CO in inter- and circumstellar ices are regarded
as an important starting point in the formation of more complex species.
Previous laboratory measurements by two groups on the hydrogenation of CO ices
resulted in controversial results on the formation rate of methanol. Our aim is
to resolve this controversy by an independent investigation of the reaction
scheme for a range of H-atom fluxes and different ice temperatures and
thicknesses. Reaction rates are determined by using a state-of-the-art ultra
high vacuum experimental setup to bombard an interstellar CO ice analog with
room temperature H atoms. The reaction of CO + H into H2CO and subsequently
CH3OH is monitored by a Fourier transform infrared spectrometer in a reflection
absorption mode. In addition, after each completed measurement a temperature
programmed desorption experiment is performed to identify the produced species.
Different H-atom fluxes, morphologies, and ice thicknesses are tested. The
formation of both formaldehyde and methanol via CO hydrogenation is confirmed
at low temperature (12-20 K). We confirm, as proposed by Hidaka et al., that
the discrepancy between the two Japanese studies is mainly due to a difference
in the applied hydrogen atom flux. The production rate of formaldehyde is found
to decrease and the penetration column to increase with temperature. In order
to fully understand the laboratory data, the experimental results are
interpreted using Monte Carlo simulations. This technique takes into account
the layered structure of CO ice. Temperature-dependent reaction barriers and
diffusion rates are inferred using this model. The model is extended to
interstellar conditions to compare with observational H2CO/CH3OH data.Comment: accepted by A. & A., 21 pages, 15 figure
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