1,083 research outputs found
High DO/HDO ratio in the inner regions of the low-mass protostar NGC1333 IRAS2A
Water plays a crucial role both in the interstellar medium and on Earth. To
constrain its formation mechanisms and its evolution through the star formation
process, the determination of the water deuterium fractionation ratios is
particularly suitable. Previous studies derived HDO/HO ratios in the warm
inner regions of low-mass protostars. We here report a detection of the DO
1-1 transition toward the low-mass protostar NGC1333 IRAS2A
with the Plateau de Bure interferometer: this represents the first
interferometric detection of DO - and only the second solar-type protostar
for which this isotopologue is detected. Using the observations of the HDO
5-6 transition simultaneously detected and three other HDO
lines previously observed, we show that the HDO line fluxes are well reproduced
with a single excitation temperature of 21821 K and a source size of
0.5 arcsec. The DO/HDO ratio is (1.20.5)
10, while the use of previous HO observations give an
HDO/HO ratio of (1.70.8) 10, i.e. a factor of 7
lower than the DO/HDO ratio. These results contradict the predictions of
current grain surface chemical models and indicate that either the surface
deuteration processes are poorly understood or that both sublimation of grain
mantles and water formation at high temperatures (230 K) take place in
the inner regions of this source. In the second scenario, the thermal
desorption of the grain mantles would explain the high DO/HDO ratio, while
water formation at high temperature would explain significant extra production
of HO leading to a decrease of the HDO/HO ratio.Comment: Accepted for publication in ApJ Letters; 12 pages, 2 figure
A Survey on the Contributions of Software-Defined Networking to Traffic Engineering
Since the appearance of OpenFlow back in 2008, software-defined networking (SDN) has gained momentum. Although there are some discrepancies between the standards developing organizations working with SDN about what SDN is and how it is defined, they all outline traffic engineering (TE) as a key application. One of the most common objectives of TE is the congestion minimization, where techniques such as traffic splitting among multiple paths or advanced reservation systems are used. In such a scenario, this manuscript surveys the role of a comprehensive list of SDN protocols in TE solutions, in order to assess how these protocols can benefit TE. The SDN protocols have been categorized using the SDN architecture proposed by the open networking foundation, which differentiates among data-controller plane interfaces, application-controller plane interfaces, and management interfaces, in order to state how the interface type in which they operate influences TE. In addition, the impact of the SDN protocols on TE has been evaluated by comparing them with the path computation element (PCE)-based architecture. The PCE-based architecture has been selected to measure the impact of SDN on TE because it is the most novel TE architecture until the date, and because it already defines a set of metrics to measure the performance of TE solutions. We conclude that using the three types of interfaces simultaneously will result in more powerful and enhanced TE solutions, since they benefit TE in complementary ways.European Commission through the Horizon 2020 Research and Innovation Programme (GN4) under Grant 691567
Spanish Ministry of Economy and Competitiveness under the Secure Deployment of Services Over SDN and NFV-based Networks Project S&NSEC under Grant TEC2013-47960-C4-3-
Constraining the abundances of complex organics in the inner regions of solar-type protostars
The high abundances of Complex Organic Molecules (COMs) with respect to
methanol, the most abundant COM, detected towards low-mass protostars, tend to
be underpredicted by astrochemical models. This discrepancy might come from the
large beam of the single-dish telescopes, encompassing several components of
the studied protostar, commonly used to detect COMs. To address this issue, we
have carried out multi-line observations of methanol and several COMs towards
the two low-mass protostars NGC1333-IRAS2A and -IRAS4A with the Plateau de Bure
interferometer at an angular resolution of 2 arcsec, resulting in the first
multi-line detection of the O-bearing species glycolaldehyde and ethanol and of
the N-bearing species ethyl cyanide towards low-mass protostars other than IRAS
16293. The high number of detected transitions from COMs (more than 40 methanol
transitions for instance) allowed us to accurately derive the source size of
their emission and the COMs column densities. The COMs abundances with respect
to methanol derived towards IRAS2A and IRAS4A are slightly, but not
substantitally, lower than those derived from previous single-dish
observations. The COMs abundance ratios do not vary significantly with the
protostellar luminosity, over five orders of magnitude, implying that low-mass
hot corinos are quite chemically rich as high-mass hot cores. Astrochemical
models still underpredict the abundances of key COMs, such as methyl formate or
di-methyl ether, suggesting that our understanding of their formation remains
incomplete.Comment: 60 pages, 10 figures, 17 tables. Accepted for publication in Ap
Can Formamide Be Formed on Interstellar Ice? An Atomistic Perspective
Interstellar formamide (NH2CHO) has recently attracted significant attention
due to its potential role as a molecular building block in the formation of
precursor biomolecules relevant for the origin of life. Its formation, whether
on the surfaces of the interstellar grains or in the gas phase, is currently
debated. The present article presents new theoretical quantum chemical
computations on possible NH2CHO formation routes in water-rich amorphous ices,
simulated by a 33-H2O-molecule cluster. We have considered three possible
routes. The first one refers to a scenario used in several current
astrochemical models, that is, the radical-radical association reaction between
NH2 and HCO. Our calculations show that formamide can indeed be formed, but in
competition with formation of NH3 and CO through a direct H transfer process.
The final outcome of the NH2 + HCO reactivity depends on the relative
orientation of the two radicals on the ice surface. We then analyzed two other
possibilities, suggested here for the first time: reaction of either HCN or CN
with water molecules of the ice mantle. The reaction with HCN has been found to
be characterized by large energy barriers and, therefore, cannot occur under
the interstellar ice conditions. On the contrary, the reaction with the CN
radical can occur, possibly leading through multiple steps to the formation of
NH2CHO. For this reaction, water molecules of the ice act as catalytic active
sites since they help the H transfers involved in the process, thus reducing
the energy barriers (compared to the gas-phase analogous reaction).
Additionally, we apply a statistical model to estimate the reaction rate
coefficient when considering the cluster of 33-H2O-molecules as an isolated
moiety with respect to the surrounding environment, i.e., the rest of the ice
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