365 research outputs found
Polycyclic aromatic hydrocarbon processing in interstellar shocks
Context: PAHs appear to be an ubiquitous interstellar dust component but the
effects of shocks waves upon them have never been fully investigated. Aims: To
study the effects of energetic (~0.01-1 keV) ion (H, He and C) and electron
collisions on PAHs in interstellar shock waves.Methods: We calculate the
ion-PAH and electron-PAH nuclear and electronic interactions, above the
threshold for carbon atom loss from a PAH, in 50-200 km/s shock waves in the
warm intercloud medium. Results: Interstellar PAHs (Nc = 50) do not survive in
shocks with velocities greater than 100 km/s and larger PAHs (Nc = 200) are
destroyed for shocks with velocities greater/equal to 125 km/s. For shocks in
the ~75 - 100 km/s range, where destruction is not complete, the PAH structure
is likely to be severely denatured by the loss of an important fraction
(20-40%) of the carbon atoms. We derive typical PAH lifetimes of the order of a
few x10^8 yr for the Galaxy. These results are robust and independent of the
uncertainties in some key parameters that have yet to be well-determined
experimentally. Conclusions: The observation of PAH emission in shock regions
implies that that emission either arises outside the shocked region or that
those regions entrain denser clumps that, unless they are completely ablated
and eroded in the shocked gas, allow dust and PAHs to survive in extreme
environments.Comment: 19 pages, 11 figures, 3 tables, typos corrected and PAH acronym in
the title substituted with full name to match version published in Astronomy
and Astrophysic
Uncertainty Quantification in Chemical Modeling
A module of PrIMe automated data-centric infrastructure, Bound-to-Bound Data Collaboration (B2BDC), was used for the analysis of systematic uncertainty and data consistency of the H2/CO reaction model (73/17). In order to achieve this purpose, a dataset of 167 experimental targets (ignition delay time and laminar flame speed) and 55 active model parameters (pre-exponent factors in the Arrhenius form of the reaction rate coefficients) was constructed. Consistency analysis of experimental data from the composed dataset revealed disagreement between models and data.
Two consistency measures were applied to identify the quality of experimental targets (Quantities of Interest, QoI): scalar consistency measure, which quantifies the tightening index of the constraints while still ensuring the existence of a set of the model parameter values whose associated modeling output predicts the experimental QoIs within the uncertainty bounds; and a newly-developed method of computing the vector consistency measure (VCM), which determines the minimal bound changes for QoIs initially identified as inconsistent, each bound by its own extent, while still ensuring the existence of a set of the model parameter values whose associated
modeling output predicts the experimental QoIs within the uncertainty bounds. The consistency analysis suggested that elimination of 45 experimental targets, 8 of which were self- inconsistent, would lead to a consistent dataset. After that the feasible parameter set was constructed through decrease uncertainty parameters for several reaction rate coefficients. This dataset was subjected for the B2BDC framework model optimization and analysis on. Forth methods of parameter optimization were applied, including those unique in the B2BDC framework. The optimized models
showed improved agreement with experimental values, as compared to the initiallyassembled
model. Moreover, predictions for experiments not included in the initial dataset were investigated. The results demonstrate benefits of applying the B2BDC methodology for development of predictive kinetic models
Soot surface growth and oxidation in laminar unsaturated-hydrocarbon/air diffusion flames
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77246/1/AIAA-2002-1116-550.pd
Geometry optimization in quantum Monte Carlo with solution mapping: Application to formaldehyde
Mechanisms for the formation of benzene in the atmosphere of Titan
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95336/1/jgre1586.pd
The CH out-of-plane bending modes of PAH molecules in astrophysical environments
We present 10-15 micron spectra of a sample of H II regions, YSOs and evolved
stars that show strong unidentified infrared emission features, obtained with
the ISO/SWS spectrograph on-board ISO. These spectra reveal a plethora of
emission features with bands at 11.0, 11.2, 12.0, 12.7, 13.5 and 14.2 micron.
These features are observed to vary considerably in relative strength to
each-other from source to source. In particular, the 10-15 micron spectra of
the evolved stars are dominated by the 11.2 micron band while for H II regions
the 12.7 is typically as strong as the 11.2 micron band. Analysing the ISO data
we find a good correlation between the 11.2 micron band and the 3.3 micron
band, and between the 12.7 micron and the 6.2 micron band. There is also a
correlation between the ratio of the UIR bands to the total dust emission and
the 12.7 over 11.2 micron ratio. Bands in the 10-15 micron spectral region are
due to CH out-of-plane (OOP) bending modes of polycyclic aromatic hydrocarbons
(PAHs). We summarise existing laboratory data and theoretical quantum chemical
calculations of these modes for neutral and cationic PAHs. Due to mode
coupling, the exact peak position of these bands depends on the number of
adjacent CH groups and hence the observed interstellar 10-15 micron spectra can
be used to determine the molecular structure of the interstellar PAHs emitting
in the different regions. We conclude that evolved stars predominantly inject
compact ~100-200 C-atom PAHs into the ISM where they are subsequently
processed, resulting in more open and uneven PAH structures.Comment: 14 pages, 9 figures, accepted for A&
Investigation of the Soot Formation in Ethylene Laminar Diffusion Flames When Diluted with Helium or Supplemented by Hydrogen
This document is the Accepted Manuscript version of a Published Work that appeared in final form in Energy and Fuels, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://dx.doi.org/10.1021/ef401970qA new optical diagnostic technique has been used to measure the spatially distributed temperatures, soot diameters, and soot volume fractions in several different ethylene laminar diffusion flames to investigate the effect of adding hydrogen and helium on the soot formation. The test results show that adding hydrogen increases the flame temperature in all regions, while adding helium does not significantly affect the flame temperature in the reaction region but does increase the flame temperature elsewhere. The flame heights when adding helium and hydrogen can be calculated using the correlation introduced by Roper if the ethylene diffusion coefficient is used. This indicates that the flame height is determined by the diffusion of ethylene molecules when the hydrogen fraction is below 20%. It was also found that either adding helium or hydrogen does not significantly affect the soot diameter but does reduce the soot volume fraction. A total of 20% of helium addition by volume was measured to reduce the total soot number by 19%, while a total of 20% of hydrogen addition reduced the total soot number by 23%. In comparison, replacing the hydrocarbon with hydrogen is much more effective in reducing soot formation. Replacement of 25% ethylene by hydrogen was measured to reduce the total soot number by 66%. Apart from demonstrating the influence of hydrogen and helium on ethylene diffusion flames, these measurements provide additional data for modelers of diffusion flames, especially those with an interest in the formation of particulate matter. © 2014 American Chemical Society
Formation of unsaturated hydrocarbons in interstellar ice analogs by cosmic rays
The formation of double and triple C-C bonds from the processing of pure
c-C6H12 (cyclohexane) and mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ices by
highly-charged, and energetic ions (219 MeV O^{7+} and 632 MeV Ni^{24+}) is
studied. The experiments simulate the physical chemistry induced by medium-mass
and heavy-ion cosmic rays in interstellar ices analogs. The measurements were
performed inside a high vacuum chamber at the heavy-ion accelerator GANIL
(Grand Accel\'erat\'eur National d'Ions Lourds) in Caen, France. The gas
samples were deposited onto a polished CsI substrate previously cooled to 13 K.
In-situ analysis was performed by a Fourier transform infrared (FTIR)
spectrometry at different ion fluences. Dissociation cross section of
cyclohexane and its half-life in astrophysical environments were determined. A
comparison between spectra of bombarded ices and young stellar sources
indicates that the initial composition of grains in theses environments should
contain a mixture of H2O, NH3, CO (or CO2), simple alkanes, and CH3OH. Several
species containing double or triple bounds were identified in the radiochemical
products, such as hexene, cyclohexene, benzene, OCN-, CO, CO2, as well as
several aliphatic and aromatic alkenes and alkynes. The results suggest an
alternative scenario for the production of unsaturated hydrocarbons and
possibly aromatic rings (via dehydrogenation processes) in interstellar ices
induced by cosmic ray bombardment.Comment: 14 pages, 10 figures, 4 tables. Accepted to be published in MNRA
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