Evolutionary optimization within an intelligent hybrid system for design integration

An intelligent hybrid approach has been developed to integrate various stages in total design, including formulation of product design specifications, conceptual design, detail design, and manufacture. The integration is achieved by blending multiple artificial intelligence (AI) techniques and CAD/CAE/CAM into a single environment. It has been applied into power transmission system design. In addition to knowledge-based systems and artificial neural networks, another AI technique, genetic algorithms (GAs), are involved in the approach. The GA is used to conduct optimization tasks: (1) searching the best combination of design parameters to obtain optimum design of gears, and (2) optimization of the architecture of the artificial neural networks used in the hybrid system. In this paper, after a brief overview of the intelligent hybrid system, the GA applications are described in detail

Integration of knowledge-based system, artificial neural networks and multimedia for gear design

Design is a complicated area consisting of a combination of rules, technical information and personal judgement. The quality of design depends highly on the designer's knowledge and experience. This system attempts to simulate the design process and to capture design expertise by combining artificial neural networks (ANNs) and knowledge based system (KBS) together with multi-media (MM). It has been applied to the design of gears. Within the system the knowledge based system handles clearly defined design knowledge, the artificial neural networks capture knowledge which is difficult to quantify and multi-media provides a user-friendly interface prompting the user to input information and to retrieve results during design process. The finished system illustrates how features of different Artificial Intelligence techniques, KBS, ANNs and MM, are combined in a hybrid manner to conduct complicated design tasks

The interstellar gas-phase chemistry of HCN and HNC

We review the reactions involving HCN and HNC in dark molecular clouds to elucidate new chemical sources and sinks of these isomers. We find that the most important reactions for the HCN-HNC system are Dissociative Recombination (DR) reactions of HCNH+ (HCNH+ + e-), the ionic CN + H3+, HCN + C+, HCN and HNC reactions with H+/He+/H3+/H3O+/HCO+, the N + CH2 reaction and two new reactions: H + CCN and C + HNC. We test the effect of the new rate constants and branching ratios on the predictions of gas-grain chemical models for dark cloud conditions. The rapid C + HNC reaction keeps the HCN/HNC ratio significantly above one as long as the carbon atom abundance remains high. However, the reaction of HCN with H3+ followed by DR of HCNH+ acts to isomerize HCN into HNC when carbon atoms and CO are depleted leading to a HCN/HNC ratio close to or slightly greater than 1. This agrees well with observations in TMC-1 and L134N taking into consideration the overestimation of HNC abundances through the use of the same rotational excitation rate constants for HNC as for HCN in many radiative transfer models.Comment: Accepted for publication in MNRA

Reactions forming C(0,+)n=2,10, Cn=2,4H(0,+) and C3H(0,+) in the gas phase: semi empirical branching ratios

The aim of this paper is to provide a new set of branching ratios for interstellar and planetary chemical networks based on a semi empirical model. We applied, instead of zero order theory (i.e. only the most exoergic decaying channel is considered), a statistical microcanonical model based on the construction of breakdown curves and using experimental high velocity collision branching ratios for their parametriza- tion. We applied the model to ion-molecule, neutral-neutral, and ion-pair reactions implemented in the few popular databases for astrochemistry such as KIDA, OSU and UMIST. We studied the reactions of carbon and hydrocarbon species with electrons, He+, H+, CH+, CH, C, and C+ leading to intermediate complexes of the type Cn=2,10, Cn=2,4 H, C3 H2, C+n=2,10, Cn=2,4 H+, or C3 H+2 . Comparison of predictions with measurements supports the validity of the model. Huge deviations with respect to database values are often obtained. Effects of the new branching ratios in time dependant chemistry for dark clouds and for photodissociation region chemistry with conditions similar to those found in the Horsehead Nebula are discussed

The gas-phase chemistry of carbon chains in dark cloud chemical models

We review the reactions between carbon chain molecules and radicals, namely Cn, CnH, CnH2, C2n+1O, CnN, HC2n+1N, with C, N and O atoms. Rate constants and branching ratios for these processes have been re-evaluated using experimental and theoretical literature data. In total 8 new species have been introduced, 41 new reactions have been proposed and 122 rate coefficients from kida.uva.2011 (Wakelam et al. 2012) have been modified. We test the effect of the new rate constants and branching ratios on the predictions of gas-grain chemical models for dark cloud conditions using two different C/O elemental ratios. We show that the new rate constants produce large differences in the predicted abundances of carbon chains since the formation of long chains is less effective. The general agreement between the model predictions and observed abundances in the dark cloud TMC-1 (CP) is improved by the new network and we find that C/O ratios of 0.7 and 0.95 both produce a similar agreement for different times. The general agreement for L134N (N) is not significantly changed. The current work specifically highlights the importance of O + CnH and N + CnH reactions. As there are very few experimental or theoretical data for the rate constants of these reactions we highlight the need for experimental studies of the O + CnH and N + CnH reactions, particularly at low temperature.Comment: Accepted for publication in MNRA

Refit to numerically problematic UMIST reaction rate coefficients

Aims. Chemical databases such as the UMIST Database for Astrochemistry (UDFA) are indispensable in the numerical modeling of astrochemical networks. Several of the listed reactions in the UDFA have properties that are problematic in numerical computations: Some are parametrized in a way that leads to extremely divergent behavior for low kinetic temperatures. Other reactions possess multiple entries that are each valid in a different temperature regime, but have no smooth transition when switching from one to another. Numerically, this introduces many difficulties.We present corrected parametrizations for these sets of reactions in the UDFA06 database. Methods. From the tabulated parametrization in UDFA, we created artificial data points and used a Levenberg-Marquardt algorithm to find a set of improved fit parameters without divergent behavior for low temperatures. For reactions with multiple entries in the database that each possess a different temperature regime, we present one joint parametrization that is designed to be valid over the whole cumulative temperature range of all individual reactions. Results. We show that it is possible to parametrize numerically problematic reactions from UDFA in a form that avoids low temperature divergence. Additionally, we demonstrate that it is possible to give a collective parametrization for reaction rate coefficients of reactions with multiple entries in UDFA. We present these new fitted values in tabulated form.Comment: accepted by A&

Sensitive survey for 13CO, CN, H2CO, and SO in the disks of T Tauri and Herbig Ae stars II: Stars in ρ\rho Oph and upper Scorpius

We attempt to determine the molecular composition of disks around young low-mass stars in the $\rho$ Oph region and to compare our results with a similar study performed in the Taurus-Auriga region. We used the IRAM 30 m telescope to perform a sensitive search for CN N=2-1 in 29 T Tauri stars located in the $\rho$ Oph and upper Scorpius regions. $^{13}$CO J=2-1 is observed simultaneously to provide an indication of the level of confusion with the surrounding molecular cloud. The bandpass also contains two transitions of ortho-H$_2$CO, one of SO, and the C$^{17}$O J=2-1 line, which provides complementary information on the nature of the emission. Contamination by molecular cloud in $^{13}$CO and even C$^{17}$O is ubiquitous. The CN detection rate appears to be lower than for the Taurus region, with only four sources being detected (three are attributable to disks). H$_2$CO emission is found more frequently, but appears in general to be due to the surrounding cloud. The weaker emission than in Taurus may suggest that the average disk size in the $\rho$ Oph region is smaller than in the Taurus cloud. Chemical modeling shows that the somewhat higher expected disk temperatures in $\rho$ Oph play a direct role in decreasing the CN abundance. Warmer dust temperatures contribute to convert CN into less volatile forms. In such a young region, CN is no longer a simple, sensitive tracer of disks, and observations with other tracers and at high enough resolution with ALMA are required to probe the gas disk population.Comment: 18 pages, 5 figures, accepted for publication in A&

The C(3P) + NH3 reaction in interstellar chemistry: II. Low temperature rate constants and modeling of NH, NH2 and NH3 abundances in dense interstellar clouds

A continuous supersonic flow reactor has been used to measure rate constants for the C + NH3 reaction over the temperature range 50 to 296 K. C atoms were created by the pulsed laser photolysis of CBr4. The kinetics of the title reaction were followed directly by vacuum ultra-violet laser induced fluorescence (VUV LIF) of C loss and through H formation. The experiments show unambiguously that the reaction is rapid at 296 K, becoming faster at lower temperatures, reaching a value of 1.8 10-10 cm3 molecule-1 s-1 at 50 K. As this reaction is not currently included in astrochemical networks, its influence on interstellar nitrogen hydride abundances is tested through a dense cloud model including gas-grain interactions. In particular, the effect of the ortho-to-para ratio of H2 which plays a crucial role in interstellar NH3 synthesis is examined

Kinetic Study of the Gas-Phase Reaction between Atomic Carbon and Acetone. Low Temperature Rate Constants and Hydrogen Atom Product Yields

The reactions of ground state atomic carbon, C(3P), are likely to be important in astrochemistry due to the high abundance levels of these atoms in the dense interstellar medium. Here we present a study of the gas-phase reaction between C(3P) and acetone, CH3COCH3. Experimentally, rate constants were measured for this process over the 50 to 296 K range using a continuous-flow supersonic reactor, while secondary measurements of H(2S) atom formation were also performed over the 75 to 296 K range to elucidate the preferred product channels. C(3P) atoms were generated by In-situ pulsed photolysis of carbon tetrabromide, while both C(3P) and H(2S) atoms were detected by pulsed laser induced fluorescence. Theoretically, quantum chemical calculations were performed to obtain the various complexes, adducts and transition states involved in the C(3P) + CH3COCH3 reaction over the 3A'' potential energy surface, allowing us to better understand the reaction pathways and help to interpret the experimental results. The derived rate constants are large, (2-3) x 10-10 cm3 s-1 , displaying only weak temperature variations; a result that is consistent with the barrierless nature of the reaction. As this reaction is not present in current astrochemical networks, its influence on simulated interstellar acetone abundances is tested using a gas-grain dense interstellar cloud model. For interstellar modelling purposes, the use of a temperature independent value for the rate constant, k(C+CH3COCH3 )= 2.2 x 10-10 cm3 s-1, is recommended. The C(3P) + CH3COCH3 reaction decreases gas-phase CH3COCH3 abundances by as much as two orders of magnitude at early and intermediate cloud ages.Comment: Accepted for publication in ACS Earth and Space Chemistry. 55 pages including S

S-bearing molecules in Massive Dense Cores

Chemical composition of the massive cores forming high-mass stars can put some constrains on the time scale of the massive star formation: sulphur chemistry is of specific interest due to its rapid evolution in warm gas and because the abundance of sulphur bearing species increases significantly with the temperature. Two mid-infrared quiet and two brighter massive cores are observed in various transitions (E_up up to 289K) of CS, OCS, H2S, SO, SO2 and of their isotopologues at mm wavelengths with the IRAM 30m and CSO telescopes. 1D modeling of the dust continuum is used to derive the density and temperature laws, which are then applied in the RATRAN code to model the observed line emission, and to derive the relative abundances of the molecules. All lines, except the highest energy SO2 transition, are detected. Infall (up to 2.9km/s) may be detected towards the core W43MM1. The inferred mass rate is 5.8-9.4 10^{-2} M_{\odot}/yr. We propose an evolutionary sequence of our sources (W43MM1-IRAS18264-1152-IRAS05358+3543-IRAS18162-2048), based on the SED analysis. The analysis of the variations in abundance ratios from source to source reveals that the SO and SO2 relative abundances increase with time, while CS and OCS decrease. Molecular ratios, such as [OCS/H2S], [CS/H2S], [SO/OCS], [SO2/OCS], [CS/SO] and [SO2/SO] may be good indicators of evolution depending on layers probed by the observed molecular transitions. Observations of molecular emission from warmer layers, hence involving higher upper energy levels are mandatory to include.Comment: 24 pages, accepted for publicatio