Results 1 - 10 of 9437
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[en] In this paper, we study the determinants of starless core temperatures in the Perseus molecular cloud. We use NH3 (1,1) and (2,2) observations to derive core temperatures (T kin) and data from the COMPLETE Survey of Star-Forming Regions and the c2d Spitzer Legacy Survey for observations of the other core and molecular cloud properties. The kinetic temperature distribution probed by NH3 is in the fairly narrow range of ∼ 9-15 K. We find that cores within the clusters IC348 and NGC1333 are significantly warmer than fieldstarless cores, and T kin is higher within regions of larger extinction-derived column density. Starless cores in the field are warmer when they are closer to Class 0/I protostars, but this effect is not seen for those cores in clusters. For field starless cores, T kin is higher in regions in which the 13CO line width and the 1.1 mm flux from the core are larger, and T kin is lower when the the peak column density within the core and average volume density of the core are larger. There is no correlation between T kin and 13CO line width, 1.1 mm flux, density, or peak column density for those cores in clusters. The temperature of the cloud material along the line of sight to the core, as measured by CO or far-infrared emission from dust, is positively correlated with core temperature when considering the collection of cores in the field and in clusters, but this effect is not apparent when the two subsamples of cores are considered separately.
[en] We present the results of a Green Bank Telescope survey of NH3(1,1), (2,2), (3,3) lines toward 631 Bolocam Galactic Plane Survey (BGPS) sources at a range of Galactic longitudes in the inner Galaxy. We have detected the NH3(1,1) line toward 72% of our targets (456), demonstrating that the high column density features identified in the BGPS and other continuum surveys accurately predict the presence of dense gas. We have determined kinematic distances and resolved the distance ambiguity for all BGPS sources detected in NH3. The BGPS sources trace the locations of the Scutum and Sagittarius spiral arms, with the number of sources. We measure the physical properties of each source and find that depending on the distance, BGPS sources are primarily clumps, with some cores and clouds. We have examined the physical properties as a function of Galactocentric distance, and find a mean gas kinetic temperature of 15.6 K, and that the NH3 column density and abundance decrease by nearly an order of magnitude. Comparing sources at similar distances demonstrates that the physical properties are indistinguishable, which suggests a similarity in clump structure across the Galactic disk. We have also compared the BGPS sources to criteria for efficient star formation presented independently by Heiderman et al. and Lada et al., and for massive star formation presented by Kauffmann et al. Forty-eight percent of our sample should be forming stars (including massive stars) with high efficiency, and 87% contain subregions that should be efficiently forming stars. Indeed, we find that 67% of the sample exhibit signs of star formation activity based on an association with a mid-infrared source.
[en] Yoon et al.1 presented an approximate mathematical model to describe ammonia removal from an experimental batch reactor system with gaseous headspace. The development of the model was initially based on assuming instantaneous equilibrium between ammonia in the aqueous and gas phases. In the model, a 'saturation factor, β' was defined as a constant and used to check whether the equilibrium assumption was appropriate. The authors used the trends established by the estimated β values to conclude that the equilibrium assumption was not valid. The authors presented valuable experimental results obtained using a carefully designed system and the model used to analyze the results accounted for the following effects: speciation of ammonia between NH3 and NH4+ as a function of pH: temperature dependence of the reactions constants; and air flow rate. In this article, an alternative model based on the exact solution of the governing mass-balance differential equations was developed and used to describe ammonia removal without relying on the use of the saturation factor. The modified model was also extended to mathematically describe the pH dependence of the ammonia removal rate, in addition to accounting for the speciation of ammonia, temperature dependence of reactions constants, and air flow rate. The modified model was used to extend the analysis of the original experimental data presented by Yoon et al.1 and the results matched the theory in an excellent manner
[en] The utilization of unused energy is important because it can afford to offer a chance to increase energy efficiency of a heat pump system. Especially the waste energy recovery by heat pump can be easily utilized in industrial field for processing water supply system. In this article, the efficient way to make a high temperature water by using the waste water as heat source. This article concerns such a cycle that its system making up has the features: a two-stage compression, a two stage expansion and two desorbers, each of which being located after an expansion valve. By comparisons of the performance with those of common two-stage compression and single expansion CAHP systems for four heating cases, the simulation studies showed that the new system has superior performance, under the working conditions of lower compressor outlet temperature and lower maximum system pressure, which is very important for the components of the system to be operated safely. And the performance can be improved further by optimum displacements of the compressors chosen. The simulations also demonstrated how changes of the sink temperature glide, maximum pressure and the performance of the system were adjusted by the circulation rate and the ammonia concentration for a given heat case
[en] We present observations of continuum emission in the mid-infrared to millimeter wavelength range, complemented with ammonia observations, of the dense core ahead of the radio Herbig-Haro (HH) object HH 80N, found in the GGD 27 region. The continuum emission in all the observed bands peaks at the same position, consistent with the presence of an embedded object, HH 80N-IRS1, within the core. The distribution of the Very Large Array ammonia emission is well correlated with that of the dust, suggesting that photochemical effects caused by the nearby HH object do not play an important role in shaping this particular molecular emission. In order to unveil the nature of HH 80N-IRS1, we analyzed the continuum data of this source, using self-consistent models of protostellar collapse. We find that a young protostar surrounded by a slowly rotating collapsing envelope of radius ∼0.08 pc and 20 Msun plus a circumstellar disk of radius ∼300 AU and 0.6 Msun provide a good fit to the observed spectral energy distribution and to the maps at 350 μm, 1.2 mm, and 3.5 mm of HH 80N-IRS1. Besides, the Atacama Pathfinder Experiment and Plateau de Bure Interferometer continuum maps at 350 μm and 3.5 mm, respectively, reveal additional clumps in the continuum emission. Given the modeling results and the observed morphology of the emission, we propose a scenario consisting of a central embedded Class 0 object, HH 80N-IRS1, with the rest of the material of the HH 80N core possibly undergoing fragmentation that may lead to the formation of several protostars.
[en] We present the chemistry, temperature, and dynamical state of a sample of 193 dense cores or core candidates in the Perseus Molecular cloud and compare the properties of cores associated with young stars and clusters with those which are not. The combination of our NH3 and CCS observations with previous millimeter, submillimeter, and Spitzer data available for this cloud enables us both to determine core properties precisely and to accurately classify cores as starless or protostellar. The properties of cores in different cluster environments and before-and-after star formation provide important constraints on simulations of star formation, particularly under the paradigm that the essence of star formation is set by the turbulent formation of prestellar cores. We separate the influence of stellar content from that of the cluster environment and find that cores within clusters have (1) higher kinetic temperatures (12.9 K versus 10.8 K) and, (2) lower fractional abundances of CCS (0.6 x 10-9 versus 2.0 x 10-9) and NH3 (1.2 x 10-8 versus 2.9 x 10-8). Cores associated with protostars have (1) slightly higher kinetic temperatures (11.9 K versus 10.6 K), (2) higher NH3 excitation temperatures (7.4 K versus 6.1 K), (3) are at higher column density (1.2 x 1022 cm-2 versus 0.6 x 1022 cm-2), have (4) slightly more nonthermal/turbulent NH3 line widths (0.14 km s-1 versus 0.11 km s-1 FWHM), have (5) higher masses (1.5 M sun versus 1.0 M sun), and have (6) lower fractional abundance of CCS (1.4 x 10-9 versus 2.4 x 10-9). All values are medians. We find that neither cluster environment nor protostellar content makes a significant difference to the dynamical state of cores as estimated by the virial parameter-most cores in each category are gravitationally bound. Only the high precision of our measurements and the size of our sample make such distinctions possible. Overall, cluster environment and protostellar content have a smaller influence on the properties of the cores than is typically assumed, and the variation within categories is larger than the differences between categories.
[en] This paper describes a simulation modeling technique for assessing the availability of an ammonia plant. Field data on failure and down times have been collected and analyzed by fitting Weibull distributions. Plant availability has been estimated using the existing plant configuration and actual failure and repair time distributions. Analytical models based on the exponential and Weibull distributions have also been discussed. Availabilities obtained from different models have been compared and found to vary slightly. The possibility of improving plant performance by modification of plant configuration and change of overhaul strategy has also been studied
[en] While choosing an inherently safer alternative may seem straightforward, sometimes what seems to be the most obvious alternative may not provide the best risk reduction. The process designer must maintain a broad perspective to be able to recognize all potential hazards when evaluating design options. All aspects of operation such as start-up, shut-down, utility failure, as well as normal operation should be considered. Choosing the inherently safer option is best accomplished early in the option selection phase of a project; however, recycle back to the option selection phase may be needed if an option is not thoroughly evaluated early in the process. In this paper, a project to supply ammonia to a catalytic reactor will be reviewed. During the course of the project, an 'inherently safer' alternative was selected and later discarded due to issues uncovered during the detail design phase. The final option chosen will be compared to (1) the original design and (2) the initial 'inherently safer' alternative. The final option was inherently safer than both the original design and the initial 'inherently safer' alternative even though the design team initially believed that it would not be
[en] This Letter outlines the experimental study of molecule dissociation effect on rf capacitive discharge burning. We show that for each ammonia pressure value there exists some threshold rf voltage value below which the dissociation degree does not exceed 3%, but at higher rf voltage it grows to 30%. Increasing NH3 dissociation degree accelerates the discharge current growth against rf voltage. The rf discharge remains in a weak-current α-mode at low as well as at high dissociation degree because all ammonia dissociation products possess ionization potentials exceeding that for NH3. -- Highlights: ► The experimental study of molecule dissociation effect on rf capacitive discharge. ► There is a threshold rf voltage below which the dissociation degree is less than 3%. ► At higher rf voltage it grows to 30%. ► Increasing NH3 dissociation degree accelerates the current growth with rf voltage. ► The rf discharge remains in a weak-current alpha-mode.