38,373 research outputs found
Non-equilibrium chemistry and cooling in the diffuse interstellar medium - I. Optically thin regime
An accurate treatment of the multiphase interstellar medium (ISM) in
hydrodynamic galaxy simulations requires that we follow not only the thermal
evolution of the gas, but also the evolution of its chemical state, including
its molecular chemistry, without assuming chemical (including ionisation)
equilibrium. We present a reaction network that can be used to solve for this
thermo-chemical evolution. Our model follows the evolution of all ionisation
states of the 11 elements that dominate the cooling rate, along with important
molecules such as H2 and CO, and the intermediate molecular species that are
involved in their formation (20 molecules in total). We include chemical
reactions on dust grains, thermal processes involving dust, cosmic ray
ionisation and heating and photochemical reactions. We focus on conditions
typical for the diffuse ISM, with densities of 10^-2 cm^-3 < nH < 10^4 cm^-3
and temperatures of 10^2 K < T < 10^4 K, and we consider a range of radiation
fields, including no UV radiation. In this paper we consider only gas that is
optically thin, while paper II considers gas that becomes shielded from the
radiation field. We verify the accuracy of our model by comparing chemical
abundances and cooling functions in chemical equilibrium with the
photoionisation code Cloudy. We identify the major coolants in diffuse
interstellar gas to be CII, SiII and FeII, along with OI and H2 at densities nH
> 10^2 cm^-3. Finally, we investigate the impact of non-equilibrium chemistry
on the cooling functions of isochorically or isobarically cooling gas. We find
that, at T < 10^4 K, recombination lags increase the electron abundance above
its equilibrium value at a given temperature, which can enhance the cooling
rate by up to two orders of magnitude. The cooling gas also shows lower H2
abundances than in equilibrium, by up to an order of magnitude.Comment: 26 pages, 13 figures, accepted for publication in MNRAS. Corrected an
error in figure 2. Supplementary material can be found at
http://noneqism.strw.leidenuniv.n
Does choice of programming language affect student understanding of programming concepts in a first year engineering course?
Most undergraduate engineering curricula include computer programming to some degree,introducing a structured language such as C, or a computational system such as MATLAB, or both. Many of these curricula include programming in first year engineering courses, integrating the solution of simple engineering problems with an introduction to programming concepts. In line with this practice, Roger Williams University has included an introduction to programming as a part of the first year engineering curriculum for many years. However, recent industry and pedagogical trends have motivated the switch from a structured language (VBA) to a computational system (MATLAB). As a part of the pilot run of this change,the course instructors felt that it would be worthwhile to verify that changing the programming language did not negatively affect students’ ability to understand key programming concepts. In particular it was appropriate to explore students’ ability to translate word problems into computer programs containing inputs, decision statements, computational processes, and outputs. To test the hypothesis that programming language does not affect students’ ability to understand programming concepts, students from consecutive years were given the same homework assignment, with the first cohort using VBA and the second using MATLAB to solve the assignment. A rubric was developed which allowed the investigators to rate assignments independent of programming language. Results from this study indicate that there is not a significant impact of the change in programming language. These results suggest that the choice of programming language likely does not matter for student understanding of programming concepts. Course instructors should feel free to select programming language based on other factors, such as market demand, cost, or the availability of pedagogical resources
Weakly Turbulent MHD Waves in Compressible Low-Beta Plasmas
In this Letter, weak turbulence theory is used to investigate interactions
among Alfven waves and fast and slow magnetosonic waves in collisionless
low-beta plasmas. The wave kinetic equations are derived from the equations of
magnetohydrodynamics, and extra terms are then added to model collisionless
damping. These equations are used to provide a quantitative description of a
variety of nonlinear processes, including "parallel" and "perpendicular" energy
cascade, energy transfer between wave types, "phase mixing," and the generation
of back-scattered Alfven waves.Comment: Accepted, Physical Review Letter
Environmental effects on magnetic fluorescent powder development of fingermarks on bird of prey feathers
A comparison study of the effects of environmental conditions on the development of latent fingermarks on raptor feathers using green magnetic fluorescent powder was undertaken using both sebaceous loaded and natural fingermark deposits. Sparrowhawk feathers were stored in indoor conditions for 60 days (Study 1), and buzzard feathers were left exposed to two different environmental conditions (hidden and visible) for 21 days (Study 2), with developments made at regular ageing periods. In Study 1, latent fingermarks were successfully developed (Grade 1–4) on the indoor feathers up to 60 days after deposition – 98.6% of the loaded deposits and 85.3% for natural deposits. Under outdoor conditions in Study 2, both loaded and natural deposits were affected by environmental exposure. Latent fingermarks were successfully developed up to 14 days after deposition on the outdoor feathers, with some occasional recovery after 21 days. The visible feathers recorded 34.7% (loaded) and 16.4% (natural) successful developments (Grade 1–4), whereas the hidden feathers recorded 46.7% (loaded) and 22.2% (natural) successful developments, suggesting that protection from the environment helps to preserve latent fingermarks on the surface of a feather. Environmental exposure accelerated the deterioration of ridge detail and the number of successful developments
Detecting the Dusty Debris of Terrestrial Planet Formation
We use a multiannulus accretion code to investigate debris disks in the
terrestrial zone, at 0.7-1.3 AU around a 1 solar mass star. Terrestrial planet
formation produces a bright dusty ring of debris with a lifetime of at least 1
Myr. The early phases of terrestrial planet formation are observable with
current facilities; the late stages require more advanced instruments with
adaptive optics.Comment: 11 pages of text, 3 figures, accepted for ApJ Letters, additional
info at http://cfa-www.harvard.edu/~kenyon/pf/terra/td
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