3,013 research outputs found
Acoustic Engineering Workstation at the College of the Ozarks
The College of the Ozarks is developing the ability to provide acoustic engineering services to customers on and off-campus. The College is the only federally recognized work college with an undergraduate engineering program, which means students do not pay tuition and are assigned workstations on campus to help defray expenses and to generate income for the College. This paper addresses the purpose and administration of the workstation and how it is unique from other service-learning programs, the perceived benefits to the student workers and the engineering program, recent workstation accomplishments, lessons learned, and future plans
Experimental evidence of spontaneous symmetry breaking in intracavity type-II second harmonic generation with triple resonance
We describe an experiment showing a spontaneous symmetry breaking phenomenon
between the intensities of the ordinary and extraordinary components of the
fundamental field in intracavity type-II harmonic generation. It is based on a
triply resonant dual cavity containing a type II phase matched
crystal pumped at the fundamental frequency . The pump beam generates
in the cavity a second harmonic mode at frequency which acts as a
pump for frequency degenerate type II parametric down conversion. Under
operating conditions which are precisely symmetric with respect to the ordinary
and extraordinary components of the fundamental wave, we have observed a
breaking of the symmetry on the intensities of these two waves in agreement
with the theoretical predictions.Comment: submitte
Appropriate indicators of rail freight activity and market share: a review of UK practice and recommendations for change
Growth in rail freight activity features strongly in contemporary transport policy at both the United Kingdom (UK) and European Union (EU) scale. Specifically, the British government set an 80 per cent growth target between 2000 and 2010, with lower (but still substantial) growth estimates being identified in late-2005. This paper assesses the appropriateness of the forms of measurement adopted for rail freight activity and argues that achieving stated growth targets or estimates will not necessarily mean that policies encouraging modal shift from road to rail have succeeded. Additional or alternative means of monitoring the level of rail freight activity are discussed, since this is an issue of fundamental importance to policy implementation and evaluation
The growth of dark matter halos: evidence for significant smooth accretion
We study the growth of dark matter halos in the concordance LCDM cosmology
using several N-body simulations of large cosmological volumes. We build merger
trees from the Millennium and Millennium-II simulations, covering a range
10^9-10^15 Msun in halo mass and 1-10^5 in merger mass ratio. Our algorithm
takes special care of halo fragmentation and ensures that the mass contribution
of each merger to halo growth is only counted once. This way the integrated
merger rate converges and we can consistently determine the contribution of
mergers of different mass ratios to halo growth. We find that all resolved
mergers, up to mass ratios of 10^5, contribute only ~60% of the total halo mass
growth, while major mergers are subdominant, e.g. mergers with mass ratios
smaller than 3:1 (10:1) contribute only ~20% (~30%). This is verified with an
analysis of two additional simulation boxes, where we follow all particles
individually throughout cosmic time. Our results are also robust against using
several halo definitions. Under the assumption that the power-law behaviour of
the merger rate at large mass ratios can be extrapolated to arbitrarily large
mass ratios, it is found that, independently of halo mass, ~40% of the mass in
halos comes from genuinely smooth accretion of dark matter that was never bound
in smaller halos. We discuss possible implications of our findings for galaxy
formation. One implication, assuming as is standard that the pristine
intergalactic medium is heated and photoionized by UV photons, is that all
halos accrete >40% of their baryons in smooth "cold" T>~10^4K gas, rather than
as warm, enriched or clumpy gas or as stars.Comment: 11 pages, 9 figures. Accepted for publication in the Astrophysical
Journa
Divergent Transcriptional Regulatory Logic at the Intersection of Tissue Growth and Developmental Patterning
The Yorkie/Yap transcriptional coactivator is a well-known regulator of cellular proliferation in both invertebrates and mammals. As a coactivator, Yorkie (Yki) lacks a DNA binding domain and must partner with sequence-specific DNA binding proteins in the nucleus to regulate gene expression; in Drosophila, the developmental regulators Scalloped (Sd) and Homothorax (Hth) are two such partners. To determine the range of target genes regulated by these three transcription factors, we performed genome-wide chromatin immunoprecipitation experiments for each factor in both the wing and eye-antenna imaginal discs. Strong, tissue-specific binding patterns are observed for Sd and Hth, while Yki binding is remarkably similar across both tissues. Binding events common to the eye and wing are also present for Sd and Hth; these are associated with genes regulating cell proliferation and âhousekeepingâ functions, and account for the majority of Yki binding. In contrast, tissue-specific binding events for Sd and Hth significantly overlap enhancers that are active in the given tissue, are enriched in Sd and Hth DNA binding sites, respectively, and are associated with genes that are consistent with each factor's previously established tissue-specific functions. Tissue-specific binding events are also significantly associated with Polycomb targeted chromatin domains. To provide mechanistic insights into tissue-specific regulation, we identify and characterize eye and wing enhancers of the Yki-targeted bantam microRNA gene and demonstrate that they are dependent on direct binding by Hth and Sd, respectively. Overall these results suggest that both Sd and Hth use distinct strategies â one shared between tissues and associated with Yki, the other tissue-specific, generally Yki-independent and associated with developmental patterning â to regulate distinct gene sets during development
Un modĂšle de croissance pour lâazobĂ©, Lophira alata, au Gabon
Azobé, Lophira alata, is a major timber species in Central Africa classified as vulnerable into the IUCN red list. To date, despite numerous measures of increment, no growth model has been published for this species. This study aims to distinguish the effects between tree size and local competition on tree growth. A growth model was fitted for azobé, using data from four sites in Gabon. The growth model was designed to be useful for forest management that means it relied on variables that could be computed using forest inventory data. A lognormal growth model with a negative response to stand density and basal area has been selected. The relation between growth and size was unimodal with a maximum at 60 cm of diameter at breast height. A significant residual social status effect on growth has been found (with a slower growth for suppressed trees) while no residual site effect was found
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Adaptive Evolution and the Birth of CTCF Binding Sites in the <i>Drosophila</i> Genome
Changes in the physical interaction between cis-regulatory DNA sequences and proteins drive the evolution of gene expression. However, it has proven difficult to accurately quantify evolutionary rates of such binding change or to estimate the relative effects of selection and drift in shaping the binding evolution. Here we examine the genome-wide binding of CTCF in four species of Drosophila separated by between âŒ2.5 and 25 million years. CTCF is a highly conserved protein known to be associated with insulator sequences in the genomes of human and Drosophila. Although the binding preference for CTCF is highly conserved, we find that CTCF binding itself is highly evolutionarily dynamic and has adaptively evolved. Between species, binding divergence increased linearly with evolutionary distance, and CTCF binding profiles are diverging rapidly at the rate of 2.22% per million years (Myr). At least 89 new CTCF binding sites have originated in the Drosophila melanogaster genome since the most recent common ancestor with Drosophila simulans. Comparing these data to genome sequence data from 37 different strains of Drosophila melanogaster, we detected signatures of selection in both newly gained and evolutionarily conserved binding sites. Newly evolved CTCF binding sites show a significantly stronger signature for positive selection than older sites. Comparative gene expression profiling revealed that expression divergence of genes adjacent to CTCF binding site is significantly associated with the gain and loss of CTCF binding. Further, the birth of new genes is associated with the birth of new CTCF binding sites. Our data indicate that binding of Drosophila CTCF protein has evolved under natural selection, and CTCF binding evolution has shaped both the evolution of gene expression and genome evolution during the birth of new genes.</p
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