The Response of First and Second Order Streams to Urban Land-Use in Maine, U.S.A.

Physical, chemical, and biological characteristics of streams draining 20 catchments in Maine, U.S.A were compared to determine the influence of increasing urban intensity on stream ecosystem structure. The catchments had varying levels of urban land-use (percentage of the total impervious area within the catchment) ranging from 1-31%. Stream habitat quality, stability, and water quality consistently decreased as the proportion of impervious surface area increased within the catchment. .Indices based on stream benthic macroinvertebrate communities showed even stronger declines as a function of increasing impervious area in the study catchments. Streams draining catchments with levels of impervious surfaces \u3c6% had higher levels of both total and Ephemeroptera + Plecoptera + Trichoptera (EPT) taxonomic richness. With increased levels of urban intensity, benthic macroinvertebrate communities in streams were characterized by decreased numbers of sensitive taxa. Taxa considered to be moderately sensitive to anthropogenic stress (e.g. Acerpenna (Ephemeroptera), Paracapnia and Allocapnia (Plecoptera), Optiosewus and Stenelmis (Coleoptera), Hydropsyche and Cheumatospyche (Trichoptera), Orthocladiinae (Diptera), and Oligochaeta) were apparently little influenced by increasing urban intensity. These patterns were similar between the Fall and Spring. Results indicated that beyond an apparent threshold of ≈6% impervious surface area in the catchment, study streams exhibited an abrupt step-like drop in macroinvertebrate community condition as indicated by a reduction in the presence of sensitive macroinvertebrate taxa. Streams with \u3c6% impervious surfaces contained invertebrate communities with average total richness of 33 taxa (Fall) and 31 taxa (Spring) and average EPT richness of 15 taxa (Fall) and 13 taxa (Spring). In contrast, none of the streams located in catchments with 6-27% impervious coverage exhibited average total richness \u3e18 taxa and average EPT richness \u3e6 taxa. Physical habitat and water quality parameters failed to indicate the mechanism resulting in degradation of the macroinvertebrate community

Herbig-Haro flows in B335

We have observed optical (Halpha and [SII]) and near-IR (S(1) line of H2) deep fields and taken optical spectra using the 2.56m NOT, as well as a near-UV deep field (U band) using the 3.58m NTT. In addition we present new SPITZER (IRAC and MIPS) mid-IR observations. We use previous Halpha and S(1) observations taken 15 and 9 years earlier to make proper motion maps. We then investigate the shock physics by matching our spectra with planar shock models. We discover six new HH objects in B335. From proper motions we find an optically bright, roughly E-W oriented group with high space velocities (200-280 km/s) and a near-IR bright, slower group (15-75 km/s) moving to the ESE. We also find a system of at least 15 H2 knots in the western lobe. This (WNW) counterflow suggests the possibility of a binary outflow source, giving rise to two outflow axes with slightly different orientations. We find that the E-W flow is symmetrical with evidence for two outbursts. We make the first detection of [OI] 6300/63 in HH119 B and Hbeta in HH119 A and B and find their extinctions to be AV~1.4 and 4.4, respectively. HH119 A is found to expand much faster than expected from linear expansion with distance from the outflow source. Using planar shock models we find shock velocities of ~60 km/s (A) and ~35 km/s (B and C). This agrees with A being of higher excitation than B and C. In our U image we detect three of the HH objects and propose that the emission arise from the [OII] 3728 line and the blue continuum. New SPITZER observations show most of the HH objects at 4.5 micron and a E-W elongated hour-glass shaped structure at the outflow source. Even at 24 micron it is not clear whether most of the light is direct or reflected.Comment: 23 pages, 15 figures, accepted in A&

Diagrammatic analysis of correlations in polymer fluids: Cluster diagrams via Edwards' field theory

A straightforward expansion of Edwards' functional integral representation of the grand partition function for a polymer liquid as an infinite set of Feynman diagrams is shown to yield a cluster expansion that is closely related to the corresponding Mayer cluster expansion developed for flexible molecules by Chandler and coworkers. The procedure initially yields a perturbative cluster expansion in which all free energies and correlation functions are expressed diagrammatically as functionals of single-molecule correlation functions of non-interacting molecules. Topological reduction yields several renormalized expansions for collective correlation functions of all orders as functionals of single-molecule correlation functions in the interacting fluid. Renormalized expansions are also obtained for a generalized Ornstein-Zernicke (OZ) direct correlation function and for intramolecular correlation functions. The application of the formalism to coarse-grained models of polymer fluids is discussed, and a loop expansion about self-consistent field theory is shown to converge for sufficiently coarse-grained models, in which monomers are strongly overlapping. The formalism is used to derive a new expression for the OZ direct correlation function and to recover known results for the 2-point intramolecular correlation function to first order in a loop expansion, for binary blends and diblock copolymer melts.Comment: 98 pages, 13 figures, accepted by Annals of Physic

Explicit factorization of external coordinates in constrained Statistical Mechanics models

If a macromolecule is described by curvilinear coordinates or rigid constraints are imposed, the equilibrium probability density that must be sampled in Monte Carlo simulations includes the determinants of different mass-metric tensors. In this work, we explicitly write the determinant of the mass-metric tensor G and of the reduced mass-metric tensor g, for any molecule, general internal coordinates and arbitrary constraints, as a product of two functions; one depending only on the external coordinates that describe the overall translation and rotation of the system, and the other only on the internal coordinates. This work extends previous results in the literature, proving with full generality that one may integrate out the external coordinates and perform Monte Carlo simulations in the internal conformational space of macromolecules. In addition, we give a general mathematical argument showing that the factorization is a consequence of the symmetries of the metric tensors involved. Finally, the determinant of the mass-metric tensor G is computed explicitly in a set of curvilinear coordinates specially well-suited for general branched molecules.Comment: 22 pages, 2 figures, LaTeX, AMSTeX. v2: Introduccion slightly extended. Version in arXiv is slightly larger than the published on

The phase diagram of water from quantum simulations

The phase diagram of water has been calculated for the TIP4PQ/2005 model, an empirical rigid non-polarisable model. The path integral Monte Carlo technique was used, permitting the incorporation of nuclear quantum effects. The coexistence lines were traced out using the Gibbs-Duhem integration method, once having calculated the free energies of the liquid and solid phases in the quantum limit, which were obtained via thermodynamic integration from the classical value by scaling the mass of the water molecule. The resulting phase diagram is qualitatively correct, being displaced to lower temperatures by 15-20K. It is found that the influence of nuclear quantum effects are correlated to the tetrahedral order parameter.Comment: 10 pages, 6 figures, 1 tabl

Universal First-passage Properties of Discrete-time Random Walks and Levy Flights on a Line: Statistics of the Global Maximum and Records

In these lecture notes I will discuss the universal first-passage properties of a simple correlated discrete-time sequence {x_0=0, x_1,x_2.... x_n} up to n steps where x_i represents the position at step i of a random walker hopping on a continuous line by drawing independently, at each time step, a random jump length from an arbitrary symmetric and continuous distribution (it includes, e.g., the Levy flights). I will focus on the statistics of two extreme observables associated with the sequence: (i) its global maximum and the time step at which the maximum occurs and (ii) the number of records in the sequence and their ages. I will demonstrate how the universal statistics of these observables emerge as a consequence of Pollaczek-Spitzer formula and the associated Sparre Andersen theorem.Comment: Lecture notes for the summer school "Fundamental Problems in Statistical Physics: XII" held at Leuven, Belgium (2009). 20 pages, 4 figures; typos corrected, a figure redrawn and new references and discussions adde

Quantum mechanical calculation of the effects of stiff and rigid constraints in the conformational equilibrium of the Alanine dipeptide

If constraints are imposed on a macromolecule, two inequivalent classical models may be used: the stiff and the rigid one. This work studies the effects of such constraints on the Conformational Equilibrium Distribution (CED) of the model dipeptide HCO-L-Ala-NH2 without any simplifying assumption. We use ab initio Quantum Mechanics calculations including electron correlation at the MP2 level to describe the system, and we measure the conformational dependence of all the correcting terms to the naive CED based in the Potential Energy Surface (PES) that appear when the constraints are considered. These terms are related to mass-metric tensors determinants and also occur in the Fixman's compensating potential. We show that some of the corrections are non-negligible if one is interested in the whole Ramachandran space. On the other hand, if only the energetically lower region, containing the principal secondary structure elements, is assumed to be relevant, then, all correcting terms may be neglected up to peptides of considerable length. This is the first time, as far as we know, that the analysis of the conformational dependence of these correcting terms is performed in a relevant biomolecule with a realistic potential energy function.Comment: 37 pages, 4 figures, LaTeX, BibTeX, AMSTe

Scale-free static and dynamical correlations in melts of monodisperse and Flory-distributed homopolymers: A review of recent bond-fluctuation model studies

It has been assumed until very recently that all long-range correlations are screened in three-dimensional melts of linear homopolymers on distances beyond the correlation length $\xi$ characterizing the decay of the density fluctuations. Summarizing simulation results obtained by means of a variant of the bond-fluctuation model with finite monomer excluded volume interactions and topology violating local and global Monte Carlo moves, we show that due to an interplay of the chain connectivity and the incompressibility constraint, both static and dynamical correlations arise on distances $r \gg \xi$. These correlations are scale-free and, surprisingly, do not depend explicitly on the compressibility of the solution. Both monodisperse and (essentially) Flory-distributed equilibrium polymers are considered.Comment: 60 pages, 49 figure