The chemical evolution of low mass prestellar cores and young stellar objects.

In this thesis we develop models of the chemical kinetics of prestellar cores, so we can identify molecules which will be able to trace various physical processes, and therefore give us more understanding of how stars form from these objects. First, we investigate non-thermal desorption mechanisms which can operate inside prestellar cores, and make estimates of the desorption efficiencies based on observational data, which before now were poorly constrained by theory. We then use our chemical models to predict molecular abundances during the collapse phase of a prestellar core. In particular we investigate the effect of different initial conditions and collapse models, so we can identify molecules which could be used to distinguish between slow and rapid star formation models. We also generate line profiles for selected molecules using a radiative transfer code, using the abundances predicted from our chemical models. We find that for CS, a double peaked line profile with a blue asymmetry can only be produced for models of core collapse with extended inward motions. This contradicts many previous interpretations of line profiles, which have explained the infall signature by collapse models with a static envelope and temperature gradient. We also explore deuterium chemistry in prestellar cores, to investigate whether core collapse by ambipolar diffusion can leave a signature on deuterium fractionation. We find that the fractionation is very sensitive to the density profile of the core, thus we propose that observations of the spatial distribution of deuterated molecules could be used to distinguish between different collapse models. Finally, we investigate the chemistry in C-shock regions associated with outflows of young stellar objects, where we also include the analysis of some new observations of outflow LI448mm, which show very strong evidence for the presence of a magnetic precursor

The sigma term and the quark number operator in QCD

We discuss the relationship of the forward matrix element of the operator $\bar\psi\psi$, related to the so-called sigma term, to the quark number. We show that in the naive quark model in the canonical formalism these quantities coincide in the limit of small average quark momenta. In the QCD parton model defined through light-front quantization this result is preserved at leading perturbative order but it receives radiative corrections. We analyze the theoretical and phenomenological consequences of this result, which provides a bridge between a current algebra quantity, the sigma term, and a deep-inelastic quantity, the parton number.Comment: 30 pages, 1 figure, DFTT-92-6 (April 1993

Electrocortical therapy for motion sickness

Given a sufficiently provocative stimulus, almost everyone can be made motion sick, with approximately one-third experiencing significant symptoms on long bus trips, on ships, or in light aircraft.1–4 Current countermeasures are either behavioral or pharmacologic. Behavioral measures include habituation/desensitization treatment protocols5 as well as positioning the head in alignment with the direction of the gravito-inertial force and maintaining a stable horizontal reference frame.5 Pharmacologic measures include antimuscarinics, H1 antihistamines, and sympathomimetics, which all detrimentally impact upon cognitive function, rendering them inappropriate for occupational use.5 All current therapies are only partially effective

Representing First-Order Logic Using Graphs

Abstract. We show how edge-labelled graphs can be used to represent first-order logic formulae. This gives rise to recursively nested structures, in which each level of nesting corresponds to the negation of a set of existentials. The model is a direct generalisation of the negative application conditions used in graph rewriting, which count a single level of nesting and are thereby shown to correspond to the fragment ∃¬∃ of first-order logic. Vice versa, this generalisation may be used to strengthen the notion of application conditions. We then proceed to show how these nested models may be flattened to (sets of) plain graphs, by allowing some structure on the labels. The resulting formulae-as-graphs may form the basis of a unification of the theories of graph transformation and predicate transformation

Relative Equilibria in the Four-Vortex Problem with Two Pairs of Equal Vorticities

We examine in detail the relative equilibria in the four-vortex problem where two pairs of vortices have equal strength, that is, \Gamma_1 = \Gamma_2 = 1 and \Gamma_3 = \Gamma_4 = m where m is a nonzero real parameter. One main result is that for m > 0, the convex configurations all contain a line of symmetry, forming a rhombus or an isosceles trapezoid. The rhombus solutions exist for all m but the isosceles trapezoid case exists only when m is positive. In fact, there exist asymmetric convex configurations when m < 0. In contrast to the Newtonian four-body problem with two equal pairs of masses, where the symmetry of all convex central configurations is unproven, the equations in the vortex case are easier to handle, allowing for a complete classification of all solutions. Precise counts on the number and type of solutions (equivalence classes) for different values of m, as well as a description of some of the bifurcations that occur, are provided. Our techniques involve a combination of analysis and modern and computational algebraic geometry

Scanning laser Doppler vibrometry of the cranium when stimulated by a B71 bone transducer

Scanning laser Doppler vibrometry (LDV) has been used extensively to investigate the movement of the middle and inner ear, but has never been used to measure vibrations from a bone transducer travelling over the skin, subcutaneous tissue and cranium in a live subject. Using three scanning laser Doppler vibrometers we measured the displacement of the cranium in 3D in a live subject when stimulated by a B71 bone transducer placed 55 mm posterior to the external auditory canal. Four pure tones (250 Hz, 500 Hz, 1000 Hz, 2000 Hz) were presented separately via the bone transducer. The displacement of the scalp was imaged in four different areas (Ipsilateral to the bone transducer in the temporoparietal region, contralateral temporoparietal region, occipital region and vertex) and linked to the phase of stimulation. Measured scalp motion was consistent with expected displacement of the underlying cranium. Rigid-body motion was the dominant mode of vibration at 250 Hz. At 1000 Hz a mass-spring effect was seen. At 500 Hz there was a transition frequency between vibration as a rigid-body and as a mass-spring. Higher frequencies (2000 Hz) showed that wave transmission was the primary vibrational mode of sound transmission over the cranium. These results broadly support previous research studies but open up potential areas of interest in the investigation of differing skull resonance frequencies

Optical Activity From Extra Dimension

Optical activity, like Faraday effect, is a rotation of the plane of polarization of propagating light in a medium and can be attributed to different sources with distinct signatures. In this note we discuss the effect of optical activity {\it{in vacuum}} due to Kaluza-Klein scalar field $\phi$, in the presence of an external electro-magnetic field. The astrophysical implication of this effect is indicated. We also point out the possibility of observing the same in laboratory conditions.Comment: Four Page

Gluon Propagator on Coarse Lattices in Laplacian Gauges

The Laplacian gauge is a nonperturbative gauge fixing that reduces to Landau gauge in the asymptotic limit. Like Landau gauge, it respects Lorentz invariance, but it is free of Gribov copies; the gauge fixing is unambiguous. In this paper we study the infrared behavior of the lattice gluon propagator in Laplacian gauge by using a variety of lattices with spacings from $a = 0.125$ to 0.35 fm, to explore finite volume and discretization effects. Three different implementations of the Laplacian gauge are defined and compared. The Laplacian gauge propagator has already been claimed to be insensitive to finite volume effects and this is tested on lattices with large volumes.Comment: RevTex 4.0, 14 pages, 9 colour figures; Correction to Reference

Mesons as qbar-q Bound States from Euclidean 2-Point Correlators in the Bethe-Salpeter Approach

We investigate the 2-point correlation function for the vector current. The gluons provide dressings for both the quark self energy as well as the vector vertex function, which are described consistently by the rainbow Dyson-Schwinger equation and the inhomogeneous ladder Bethe-Salpeter equation. The form of the gluon propagator at low momenta is modeled by a 2-parameter ansatz fitting the weak pion decay constant. The quarks are confined in the sense that the quark propagator does not have a pole at timelike momenta. We determine the ground state mass in the vector channel from the Euclidean time Fourier transform of the correlator, which has an exponential falloff at large times. The ground state mass lies around 590 MeV and is almost independent of the model form for the gluon propagator. This method allows us to stay in Euclidean space and to avoid analytic continuation of the quark or gluon propagators into the timelike region.Comment: 21 pages (REVTEX), 8 Postscript figure