2,014 research outputs found

    Damping of MHD turbulence in partially ionized gas and the observed difference of velocities of neutrals and ions

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    Theoretical and observational studies on the turbulence of the interstellar medium developed fast in the past decades. The theory of supersonic magnetized turbulence, as well as the understanding of projection effects of observed quantities, are still in progress. In this work we explore the characterization of the turbulent cascade and its damping from observational spectral line profiles. We address the difference of ion and neutral velocities by clarifying the nature of the turbulence damping in the partially ionized. We provide theoretical arguments in favor of the explanation of the larger Doppler broadening of lines arising from neutral species compared to ions as arising from the turbulence damping of ions at larger scales. Also, we compute a number of MHD numerical simulations for different turbulent regimes and explicit turbulent damping, and compare both the 3-dimensional distributions of velocity and the synthetic line profile distributions. From the numerical simulations, we place constraints on the precision with which one can measure the 3D dispersion depending on the turbulence sonic Mach number. We show that no universal correspondence between the 3D velocity dispersions measured in the turbulent volume and minima of the 2D velocity dispersions available through observations exist. For instance, for subsonic turbulence the correspondence is poor at scales much smaller than the turbulence injection scale, while for supersonic turbulence the correspondence is poor for the scales comparable with the injection scale. We provide a physical explanation of the existence of such a 2D-3D correspondence and discuss the uncertainties in evaluating the damping scale of ions that can be obtained from observations. However, we show that the statistics of velocity dispersion from observed line profiles can provide the spectral index and the energy transfer rate of turbulence. Also, comparing two similar simulations with different viscous coefficients it was possible to constrain the turbulent cut-off scale. This may especially prove useful since it is believed that ambipolar diffusion may be one of the dominant dissipative mechanism in star-forming regions. In this case, the determination of the ambipolar diffusion scale may be used as a complementary method for the determination of magnetic field intensity in collapsing cores. We discuss the implications of our findings in terms of a new approach to magnetic field measurement proposed by Li & Houde (2008).Comment: ApJ accepte

    The Carter Constant for Inclined Orbits About a Massive Kerr Black Hole: I. circular orbits

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    In an extreme binary black hole system, an orbit will increase its angle of inclination (i) as it evolves in Kerr spacetime. We focus our attention on the behaviour of the Carter constant (Q) for near-polar orbits; and develop an analysis that is independent of and complements radiation reaction models. For a Schwarzschild black hole, the polar orbits represent the abutment between the prograde and retrograde orbits at which Q is at its maximum value for given values of latus rectum (l) and eccentricity (e). The introduction of spin (S = |J|/M2) to the massive black hole causes this boundary, or abutment, to be moved towards greater orbital inclination; thus it no longer cleanly separates prograde and retrograde orbits. To characterise the abutment of a Kerr black hole (KBH), we first investigated the last stable orbit (LSO) of a test-particle about a KBH, and then extended this work to general orbits. To develop a better understanding of the evolution of Q we developed analytical formulae for Q in terms of l, e, and S to describe elliptical orbits at the abutment, polar orbits, and last stable orbits (LSO). By knowing the analytical form of dQ/dl at the abutment, we were able to test a 2PN flux equation for Q. We also used these formulae to numerically calculate the di/dl of hypothetical circular orbits that evolve along the abutment. From these values we have determined that di/dl = -(122.7S - 36S^3)l^-11/2 -(63/2 S + 35/4 S^3) l^-9/2 -15/2 S l^-7/2 -9/2 S l^-5/2. Thus the abutment becomes an important analytical and numerical laboratory for studying the evolution of Q and i in Kerr spacetime and for testing current and future radiation back-reaction models for near-polar retrograde orbits.Comment: 51 pages, 8 figures, accepted by Classical and Quantum Gravity on September 22nd, 201

    Learning and adaptation in speech production without a vocal tract

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    How is the complex audiomotor skill of speaking learned? To what extent does it depend on the specific characteristics of the vocal tract? Here, we developed a touchscreen-based speech synthesizer to examine learning of speech production independent of the vocal tract. Participants were trained to reproduce heard vowel targets by reaching to locations on the screen without visual feedback and receiving endpoint vowel sound auditory feedback that depended continuously on touch location. Participants demonstrated learning as evidenced by rapid increases in accuracy and consistency in the production of trained targets. This learning generalized to productions of novel vowel targets. Subsequent to learning, sensorimotor adaptation was observed in response to changes in the location-sound mapping. These findings suggest that participants learned adaptable sensorimotor maps allowing them to produce desired vowel sounds. These results have broad implications for understanding the acquisition of speech motor control.Published versio

    Simultaneous Determination of the Cosmic Ray Ionization Rate and Fractional Ionization in DR21(OH)

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    We present a new method for the simultaneous calculation of the cosmic ray ionization rate, zeta(H2), and the ionization fraction, chi(e), in dense molecular clouds. A simple network of chemical reactions dominant in the creation and destruction of HCNH+ and HCO+ is used in conjunction with observed pairs of rotational transitions of several molecular species in order to determine the electron abundance and the H3+ abundance. The cosmic ray ionization rate is then calculated by taking advantage of the fact that, in dark clouds, it governs the rate of creation of H3+. We apply this technique to the case of the star-forming region DR21(OH), where we successfully detected the (J=3-2) and (J=4-3) rotational transitions of HCNH+. We also determine the C and O isotopic ratios in this source to be 12C/13C=63+-4 and 16O/18O=318+-64, which are in good agreement with previous measurements in other clouds. The significance of our method lies in the ability to determine N(H3+) and chi(e) directly from observations, and estimate zeta(H2) accordingly. Our results, zeta(H2)=3.1x10^(-18) 1/s and chi(e)=3.2x10^(-8), are consistent with recent determinations in other objects.Comment: 22 pages, including 3 figure

    Controlling a Telescope Chopping Secondary Mirror Assembly Using a Signal Deconvolution Technique

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    We describe a technique for improving the response of a telescope chopping secondary mirror assembly by using a signal processing method based on the Lucy deconvolution technique. This technique is general and could be used for any systems, linear or non-linear, where the transfer function(s) can be measured with sufficient precision. We demonstrate how the method was implemented and show results obtained at the Caltech Submillimeter Observatory using different chop throw amplitudes and frequencies. No intervention from the telescope user is needed besides the selection of the chop throw amplitude and frequency. All the calculations are done automatically once the appropriate command is issued from the user interface of the observatory's main computer.Comment: 17 pages, 6 figure

    Polarisation Observations of H2_{2}O JK1K1=532441J_{K_{-1}K_{1}} = 5_{32} - 4_{41} 620.701 GHz Maser Emission with Herschel/HIFI in Orion KL

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    Context. The high intensities and narrow bandwidths exhibited by some astronomical masers make them ideal tools for studying star-forming giant molecular clouds. The water maser transition JK1K1=532441J_{K_{-1}K_{1}}=5_{32}-4_{41} at 620.701 GHz can only be observed from above Earth's strongly absorbing atmosphere; its emission has recently been detected from space. Aims. We sought to further characterize the star-forming environment of Orion KL by investigating the linear polarisation of a source emitting a narrow 620.701 GHz maser feature with the heterodyne spectrometer HIFI on board the Herschel Space Observatory. Methods. High-resolution spectral datasets were collected over a thirteen month period beginning in 2011 March, to establish not only the linear polarisation but also the temporal variability of the source. Results. Within a 3σ3\sigma uncertainty, no polarisation was detected to an upper limit of approximately 2%. These results are compared with coeval linear polarisation measurements of the 22.235 GHz JK1K1=616523J_{K_{-1}K_{1}}=6_{16}-5_{23} maser line from the Effelsberg 100-m radio telescope, typically a much stronger maser transition. Although strongly polarised emission is observed for one component of the 22.235 GHz maser at 7.2 km s1^{-1}, a weaker component at the same velocity as the 620.701 GHz maser at 11.7 km s1^{-1} is much less polarised.Comment: Accepted for publication in A&

    A Study of Elliptical Last Stable Orbits About a Massive Kerr Black Hole

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    The last stable orbit (LSO) of a compact object (CO) is an important boundary condition when performing numerical analysis of orbit evolution. Although the LSO is already well understood for the case where a test-particle is in an elliptical orbit around a Schwarzschild black hole (SBH) and for the case of a circular orbit about a Kerr black hole (KBH) of normalised spin, S (|J|/M^2, where J is the spin angular momentum of the KBH); it is worthwhile to extend our knowledge to include elliptical orbits about a KBH. This extension helps to lay the foundation for a better understanding of gravitational wave (GW) emission. The mathematical developments described in this work sprang from the use of an effective potential (V) derived from the Kerr metric, which encapsulates the Lense-Thirring precession. That allowed us to develop a new form of analytical expression to calculate the LSO Radius for circular orbits (R_LSO) of arbitrary KBH spin. We were then able to construct a numerical method to calculate the latus rectum (l_LSO) for an elliptical LSO. Abstract Formulae for E^2 (square of normalised orbital energy) and L^2 (square of normalised orbital angular momentum) in terms of eccentricity, e, and latus rectum, l, were previously developed by others for elliptical orbits around an SBH and then extended to the KBH case; we used these results to generalise our analytical l_LSO equations to elliptical orbits. LSO data calculated from our analytical equations and numerical procedures, and those previously published, are then compared and found to be in excellent agreement.Comment: 42 pages, 9 figures, accepted for publication in Classical and Quantum Gravit

    Corporate Wellness Programs: A Cost Containment Strategy

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    The scope of this report involves a comprehensive look at the corporate trend toward wellness in the workplace. Because of the severity of rising health care costs and subsequent impact on profitability, corporations are turning toward containment strategies that enable cost increases to be controlled. An indepth study was conducted on existing corporate wellness programs and their effectiveness in terms of reducing health care costs and improving overall productivity. The information presented in this report was collected through extensive secondary research. Overwhelming evidence exists to support the belief that corporate well ness programs are an effective cost containment strategy against rising health care costs. Successful wellness programs have proven not only that healthy employees use the health care system less often, but that they are also more productive and absent less frequently than unhealthy employees. These direct and indirect benefits translate into reduced medical costs and improved productivity for the company.Business Administratio
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