2,176 research outputs found
The Kinematics of High Proper Motion Halo White Dwarfs
We analyse the kinematics of the entire spectroscopic sample of 99 recently
discovered high proper-motion white dwarfs by Oppenheimer et al. using a
maximum-likelihood analysis, and discuss the claim that the high-velocity white
dwarfs are members of a halo population with a local density at least ten times
greater than traditionally assumed. We argue that the observations, as
reported, are consistent with the presence of an almost undetected thin disc
plus a thick disc, with densities as conventionally assumed. In addition, there
is a kinematically distinct, flattened, halo population at the more than 99%
confidence level. Surprisingly, the thick disc and halo populations are
indistinguishable in terms of luminosity, color and apparent age (1-10 Gyr).
Adopting a bimodal, Schwarzschild model for the local velocity ellipsoid, with
the ratios sigma_U:sigma_V:sigma_W=1:2/3:1/2, we infer radial velocity
dispersions of sigma_U=62(+8/-10) km/s and 150(+80/-40) km/s (90% C.L.) for the
local thick disc and halo populations, respectively. The thick disc result
agrees with the empirical relation between asymmetric drift and radial velocity
dispersion, inferred from local stellar populations. The local thick-disc plus
halo density of white dwarfs is n^{td+h}=(1.9+-0.5)x10^-3 pc^-3 (90% C.L.), of
which n^{h}=1.1(+2.1/-0.7)x10^-4 pc^-3 (90% C.L.) belongs to the halo, a
density about five times higher than previously thought. (Abridged)Comment: 19 pages, 11 figures; submitted to MNRA
Relativistic Photon Mediated Shocks
A system of equations governing the structure of a steady, relativistic
radiation dominated shock is derived, starting from the general form of the
transfer equation obeyed by the photon distribution function. Closure is
obtained by truncating the system of moment equations at some order. The
anisotropy of the photon distribution function inside the shock is shown to
increase with increasing shock velocity, approaching nearly perfect beaming at
upstream Lorentz factors . Solutions of the shock equations are
presented for some range of upstream conditions. These solutions are shown to
converge as the truncation order is increased.Comment: 5 pages, a shorter version will appear in PR
White Dwarfs: Contributors and Tracers of the Galactic Dark-Matter Halo
We examine the claim by Oppenheimer et al. (2001) that the local halo density
of white dwarfs is an order of magnitude higher than previously thought. As it
stands, the observational data support the presence of a kinematically distinct
population of halo white dwarfs at the >99% confidence level. A
maximum-likelihood analysis gives a radial velocity dispersion of
sigma^h_U=150(+80/-40) km/s and an asymmetric drift of v_a^h=176(+102/-80)
km/s, for a Schwarzschild velocity distribution function with
sigma_U:sigma_V:sigma_W=1:2/3:1/2. Halo white dwarfs have a local number
density of 1.1(+2.1/-0.7)x10^-4 pc-3, which amounts to 0.8(+1.6/-0.5) per cent
of the nominal local dark-matter halo density and is 5.0(+9.5/-3.2) times
higher and thus only marginally in agreement with previous estimates (all
errors indicate the 90% C.L.). We discuss several direct consequences of this
white-dwarf population (e.g. microlensing) and postulate a potential mechanism
to eject young white dwarfs from the disc to the halo, through the orbital
instabilities in triple or multiple stellar systems.Comment: 5 pages, to appear in the proceedings of the Yale Cosmology Workshop
"The Shapes of Galaxies and their Halos" (ed. Priya Natarajan); revised
numerical results, using a corrected likelihood function (thanks to David
Graff and Andy Gould); general conclusions remain simila
Thermo-Rotational Instability in Plasma Disks Around Compact Objects
Differentially rotating plasma disks, around compact objects, that are
imbedded in a ``seed'' magnetic field are shown to develop vertically localized
ballooning modes that are driven by the combined radial gradient of the
rotation frequency and vertical gradients of the plasma density and
temperature. When the electron mean free path is shorter than the disk height
and the relevant thermal conductivity can be neglected, the vertical particle
flows produced by of these modes have the effect to drive the density and
temperature profiles toward the ``adiabatic condition'' where
. Here is the plasma temperature and
the particle density. The faster growth rates correspond to steeper
temperature profiles such as those produced by an internal
(e.g., viscous) heating process. In the end, ballooning modes excited for
various values of can lead to the evolution of the disk into a
different current carrying configuration such as a sequence of plasma rings
Stability Properties of Strongly Magnetized Spine Sheath Relativistic Jets
The linearized relativistic magnetohydrodynamic (RMHD) equations describing a
uniform axially magnetized cylindrical relativistic jet spine embedded in a
uniform axially magnetized relativistically moving sheath are derived. The
displacement current is retained in the equations so that effects associated
with Alfven wave propagation near light speed can be studied. A dispersion
relation for the normal modes is obtained. Analytical solutions for the normal
modes in the low and high frequency limits are found and a general stability
condition is determined. A trans-Alfvenic and even a super-Alfvenic
relativistic jet spine can be stable to velocity shear driven Kelvin-Helmholtz
modes. The resonance condition for maximum growth of the normal modes is
obtained in the kinetically and magnetically dominated regimes. Numerical
solution of the dispersion relation verifies the analytical solutions and is
used to study the regime of high sound and Alfven speeds.Comment: 42 pages includes 7 figures, to appear in Ap
Determinants of Farm Policies in the United States, 1996-2008
This paper focuses on the political economy of U.S. farm policy since the Uruguay Round trade negotiations concluded in 1994 and established the WTO. The continued ability of the powerful farm lobby in the United States to elicit support in the political arena is evident from this analysis. Yet there have been some substantial changes in policy that have reduced their distortionary effects, as well as some setbacks to liberalizing reform. New Doha Round commitments could put further constraints on subsidies provided by some U.S. policy instruments. And despite the ability of the farm lobby to retain its support programs through 2012, there are several political uncertainties about the alignments that have allowed U.S. farm support to endure.Distorted incentives, agricultural and trade policy reforms, national agricultural development, United States farm policies, political economy of agricultural distortions, Agricultural and Food Policy, International Relations/Trade, F13, F14, Q17, Q18, O13, P22,
Finite Element Simulation of Saturated-Unsaturated Subsurface Flow
A two-dimensional transient model for flow through saturated-unsaturated porous media is developed, The model numerically solves the pressure head dependent or moisture content dependent form of Richard\u27s equation. The model code uses isoparametric quadratic triangular and/or quadrilateral finite elements for the geometric representation and for the weak Galerkin spacial integrations. An implicit, unconditionally stable single-step numerical time integration scheme with an oscilliatory noise reduction option is utilized for the temporal discretization. The highly efficient symmetric skyline (profile) solution scheme is used to solve the resulting simultaneous equations. The nonlinear subsurface flow parameters are approximated using cubic spline interpolation. The element material properties can be independently defined thus permitting the modelling of layered geologic formations, Derivative smoothing is presented for the post-calculation of Darcian velocities. Currently, the program is limited to time varying specification of pressure head or moisture content and fluxes. Several sample problems are presented illustrating the accuracy and validity of the developed model
THE CURRENT WTO AGRICULTURAL NEGOTIATIONS: OPTIONS FOR PROGRESS; SYNTHESIS
World Trade Organization, International Relations/Trade,
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