3,207 research outputs found
Neutron activation analysis traces copper artifacts to geographical point of origin
Impurities remaining in the metallic copper are identified and quantified by spectrographic and neutron activation analysis. Determination of the type of ore used for the copper artifact places the geographic point of origin of the artifact
Speeding Up Computer Simulations: The Transition Observable Method
A method is presented which allows for a tremendous speed-up of computer
simulations of statistical systems by orders of magnitude. This speed-up is
achieved by means of a new observable, while the algorithm of the simulation
remains unchanged.Comment: 20 pages, 6 figures Submitted to Phys.Rev.E (August 1999) Replacement
due to some minor change
Phase transitions and configuration space topology
Equilibrium phase transitions may be defined as nonanalytic points of
thermodynamic functions, e.g., of the canonical free energy. Given a certain
physical system, it is of interest to understand which properties of the system
account for the presence of a phase transition, and an understanding of these
properties may lead to a deeper understanding of the physical phenomenon. One
possible approach of this issue, reviewed and discussed in the present paper,
is the study of topology changes in configuration space which, remarkably, are
found to be related to equilibrium phase transitions in classical statistical
mechanical systems. For the study of configuration space topology, one
considers the subsets M_v, consisting of all points from configuration space
with a potential energy per particle equal to or less than a given v. For
finite systems, topology changes of M_v are intimately related to nonanalytic
points of the microcanonical entropy (which, as a surprise to many, do exist).
In the thermodynamic limit, a more complex relation between nonanalytic points
of thermodynamic functions (i.e., phase transitions) and topology changes is
observed. For some class of short-range systems, a topology change of the M_v
at v=v_t was proved to be necessary for a phase transition to take place at a
potential energy v_t. In contrast, phase transitions in systems with long-range
interactions or in systems with non-confining potentials need not be
accompanied by such a topology change. Instead, for such systems the
nonanalytic point in a thermodynamic function is found to have some
maximization procedure at its origin. These results may foster insight into the
mechanisms which lead to the occurrence of a phase transition, and thus may
help to explore the origin of this physical phenomenon.Comment: 22 pages, 6 figure
The close T Tauri binary system V4046 Sgr: Rotationally modulated X-ray emission from accretion shocks
We report initial results from a quasi-simultaneous X-ray/optical observing
campaign targeting V4046 Sgr, a close, synchronous-rotating classical T Tauri
star (CTTS) binary in which both components are actively accreting. V4046 Sgr
is a strong X-ray source, with the X-rays mainly arising from high-density (n_e
~ 10^(11-12) cm^(-3)) plasma at temperatures of 3-4 MK. Our multiwavelength
campaign aims to simultaneously constrain the properties of this X-ray emitting
plasma, the large scale magnetic field, and the accretion geometry. In this
paper, we present key results obtained via time-resolved X-ray grating spectra,
gathered in a 360 ks XMM-Newton observation that covered 2.2 system rotations.
We find that the emission lines produced by this high-density plasma display
periodic flux variations with a measured period, 1.22+/-0.01 d, that is
precisely half that of the binary star system (2.42 d). The observed rotational
modulation can be explained assuming that the high-density plasma occupies
small portions of the stellar surfaces, corotating with the stars, and that the
high-density plasma is not azimuthally symmetrically distributed with respect
to the rotational axis of each star. These results strongly support models in
which high-density, X-ray-emitting CTTS plasma is material heated in accretion
shocks, located at the base of accretion flows tied to the system by magnetic
field lines.Comment: paper accepted by Ap
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