7,385 research outputs found
Orbits and masses in the young triple system TWA 5
We aim to improve the orbital elements and determine the individual masses of
the components in the triple system TWA 5.
Five new relative astrometric positions in the H band were recorded with the
adaptive optics system at the Very Large Telescope (VLT). We combine them with
data from the literature and a measurement in the Ks band. We derive an
improved fit for the orbit of TWA 5Aa-b around each other. Furthermore, we use
the third component, TWA 5B, as an astrometric reference to determine the
motion of Aa and Ab around their center of mass and compute their mass ratio.
We find an orbital period of 6.03+/-0.01 years and a semi-major axis of
63.7+/-0.2 mas (3.2+/-0.1 AU). With the trigonometric distance of 50.1+/-1.8
pc, this yields a system mass of 0.9+/-0.1 Msun, where the error is dominated
by the error of the distance. The dynamical mass agrees with the system mass
predicted by a number of theoretical models if we assume that TWA5 is at the
young end of the age range of the TW Hydrae association.
We find a mass ratio of M_Ab / M_Aa = 1.3 +0.6/-0.4, where the less luminous
component Ab is more massive. This result is likely to be a consequence of the
large uncertainties due to the limited orbital coverage of the observations.Comment: 9 pages, 8 figures, accepted by Astronomy and Astrophysic
Association of molecules using a resonantly modulated magnetic field
We study the process of associating molecules from atomic gases using a
magnetic field modulation that is resonant with the molecular binding energy.
We show that maximal conversion is obtained by optimising the amplitude and
frequency of the modulation for the particular temperature and density of the
gas. For small modulation amplitudes, resonant coupling of an unbound atom pair
to a molecule occurs at a modulation frequency corresponding to the sum of the
molecular binding energy and the relative kinetic energy of the atom pair. An
atom pair with an off-resonant energy has a probability of association which
oscillates with a frequency and time-varying amplitude which are primarily
dependent on its detuning. Increasing the amplitude of the modulation tends to
result in less energetic atom pairs being resonantly coupled to the molecular
state, and also alters the dynamics of the transfer from continuum states with
off-resonant energies. This leads to maxima and minima in the total conversion
from the gas as a function of the modulation amplitude. Increasing the
temperature of the gas leads to an increase in the modulation frequency
providing the best fit to the thermal distribution, and weakens the resonant
frequency dependence of the conversion. Mean-field effects can alter the
optimal modulation frequency and lead to the excitation of higher modes. Our
simulations predict that resonant association can be effective for binding
energies of order MHz.Comment: 8 pages latex, figures revised, references updated and typos
correcte
Encoding databases satisfying a given set of dependencies
Consider a relation schema with a set of dependency constraints. A fundamental question is what is the minimum space where the possible instances of the schema can be "stored". We study the following model. Encode the instances by giving a function which maps the set of possible instances into the set of words of a given length over the binary alphabet in a decodable way. The problem is to find the minimum length needed. This minimum is called the information content of the database. We investigate several cases where the set of dependency constraints consist of relatively simple sets of functional or multivalued dependencies. We also consider the following natural extension. Is it possible to encode the instances such a way that small changes in the instance cause a small change in the code. © 2012 Springer-Verlag
Feshbach resonances and collapsing Bose-Einstein condensates
We investigate the quantum state of burst atoms seen in the recent Rb-85
experiments at JILA. We show that the presence of a resonance scattering state
can lead to a pairing instability generating an outflow of atoms with energy
comparable to that observed. A resonance effective field theory is used to
study this dynamical process in an inhomogeneous system with spherical
symmetry
A simple model of epitaxial growth
A discrete solid-on-solid model of epitaxial growth is introduced which, in a
simple manner, takes into account the effect of an Ehrlich-Schwoebel barrier at
step edges as well as the local relaxation of incoming particles. Furthermore a
fast step edge diffusion is included in 2+1 dimensions. The model exhibits the
formation of pyramid-like structures with a well-defined constant inclination
angle. Two regimes can be distinguished clearly: in an initial phase (I) a
definite slope is selected while the number of pyramids remains unchanged. Then
a coarsening process (II) is observed which decreases the number of islands
according to a power law in time. Simulations support self-affine scaling of
the growing surface in both regimes. The roughness exponent is alpha =1 in all
cases. For growth in 1+1 dimensions we obtain dynamic exponents z = 2 (I) and z
= 3 (II). Simulations for d=2+1 seem to be consistent with z= 2 (I) and z= 2.3
(II) respectively.Comment: 8 pages Latex2e, 4 Postscript figures included, uses packages
a4wide,epsfig,psfig,amsfonts,latexsy
The Nucleon Spectral Function at Finite Temperature and the Onset of Superfluidity in Nuclear Matter
Nucleon selfenergies and spectral functions are calculated at the saturation
density of symmetric nuclear matter at finite temperatures. In particular, the
behaviour of these quantities at temperatures above and close to the critical
temperature for the superfluid phase transition in nuclear matter is discussed.
It is shown how the singularity in the thermodynamic T-matrix at the critical
temperature for superfluidity (Thouless criterion) reflects in the selfenergy
and correspondingly in the spectral function. The real part of the on-shell
selfenergy (optical potential) shows an anomalous behaviour for momenta near
the Fermi momentum and temperatures close to the critical temperature related
to the pairing singularity in the imaginary part. For comparison the selfenergy
derived from the K-matrix of Brueckner theory is also calculated. It is found,
that there is no pairing singularity in the imaginary part of the selfenergy in
this case, which is due to the neglect of hole-hole scattering in the K-matrix.
From the selfenergy the spectral function and the occupation numbers for finite
temperatures are calculated.Comment: LaTex, 23 pages, 21 PostScript figures included (uuencoded), uses
prc.sty, aps.sty, revtex.sty, psfig.sty (last included
Random Hierarchical Matrices: Spectral Properties and Relation to Polymers on Disordered Trees
We study the statistical and dynamic properties of the systems characterized
by an ultrametric space of states and translationary non-invariant symmetric
transition matrices of the Parisi type subjected to "locally constant"
randomization. Using the explicit expression for eigenvalues of such matrices,
we compute the spectral density for the Gaussian distribution of matrix
elements. We also compute the averaged "survival probability" (SP) having sense
of the probability to find a system in the initial state by time . Using the
similarity between the averaged SP for locally constant randomized Parisi
matrices and the partition function of directed polymers on disordered trees,
we show that for times (where is some critical
time) a "lacunary" structure of the ultrametric space occurs with the
probability . This means that the escape from some bounded
areas of the ultrametric space of states is locked and the kinetics is confined
in these areas for infinitely long time.Comment: 7 pages, 2 figures (the paper is essentially reworked
The phase diagram of the betaine arsenate-phosphate mixed crystal system by measurement of the dielectric hysteresis and current curves
The ferroelectric (FE) betaine arsenate (BA) and the antiferroelectric
(AF) betaine phosphate (BP) are structurally very closely related and
mixed crystals of BAxBP1-x can be grown over the whole concentration
range. The concentration (x) – temperature (T) – pressure (P) phase
diagram of the system is characterized by a transition from an AF to a
FE phase along with the appearance of some mixed regions on changing
concentration. Depending on concentration, one or two relaxation
processes appear in the low temperature region of the phase diagram.
Measurement of the hysteresis loops and the current curves have been
carried out along the (x-T-P)-phase diagram of this system. Multiple
loops have been observed in various regions of the phase diagram. The
correlation of the different contributions of the polarization and the temperature behavior of their critical fields with the main features of the
temperature and frequency dependent dielectric constant enables
clarification of the rich phase diagram obtained in this mixed
compounds in terms of the ferro- and/or antiferroelectric interactions
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