975 research outputs found
Enhanced Dimer Relaxation in an Atomic/Molecular BEC
We derive a universal formula for the rate constant \beta for relaxation of a
shallow dimer into deeply-bound diatomic molecules in the case of atoms with a
large scattering length a. We show that \beta is determined by a and by two
3-body parameters that also determine the binding energies and widths of Efimov
states. The rate constant \beta scales like \hbar a/m near the resonance, but
the coefficient is a periodic function of ln(a) that may have resonant
enhancement at values of a that differ by multiples of 22.7.Comment: 5 pages, revtex4, 2 PS figures, title changed, final versio
Astrometric Positions and Proper Motions of 19 Radio Stars
We have used the Very Large Array, linked with the Pie Town Very Long
Baseline Array antenna, to determine astrometric positions of 19 radio stars in
the International Celestial Reference Frame (ICRF). The positions of these
stars were directly linked to the positions of distant quasars through phase
referencing observations. The positions of the ICRF quasars are known to 0.25
mas, thus providing an absolute reference at the angular resolution of our
radio observations. Average values for the errors in our derived positions for
all sources were 13 mas and 16 mas in R.A. and declination respectively, with
accuracies approaching 1-2 mas for some of the stars observed. Differences
between the ICRF positions of the 38 quasars, and those measured from our
observations showed no systematic offsets, with mean values of -0.3 mas in R.A.
and -1.0 mas in declination. Standard deviations of the quasar position
differences of 17 mas and 11 mas in R.A. and declination respectively, are
consistent with the mean position errors determined for the stars. Our measured
positions were combined with previous Very Large Array measurements taken from
1978-1995 to determine the proper motions of 15 of the stars in our list. With
mean errors of approximately 1.6 mas/yr, the accuracies of our proper motions
approach those derived from Hipparcos, and for a few of the stars in our
program, are better than the Hipparcos values. Comparing the positions of our
radio stars with the Hipparcos catalog, we find that at the epoch of our
observations, the two frames are aligned to within formal errors of
approximately 3 mas. This result confirms that the Hipparcos frame is inertial
at the expected level.Comment: 20 pages, 9 figures Accepted by the Astronomical Journal, 2003 March
1
Nongaussian fluctuations arising from finite populations: Exact results for the evolutionary Moran process
The appropriate description of fluctuations within the framework of
evolutionary game theory is a fundamental unsolved problem in the case of
finite populations. The Moran process recently introduced into this context
[Nowak et al., Nature (London) 428, 646 (2004)] defines a promising standard
model of evolutionary game theory in finite populations for which analytical
results are accessible. In this paper, we derive the stationary distribution of
the Moran process population dynamics for arbitrary games for the
finite size case. We show that a nonvanishing background fitness can be
transformed to the vanishing case by rescaling the payoff matrix. In contrast
to the common approach to mimic finite-size fluctuations by Gaussian
distributed noise, the finite size fluctuations can deviate significantly from
a Gaussian distribution.Comment: 4 pages (2 figs). Published in Physical Review E (Rapid
Communications
Coevolutionary Dynamics: From Finite to Infinite Populations
Traditionally, frequency dependent evolutionary dynamics is described by
deterministic replicator dynamics assuming implicitly infinite population
sizes. Only recently have stochastic processes been introduced to study
evolutionary dynamics in finite populations. However, the relationship between
deterministic and stochastic approaches remained unclear. Here we solve this
problem by explicitly considering large populations. In particular, we identify
different microscopic stochastic processes that lead to the standard or the
adjusted replicator dynamics. Moreover, differences on the individual level can
lead to qualitatively different dynamics in asymmetric conflicts and, depending
on the population size, can even invert the direction of the evolutionary
process.Comment: 4 pages (2 figs included). Published in Phys. Rev. Lett., December
200
Asymmetry in the Spectrum of High-Velocity H2O Maser Emission Features in Active Galactic Nuclei
We suggest a mechanism for the amplification of high-velocity water-vapor
maser emission features from the central regions of active galactic nuclei. The
model of an emitting accretion disk is considered. The high-velocity emission
features originate in the right and left wings of the Keplerian disk. The
hyperfine splitting of the signal levels leads to an asymmetry in the spectral
profile of the water vapor maser line at a frequency of 22.235 GHz. We show
that the gain profile asymmetry must lead to an enhanced brightness of the
blueshifted high-velocity emission features compared to the redshifted ones.
Such a situation is observed in the source UGC 3789.Comment: 11 pages 3 figure
A paper based graphene-nanocauliflower hybrid composite for point of care biosensing
Graphene paper has diverse applications in printed circuit board electronics, bioassays, 3D cell culture, and biosensing. Although development of nanometal-graphene hybrid composites is commonplace in the sensing literature, to date there are only a few examples of nanometal-decorated graphene paper for use in biosensing. In this manuscript, we demonstrate the synthesis and application of Pt nano cauliflower-functionalized graphene paper for use in electrochemical biosensing of small molecules (glucose, acetone, methanol) or detection of pathogenic bacteria (Escherichia coli O157:H7). Raman spectroscopy, scanning electron microscopy and energy dispersive spectroscopy were used to show that graphene oxide deposited on nanocellulose crystals was partially reduced by both thermal and chemical treatment. Fractal platinum nanostructures were formed on the reduced graphene oxide paper, producing a conductive paper with an extremely high electroactive surface area, confirmed by cyclic voltammetry and electrochemical impedance spectroscopy. To show the broad applicability of the material, the platinum surface was functionalized with three different biomaterials: 1) glucose oxidase (via chitosan encapsulation); 2) a DNA aptamer (via covalent linking), or 3) a chemosensory protein (via his linking). We demonstrate the application of this device for point of care biosensing. The detection limit for both glucose (0.08 ± 0.02 μM) and E. coli O157:H7 (1.3 ± 0.1 CFU mL-1) were competitive with, or superior to, previously reported devices in the biosensing literature. The response time (6 sec for glucose and 10 min for E. coli) were also similar to silicon biochip and commercial electrode sensors. The results demonstrate that the nanocellulose-graphene-nanoplatinum material is an excellent paper-based platform for development of electrochemical biosensors targeting small molecules or whole cells for use in point of care biosensing
Cool carbon stars in the halo: new very red or distant objects
The goal of this paper is to present and analyse a new sample of cool carbon
(C)stars located in the halo. Twenty three new C stars were discovered. Spectra
are typical of N-type stars with C2 and CN bands and sometimes Halpha in
emission. ... Four objects are particularly red with J-K > 3, with 2 located at
more than 5 kpc. from the Galactic plane. Eight additional objects with similar
properties are found in the literature and our previous works. These 12 C stars
could be useful to study mass loss at low metallicity. Two objects are at
distances of 95 and 110 kpc. They are located in the region with galactocentric
Z < -60 kpc in which the model of Law et al. predicts the Sgr stream to have a
loop. (Abstact abridged)Comment: 16 pages, 12 figures, accepted by A
Observational Evidence for Massive Black Holes in the Centers of Active Galaxies
Naturally occurring water vapor maser emission at 1.35 cm wavelength provides
an accurate probe for the study of accretion disks around highly compact
objects, thought to be black holes, in the centers of active galaxies. Because
of the exceptionally fine angular resolution, 200 microarcseconds, obtainable
with very long baseline interferometry, accompanied by high spectral
resolution, < 0.1 km/s, the dynamics and structures of these disks can be
probed with exceptional clarity. The data on the galaxy NGC4258 are discussed
here in detail. The mass of the black hole binding the accretion disk is 3.9
times 10^7 solar masses. Although the accretion disk has a rotational period of
about 800 years, the physical motions of the masers have been directly measured
with VLBI over a period of a few years. These measurements also allow the
distance from the earth to the black hole to be estimated to an accuracy of 4
percent. The status of the search for other maser/black hole candidates is also
discussed.Comment: 24 pages, 11 figures, latex, uses aaspp4 style file. To be published
in the Journal of Astronomy and Astrophysics (India), proceedings of the
Discussion Meeting on the Physics of Black Holes, Bangalore, India: December
199
VLA+PT Astrometry of 46 Radio Stars
We have used the Very Large Array (VLA), linked with the Pie Town Very Long
Baseline Array antenna, to determine astrometric positions of 46 radio stars in
the International Celestial Reference Frame (ICRF). Positions were obtained in
the ICRF directly through phase referencing of the stars to nearby ICRF quasars
whose positions are accurate at the 0.25 mas level. Radio star positions are
estimated to be accurate at the 10 mas level, with position errors approaching
a few milli-arcseconds for some of the stars observed. Our measured positions
were combined with previous measurements taken from as early as 1978 to obtain
proper motion estimates for all 46 stars with average uncertainties of ~1.7
mas/yr. We compared our radio star positions and proper motions with the
Hipparcos Catalogue data, and find consistency in the reference frames produced
by each data set on the 1-sigma level, with errors of ~2.7 mas per axis for the
reference frame orientation angles at our mean epoch of 2003.78. No significant
spin is found between our radio data frame and the Hipparcos Celestial
Reference Frame (HCRF) with largest rotation rates of +0.55 and -0.41 mas/yr
around the x and z axes, respectively, with 1-sigma errors of 0.36 mas/yr.
Thus, our results are consistent with a non-rotating Hipparcos frame with
respect to the ICRF.Comment: 29 pages, 8 figures, accepted for publication in the Astronomical
Journa
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