9,707 research outputs found
Critical Rotation of an Annular Superfluid Bose Gas
We analyze the excitation spectrum of a superfluid Bose-Einstein condensate
rotating in a ring trap. We identify two important branches of the spectrum
related to outer and inner edge surface modes that lead to the instability of
the superfluid. Depending on the initial circulation of the annular condensate,
either the outer or the inner modes become first unstable. This instability is
crucially related to the superfluid nature of the rotating gas. In particular
we point out the existence of a maximal circulation above which the superflow
decays spontaneously, which cannot be explained by invoking the average speed
of sound.Comment: 5 pages, 5 figures, PRA Rapid Com
Performance of astrometric detection of a hotspot orbiting on the innermost stable circular orbit of the galactic centre black hole
The galactic central black hole Sgr A* exhibits outbursts of radiation in the
near infrared (so-called IR flares). One model of these events consists in a
hotspot orbiting on the innermost stable circular orbit (ISCO) of the hole.
These outbursts can be used as a probe of the central gravitational potential.
One main scientific goal of the second generation VLTI instrument GRAVITY is to
observe these flares astrometrically. Here, the astrometric precision of
GRAVITY is investigated in imaging mode, which consists in analysing the image
computed from the interferometric data. The capability of the instrument to put
in light the motion of a hotspot orbiting on the ISCO of our central black hole
is then discussed.
We find that GRAVITY's astrometric precision for a single star in imaging
mode is smaller than the Schwarzschild radius of Sgr A*. The instrument can
also demonstrate that a body orbiting on the last stable orbit of the black
hole is indeed moving. It yields a typical size of the orbit, if the source is
as bright as m_K=14.
These results show that GRAVITY allows one to study the close environment of
Sgr A*. Having access to the ISCO of the central massive black hole probably
allows constraining general relativity in its strong regime. Moreover, if the
hotspot model is appropriate, the black hole spin can be constrained.Comment: 13 pages, 11 figures ; accepted by MNRA
An optical study of interdiffusion in ZnSe/ZnCdSe
Copyright 1996 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. This article appeared in Applied Physics Letters 69, 1579 (1996) and may be found at
The evaluation of Education Maintenance Allowance Pilots: three years' evidence: a quantitative evaluation
This is the third report of the longitudinal quantitative evaluation of Education Maintenance Allowance (EMA) pilots and the first since the government announced that EMA is to be rolled out nationally from 2004. The evaluation was commissioned in 1999, by the Department for Education and Skills (DfES) from a consortium of research organisations, led by the Centre for Research in Social Policy (CRSP) and including the National Centre for Social Research, the Institute for Fiscal Studies (IFS) and the National Institute for Careers Education and Counselling (NICEC).
The statistical evaluation design is a longitudinal cohort study involving large random sample surveys of young people (and their parents) in 10 EMA pilot areas and eleven control areas. Two cohorts of young people were selected from Child Benefit records. The first cohort of young people left compulsory schooling in the summer of 1999 and they, and their parents, were interviewed between October 1999 and April 2000 (Year 12 interview). A second interview was carried out with these young people between October 2000 and April 2001 (Year 13 interview). The second cohort left compulsory education the following summer of 2000 and young people, and their parents, were first interviewed between October 2000 and April 2001.
The report uses both propensity score matching (PSM) and descriptive techniques, each of which brings their own particular strengths to the analysis
Two-point phase correlations of a one-dimensional bosonic Josephson junction
We realize a one-dimensional Josephson junction using quantum degenerate Bose
gases in a tunable double well potential on an atom chip. Matter wave
interferometry gives direct access to the relative phase field, which reflects
the interplay of thermally driven fluctuations and phase locking due to
tunneling. The thermal equilibrium state is characterized by probing the full
statistical distribution function of the two-point phase correlation.
Comparison to a stochastic model allows to measure the coupling strength and
temperature and hence a full characterization of the system
Rubidium-87 Bose-Einstein condensate in an optically plugged quadrupole trap
We describe an experiment to produce 87Rb Bose-Einstein condensates in an
optically plugged magnetic quadrupole trap, using a blue-detuned laser. Due to
the large detuning of the plug laser with respect to the atomic transition, the
evaporation has to be carefully optimized in order to efficiently overcome the
Majorana losses. We provide a complete theoretical and experimental study of
the trapping potential at low temperatures and show that this simple model
describes well our data. In particular we demonstrate methods to reliably
measure the trap oscillation frequencies and the bottom frequency, based on
periodic excitation of the trapping potential and on radio-frequency
spectroscopy, respectively. We show that this hybrid trap can be operated in a
well controlled regime that allows a reliable production of degenerate gases.Comment: 13 pages, 8 figure
Imaging the spotty surface of Betelgeuse in the H band
This paper reports on H-band interferometric observations of Betelgeuse made
at the three-telescope interferometer IOTA. We image Betelgeuse and its
asymmetries to understand the spatial variation of the photosphere, including
its diameter, limb darkening, effective temperature, surrounding brightness,
and bright (or dark) star spots. We used different theoretical simulations of
the photosphere and dusty environment to model the visibility data. We made
images with parametric modeling and two image reconstruction algorithms: MIRA
and WISARD. We measure an average limb-darkened diameter of 44.28 +/- 0.15 mas
with linear and quadratic models and a Rosseland diameter of 45.03 +/- 0.12 mas
with a MARCS model. These measurements lead us to derive an updated effective
temperature of 3600 +/- 66 K. We detect a fully-resolved environment to which
the silicate dust shell is likely to contribute. By using two imaging
reconstruction algorithms, we unveiled two bright spots on the surface of
Betelgeuse. One spot has a diameter of about 11 mas and accounts for about 8.5%
of the total flux. The second one is unresolved (diameter < 9 mas) with 4.5% of
the total flux. Resolved images of Betelgeuse in the H band are asymmetric at
the level of a few percent. The MOLsphere is not detected in this wavelength
range. The amount of measured limb-darkening is in good agreement with model
predictions. The two spots imaged at the surface of the star are potential
signatures of convective cells.Comment: 10 pages, 10 figures, accepted for publication in A&A, references
adde
GCIRS 7, a pulsating M1 supergiant at the Galactic centre. Physical properties and age
The stellar population in the central parsec of the Galaxy is dominated by an
old (several Gyr) population, but young, massive stars dominate the luminosity
function. We have studied the most luminous of these stars, GCIRS 7, in order
to constrain the age of the recent star formation event in the Galactic Centre
and to characterise it as an interferometric reference for observations of the
Galactic Centre with the instrument GRAVITY, which will equip the Very Large
Telescope Interferometer in the near future. We present the first H-band
interferometric observations of GCIRS 7, obtained using the PIONIER visitor
instrument on the VLTI using the four 8.2-m unit telescopes. In addition, we
present unpublished K-band VLTI/AMBER data, build JHKL light-curves based on
data spanning 4 decades, and measured the star's effective temperature using
SINFONI spectroscopy. GCIRS 7 is marginally resolved at H-band (in 2013:
uniform-disk diameter=1.076+/-0.093mas, R=960+/-92Rsun at 8.33+/-0.35kpc). We
detect a significant circumstellar contribution at K-band. The star and its
environment are variable in brightness and in size. The photospheric H-band
variations are well modelled with two periods: P0~470+/-10 days (amplitude
~0.64mag) and long secondary period LSP~2700-2850 days (~1.1mag). As measured
from CO equivalent width, =3600+/-195K. The size, periods, luminosity
(=-8.44+/-0.22) and effective temperature are consistent with an M1
supergiant with an initial mass of 22.5+/-2.5Msun and an age of 6.5-10Myr
(depending on rotation). This age is in remarkable agreement with most
estimates for the recent star formation event in the central parsec. Caution
should be taken when using this star as an interferometric reference as it is
variable in size, is surrounded by a variable circumstellar environment and
large convection cells may form on its photosphere.Comment: Accepted for publication in A&A. 10 pages, 12 figure
Determination of Fluorescence Polarization and Absorption Anisotropy in Molecular Complexes Having Threefold Rotational Symmetry
The current work concerns investigation of the polarization properties of complex molecular ensembles exhibiting threefold (C3) rotational symmetry, particularly with regard to the interplay between their structure and dynamics of internal energy transfer. We assume that the molecules or chromophores in such complexes possess strongly overlapped spectra both for absorption and fluorescence. Such trimeric structures are widely found in biological preparations, as for example the trimer of C-phycocyanin (C-PC). Higher order aggregates, e.g. hex-amers and three-hexamer rods, are also investigated and compared with the trimer case. The theory addresses both steady-state and 8-pulse excitation and establishes some links between them. Monochromophoric, bichro-mophoric and trichromophoric molecular complexes are individually examined. For steady-state excitation, analytical formulas are reported for the degree of fluorescence polarization and absorption anisotropy. It is shown that the polarization is dependent on the chromophore inclination relative to the symmetry axis, the relative efficiencies of absorption and fluorescence by chromophores of different spectral types, and the rates of energy equilibration. To assess the validity of the theory, it has been applied to C-PC aggregates. Here it was found that different C-PC aggregates provide practically identical polarization response. For S-pulse excitation we give analytical formulas for determination of the fluorescence depolarization, and also the depolarization associated with absorption recovery, both for a monochromophoric trimer and some particular cases of bichromophoric trimer. More complicated systems are analyzed by computer modeling. Thus it transpires that the initial polarization anisotropy r(t = 0) takes the value 0.4 for all considered aggregates; the long-time limit r(t ââ) has about the same value as is associated with steady-state excitation. We also show that with steady-state excitation the degree of fluorescence polarization is practically equal for various C3 aggregates of C-PC, and that the major factor determining the polarization is the chromophore orientation relative to the symmetry axis
Stochastic optimization of a cold atom experiment using a genetic algorithm
We employ an evolutionary algorithm to automatically optimize different
stages of a cold atom experiment without human intervention. This approach
closes the loop between computer based experimental control systems and
automatic real time analysis and can be applied to a wide range of experimental
situations. The genetic algorithm quickly and reliably converges to the most
performing parameter set independent of the starting population. Especially in
many-dimensional or connected parameter spaces the automatic optimization
outperforms a manual search.Comment: 4 pages, 3 figure
- âŠ