21,934 research outputs found
Quantum behaviour of open pumped and damped Bose-Hubbard trimers
We propose and analyse analogs of optical cavities for atoms using three-well
inline Bose-Hubbard models with pumping and losses. With one well pumped and
one damped, we find that both the mean-field dynamics and the quantum
statistics show a qualitative dependence on the choice of damped well. The
systems we analyse remain far from equilibrium, although most do enter a
steady-state regime. We find quadrature squeezing, bipartite and tripartite
inseparability and entanglement, and states exhibiting the EPR paradox,
depending on the parameter regimes. We also discover situations where the
mean-field solutions of our models are noticeably different from the quantum
solutions for the mean fields. Due to recent experimental advances, it should
be possible to demonstrate the effects we predict and investigate in this
article.Comment: 20 pages, 8 figures, theoretica
Effects of ultrasound on Transforming Growth Factor-beta genes in bone cells
Therapeutic ultrasound (US) is a widely used form of biophysical stimulation that is increasingly applied to promote fracture healing. Transforming growth factor-beta (TGF-beta), which is encoded by three related but different genes, is known to play a major part in bone growth and repair. However, the effects of US on the expression of the TGF-beta genes and the physical acoustic mechanisms involved in initiating changes in gene expression in vitro, are not yet known. The present study demonstrates that US had a differential effect on these TGF-beta isoforms in a human osteoblast cell line, with the highest dose eliciting the most pronounced up-regulation of both TGF-beta1 and TGF-beta3 at 1 hour after treatment and thereafter declining. In contrast, US had no effect on TGF-beta2 expression. Fluid streaming rather than thermal effects or cavitation was found to be the most likely explanation for the gene responses observed in vitro
Really Cool Stars and the Star Formation History at the Galactic Center
We present R=550 to 1200 near infrared H and K spectra for a magnitude
limited sample of 79 asymptotic giant branch and cool supergiant stars in the
central ~ 5 pc (diameter) of the Galaxy. We use a set of similar spectra
obtained for solar neighborhood stars with known Teff and Mbol that is in the
same range as the Galactic center (GC) sample to derive Teff and Mbol for the
GC sample. We then construct the Hertzsprung--Russell (HRD) diagram for the GC
sample. Using an automated maximum likelihood routine, we derive a coarse star
formation history of the GC. We find (1) roughly 75% of the stars formed in the
central few pc are older than 5 Gyr; (2) the star formation rate (SFR) is
variable over time, with a roughly 4 times higher star formation rate in the
last 100 Myr compared to the average SFR; (3) our model can only match
dynamical limits on the total mass of stars formed by limiting the IMF to
masses above 0.7 M. This could be a signature of mass segregation or of
the bias toward massive star formation from the unique star formation
conditions in the GC; (4) blue supergiants account for 12 % of the total sample
observed, and the ratio of red to blue supergiants is roughly 1.5; (5) models
with isochrones with [Fe/H] = 0.0 over all ages fit the stars in our HRD better
than models with lower [Fe/H] in the oldest age bins, consistent with the
finding of Ramirez et al. (2000) that stars with ages between 10 Myr and 1 Gyr
have solar [Fe/H].Comment: ApJ, accepted. Latex, 65 pages including 19 figure
Absolute dimensions of eclipsing binaries. XVII. A metal-weak F-type system, perhaps with preference for Y = 0.23-0.24
V1130 Tau is a bright (m_V = 6.56), nearby (71 +/- 2 pc) detached system with
a circular orbit (P = 0.80d). The components are deformed with filling factors
above 0.9. Their masses and radii have been established to 0.6-0.7%. We derive
a [Fe/H] abundance of -0.25 +/- 0.10. The measured rotational velocities, 92.4
+/- 1.1 (primary) and 104.7 +/- 2.7 (secondary) km/s, are in fair agreement
with synchronization. The larger 1.39 Msun secondary component has evolved to
the middle of the main-sequence band and is slightly cooler than the 1.31 Msun
primary. Yonsai-Yale, BaSTI, and Granada evolutionary models for the observed
metal abundance and a 'normal' He content of Y = 0.25-0.26, marginally
reproduce the components at ages between 1.8 and 2.1 Gyr. All such models are,
however, systematically about 200 K hotter than observed and predict ages for
the more massive component, which are systematically higher than for the less
massive component. These trends can not be removed by adjusting the amount of
core overshoot or envelope convection level, or by including rotation in the
model calculations. They may be due to proximity effects in V1130 Tau, but on
the other hand, we find excellent agreement for 2.5-2.8 Gyr Granada models with
a slightly lower Y of 0.23-0.24. V1130 Tau is a valuable addition to the very
few well-studied 1-2 Msun binaries with component(s) in the upper half of the
main-sequence band, or beyond. The stars are not evolved enough to provide new
information on the dependence of core overshoot on mass (and abundance), but
might - together with a larger sample of well-detached systems - be useful for
further tuning of the helium enrichment law.Comment: Accepted for publication in Astronomy & Astrophysic
Production of relativistic positronium in collisions of photons and electrons with nuclei and atoms
We consider the production of ultrarelativistic positronium (Ps) in and processes where is an atom or a nucleus
with charge . For the photoproduction of para- and ortho-Ps and the
electroproduction of para-Ps we obtain the most complete description compared
with previous works. It includes high order corrections and
polarization effects. The accuracy of the obtained cross sections is determined
by omitted terms of the order of the inverse Ps Lorentz factor squared. The
studied high order multi-photon electroproduction of ortho-Ps dominates for the
collision of electrons with heavy atoms over the bremsstrahlung production from
the electron via a virtual photon proposed by Holvik and Olsen. Our results
complete and correct the studies of those authors.Comment: 19 pages, 9 figures, RevTex; v2: minor corrections for the accuracy
of the results, a discussion of the literature added in a footnote, one
additional reference; v3: diagram of Fig.2 correcte
Coulomb corrections to bremsstrahlung in electric field of heavy atom at high energies
The differential and partially integrated cross sections are considered for
bremsstrahlung from high-energy electrons in atomic field with the exact
account of this field. The consideration exploits the quasiclassical electron
Green's function and wave functions in an external electric field. It is shown
that the Coulomb corrections to the differential cross section are very
susceptible to screening. Nevertheless, the Coulomb corrections to the cross
section summed up over the final-electron states are independent of screening
in the leading approximation over a small parameter ( is
a screening radius, is the electron mass, ). Bremsstrahlung from
an electron beam of the finite size on heavy nucleus is considered as well.
Again, the Coulomb corrections to the differential probability are very
susceptible to the beam shape, while those to the probability integrated over
momentum transfer are independent of it, apart from the trivial factor, which
is the electron-beam density at zero impact parameter. For the Coulomb
corrections to the bremsstrahlung spectrum, the next-to-leading terms with
respect to the parameters ( is the electron energy) and
are obtained.Comment: 13 pages, 4 figure
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