153 research outputs found
Geodesics of Random Riemannian Metrics
We analyze the disordered Riemannian geometry resulting from random
perturbations of the Euclidean metric. We focus on geodesics, the paths traced
out by a particle traveling in this quenched random environment. By taking the
point of the view of the particle, we show that the law of its observed
environment is absolutely continuous with respect to the law of the random
metric, and we provide an explicit form for its Radon-Nikodym derivative. We
use this result to prove a "local Markov property" along an unbounded geodesic,
demonstrating that it eventually encounters any type of geometric phenomenon.
We also develop in this paper some general results on conditional Gaussian
measures. Our Main Theorem states that a geodesic chosen with random initial
conditions (chosen independently of the metric) is almost surely not
minimizing. To demonstrate this, we show that a minimizing geodesic is
guaranteed to eventually pass over a certain "bump surface," which locally has
constant positive curvature. By using Jacobi fields, we show that this is
sufficient to destabilize the minimizing property.Comment: 55 pages. Supplementary material at arXiv:1206.494
Collisional stability of fermionic Feshbach molecules
Using a Feshbach resonance, we create ultracold fermionic molecules starting
from a Bose-Fermi atom gas mixture. The resulting mixture of atoms and weakly
bound molecules provides a rich system for studying few-body collisions because
of the variety of atomic collision partners for molecules; either bosonic,
fermionic, or distinguishable atoms. Inelastic loss of the molecules near the
Feshbach resonance is dramatically affected by the quantum statistics of the
colliding particles and the scattering length. In particular, we observe a
molecule lifetime as long as 100 ms near the Feshbach resonance.Comment: 4 pages, 4 figures, 1 tabl
FIRST "Winged" and X-shaped Radio Source Candidates: II. New Redshifts
We report optical spectroscopic observations of X-shaped radio sources with
the Hobby-Eberly Telescope and Multiple-Mirror Telescope, focused on the sample
of candidates from the FIRST survey presented in Paper I (Cheung 2007). A total
of 27 redshifts were successfully obtained, 21 of which are new, including that
of a newly identified candidate source of this type which is presented here.
With these observations, the sample of candidates from Paper I is over 50%
spectroscopically identified. Two new broad emission-lined X-shaped radio
sources are revealed, while no emission lines were detected in about one third
of the observed sources; a detailed study of the line properties is deferred to
a future paper. Finally, to explore their relation to the Fanaroff-Riley
division, the radio luminosities and host galaxy absolute magnitudes of a
spectroscopically identified sample of 50 X-shaped radio galaxies are
calculated to determine their placement in the Owen-Ledlow plane.Comment: emulateapj style, 10 pages, 4 figures, 2 tables; ApJS accepted with
minor revision from submitted version (v1
WebFR3D—a server for finding, aligning and analyzing recurrent RNA 3D motifs
WebFR3D is the on-line version of ‘Find RNA 3D’ (FR3D), a program for annotating atomic-resolution RNA 3D structure files and searching them efficiently to locate and compare RNA 3D structural motifs. WebFR3D provides on-line access to the central features of FR3D, including geometric and symbolic search modes, without need for installing programs or downloading and maintaining 3D structure data locally. In geometric search mode, WebFR3D finds all motifs similar to a user-specified query structure. In symbolic search mode, WebFR3D finds all sets of nucleotides making user-specified interactions. In both modes, users can specify sequence, sequence–continuity, base pairing, base-stacking and other constraints on nucleotides and their interactions. WebFR3D can be used to locate hairpin, internal or junction loops, list all base pairs or other interactions, or find instances of recurrent RNA 3D motifs (such as sarcin–ricin and kink-turn internal loops or T- and GNRA hairpin loops) in any PDB file or across a whole set of 3D structure files. The output page provides facilities for comparing the instances returned by the search by superposition of the 3D structures and the alignment of their sequences annotated with pairwise interactions. WebFR3D is available at http://rna.bgsu.edu/webfr3d
Free Expansion of a Weakly-interacting Dipolar Fermi Gas
We theoretically investigate a polarized dipolar Fermi gas in free expansion.
The inter-particle dipolar interaction deforms phase-space distribution in trap
and also in the expansion. We exactly predict the minimal quadrupole
deformation in the expansion for the high-temperature Maxwell-Boltzmann and
zero-temperature Thomas-Fermi gases in the Hartree-Fock and Landau-Vlasov
approaches. In conclusion, we provide a proper approach to develop the
time-of-flight method for the weakly-interacting dipolar Fermi gas and also
reveal a scaling law associated with the Liouville's theorem in the long-time
behaviors of the both gases
Ultracold dense gas of deeply bound heteronuclear molecules
Recently, the quest for an ultracold and dense ensemble of polar molecules
has attracted strong interest. Polar molecules have bright prospects for novel
quantum gases with long-range and anisotropic interactions, for quantum
information science, and for precision measurements. However, high-density
clouds of ultracold polar molecules have so far not been produced. Here, we
report a key step towards this goal. Starting from an ultracold dense gas of
heteronuclear 40K-87Rb Feshbach molecules with typical binding energies of a
few hundred kHz and a negligible dipole moment, we coherently transfer these
molecules into a vibrational level of the ground-state molecular potential
bound by >10 GHz. We thereby increase the binding energy and the expected
dipole moment of the 40K-87Rb molecules by more than four orders of magnitude
in a single transfer step. Starting with a single initial state prepared with
Feshbach association, we achieve a transfer efficiency of 84%. While dipolar
effects are not yet observable, the presented technique can be extended to
access much more deeply bound vibrational levels and ultimately those
exhibiting a significant dipole moment. The preparation of an ultracold quantum
gas of polar molecules might therefore come within experimental reach.Comment: 5 pages, 5 figure
Strong molecular hydrogen emission and kinematics of the multiphase gas in radio galaxies with fast jet-driven outflows
Observations of ionized and neutral gas outflows in radio-galaxies (RGs)
suggest that AGN radio jet feedback has a galaxy-scale impact on the host ISM,
but it is still unclear how the molecular gas is affected. We present deep
Spitzer IRS spectroscopy of 8 RGs that show fast HI outflows. All of these
HI-outflow RGs have bright H2 mid-IR lines that cannot be accounted for by UV
or X-ray heating. This suggests that the radio jet, which drives the HI
outflow, is also responsible for the shock-excitation of the warm H2 gas. In
addition, the warm H2 gas does not share the kinematics of the ionized/neutral
gas. The mid-IR ionized gas lines are systematically broader than the H2 lines,
which are resolved by the IRS (with FWHM up to 900km/s) in 60% of the detected
H2 lines. In 5 sources, the NeII line, and to a lesser extent the NeIII and NeV
lines, exhibit blue-shifted wings (up to -900km/s with respect to the systemic
velocity) that match the kinematics of the outflowing HI or ionized gas. The H2
lines do not show broad wings, except tentative detections in 3 sources. This
shows that, contrary to the HI gas, the H2 gas is inefficiently coupled to the
AGN jet-driven outflow of ionized gas. While the dissipation of a small
fraction (<10%) of the jet kinetic power can explain the dynamical heating of
the molecular gas, our data show that the bulk of the warm molecular gas is not
expelled from these galaxies.Comment: 26 pages, 15 figures, Accepted for ublication in Ap
CSSs in a sample of B2 radio sources of intermediate strength
We present radio observations of 19 candidate compact steep-spectrum (CSS)
objects selected from a well-defined, complete sample of 52 B2 radio sources of
intermediate strength. These observations were made with the VLA A-array at
4.835 GHz. The radio structures of the entire sample are summarised and the
brightness asymmetries within the compact sources are compared with those of
the more extended ones, as well as with those in the 3CRR sample and the CSSs
from the B3-VLA sample. About 25 per cent of the CSS sources exhibit large
brightness asymmetries, with a flux density ratio for the opposing lobes of
5, possibly due to interaction of the jets with infalling material. The
corresponding percentage for the larger-sized objects is only about 5 per cent.
We also investigate possible dependence of the flux density asymmetry of the
lobes on redshift, since this might be affected by more interactions and
mergers in the past. No such dependence is found. A few individual objects of
interest are discussed in the paper.Comment: 10 pages, 7 figures, 2 tables; accepted for publication in Astronomy
and Astrophysic
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