TWO NEW SHORT-PERIOD CEPHEIDS

The General Catalogue of Variable Stars gives periods of slightly less than three-quarters of a day for the stars NO Cas and CN Tau. However, new photometry demonstrates that their periods are actually 2.6 and 1.8 days, respectively, and they are thus classical Cepheids. Fourier decompositions of their light curves are performed, and they are found to be members of a class of Cepheids with periods less than three days which may be related to the s-Cepheids. These two stars represent the shortest and longest known members of this class and thus are very useful in defining its properties in the Fourier diagrams

Ab initio Quantum and ab initio Molecular Dynamics of the Dissociative Adsorption of Hydrogen on Pd(100)

The dissociative adsorption of hydrogen on Pd(100) has been studied by ab initio quantum dynamics and ab initio molecular dynamics calculations. Treating all hydrogen degrees of freedom as dynamical coordinates implies a high dimensionality and requires statistical averages over thousands of trajectories. An efficient and accurate treatment of such extensive statistics is achieved in two steps: In a first step we evaluate the ab initio potential energy surface (PES) and determine an analytical representation. Then, in an independent second step dynamical calculations are performed on the analytical representation of the PES. Thus the dissociation dynamics is investigated without any crucial assumption except for the Born-Oppenheimer approximation which is anyhow employed when density-functional theory calculations are performed. The ab initio molecular dynamics is compared to detailed quantum dynamical calculations on exactly the same ab initio PES. The occurence of quantum oscillations in the sticking probability as a function of kinetic energy is addressed. They turn out to be very sensitive to the symmetry of the initial conditions. At low kinetic energies sticking is dominated by the steering effect which is illustrated using classical trajectories. The steering effects depends on the kinetic energy, but not on the mass of the molecules. Zero-point effects lead to strong differences between quantum and classical calculations of the sticking probability. The dependence of the sticking probability on the angle of incidence is analysed; it is found to be in good agreement with experimental data. The results show that the determination of the potential energy surface combined with high-dimensional dynamical calculations, in which all relevant degrees of freedon are taken into account, leads to a detailed understanding of the dissociation dynamics of hydrogen at a transition metal surface.Comment: 15 pages, 9 figures, subm. to Phys. Rev.

Self-Trapping, Quantum Tunneling and Decay Rates for a Bose Gas with Attractive Nonlocal Interaction

We study the Bose-Einstein condensation for a cloud of $^7$Li atoms with attractive nonlocal (finite-range) interaction in a harmonic trap. In addition to the low-density metastable branch, that is present also in the case of local interaction, a new stable branch appears at higher densities. For a large number of atoms, the size of the cloud in the stable high-density branch is independent of the trap size and the atoms are in a macroscopic quantum self-trapped configuration. We analyze the macroscopic quantum tunneling between the low-density metastable branch and the high-density one by using the istanton technique. Moreover we consider the decay rate of the Bose condensate due to inelastic two- and three-body collisions.Comment: 5 pages, 4 figures, submitted to Phys. Rev.

Macroscopic Quantum Tunneling of a Bose-Einstein Condensate with Attractive Interaction

A Bose-Einstein condensate with attractive interaction can be metastable if it is spatially confined and if the number of condensate bosons $N_0$ is below a certain critical value $N_{\rm c}$. By applying a variational method and the instanton techinique to the Gross-Pitaevskii energy functional, we find analytically the frequency of the collective excitation and the rate of macroscopic quantum tunneling (MQT). We show that near the critical point the tunneling exponent vanishes according to $(1-N_0/N_c)^\frac{5}{4}$ and that MQT can be a dominant decay mechanism of the condensate for $N_0$ very close to $N_{\rm c}$.Comment: RevTex 4 pages with 1 postscript figure. Accepted for publication in Physical Review Letter

A Variational Sum-Rule Approach to Collective Excitations of a Trapped Bose-Einstein Condensate

It is found that combining an excitation-energy sum rule with Fetter's trial wave function gives almost exact low-lying collective-mode frequencies of a trapped Bose-Einstein condensate at zero temperature.Comment: 11 pages, 2 figures, Revte

Stability analysis of the D-dimensional nonlinear Schroedinger equation with trap and two- and three-body interactions

Considering the static solutions of the D-dimensional nonlinear Schroedinger equation with trap and attractive two-body interactions, the existence of stable solutions is limited to a maximum critical number of particles, when D is greater or equal 2. In case D=2, we compare the variational approach with the exact numerical calculations. We show that, the addition of a positive three-body interaction allows stable solutions beyond the critical number. In this case, we also introduce a dynamical analysis of the conditions for the collapse.Comment: 6 pages, 7 figure

Semiclassical Solution of the Quantum Hydrodynamic Equation for Trapped Bose-condensed Gas in the l=0 Case

In this paper the quantum hydrodynamic equation describing the collective, low energy excitations of a dilute atomic Bose gas in a given trapping potential is investigated with the JWKB semiclassical method. In the case of spherically symmetric harmonic confining potential a good agreement is shown between the semiclassical and the exact energy eigenvalues as well as wave functions. It is also demonstrated that for larger quantum numbers the calculation of the semiclassical wave function is numerically more stable than the exact polynomial with large alternating coefficients.Comment: 12 pages, 7 figure

Intra-arterial Onyx Embolization of Vertebral Body Lesions

While Onyx embolization of cerebrospinal arteriovenous shunts is well-established, clinical researchers continue to broaden applications to other vascular lesions of the neuraxis. This report illustrates the application of Onyx (eV3, Plymouth, MN) embolization to vertebral body lesions, specifically, a vertebral hemangioma and renal cell carcinoma vertebral body metastatic lesion

Stability of a vortex in a small trapped Bose-Einstein condensate

A second-order expansion of the Gross-Pitaevskii equation in the interaction parameter determines the thermodynamic critical angular velocity Omega_c for the creation of a vortex in a small axisymmetric condensate. Similarly, a second-order expansion of the Bogoliubov equations determines the (negative) frequency omega_a of the anomalous mode. Although Omega_c = -omega_a through first order, the second-order contributions ensure that the absolute value |omega_a| is always smaller than the critical angular velocity Omega_c. With increasing external rotation Omega, the dynamical instability of the condensate with a vortex disappears at Omega*=|omega_a|, whereas the vortex state becomes energetically stable at the larger value Omega_c. Both second-order contributions depend explicitly on the axial anisotropy of the trap. The appearance of a local minimum of the free energy for a vortex at the center determines the metastable angular velocity Omega_m. A variational calculation yields Omega_m=|\omega_a| to first order (hence Omega_m also coincides with the critical angular velocity Omega_c to this order). Qualitatively, the scenario for the onset of stability in the weak-coupling limit is the same as that found in the strong-coupling (Thomas-Fermi) limit.Comment: 8 pages, RevTe

Mean-field analysis of collapsing and exploding Bose-Einstein condensates

The dynamics of collapsing and exploding trapped Bose-Einstein condensat es caused by a sudden switch of interactions from repulsive to attractive a re studied by numerically integrating the Gross-Pitaevskii equation with atomic loss for an axially symmetric trap. We investigate the decay rate of condensates and the phenomena of bursts and jets of atoms, and compare our results with those of the experiments performed by E. A. Donley {\it et al.} [Nature {\bf 412}, 295 (2001)]. Our study suggests that the condensate decay and the burst production is due to local intermittent implosions in the condensate, and that atomic clouds of bursts and jets are coherent. We also predict nonlinear pattern formation caused by the density instability of attractive condensates.Comment: 7 pages, 8 figures, axi-symmetric results are adde