39,053 research outputs found
Thermal instabilities in protogalactic clouds
The means by which a protogalaxy can fragment to form the first generation of stars and globular clusters remains an important problem in astrophysics. Gravitational instabilities grow on timescales too long to drive fragmentation before the background density grows by many orders of magnitude (see Murray and Lin 1989a, and references therein). Thermal instability provides a much more likely mechanism. After its initial collapse, a protogalactic cloud is expected to be shock heated to its virial temperature approx. 10(exp 6) K. Cooling by H and He+ below 10(exp 6) K has a negative slope, so that the cloud is subject to strong thermal instabilities. Density enhancements may then grow rapidly, fragmenting the protogalaxy as it cools to lower temperatures. The role of dynamical effects upon the growth of perturbations is considered here. The method used is similar to that used in Murray and Lin (1989a; see also the Erratum to appear September 15), which examined the growth of thermal instabilities with a one-dimensional Lagrangian hydrodynamics code, written for spherical symmetry. Perturbed regions therefore take the form of shells. The dynamical variables are integrated explicitly, while the temperature, ionization fraction, and molecular fraction are integrated implicitly, and account is taken for non-equilibrium values of these quantities
Are Stars with Planets Polluted?
We compare the metallicities of stars with radial velocity planets to the
metallicity of a sample of field dwarfs. We confirm recent work indicating that
the stars-with-planet sample as a whole is iron rich. However, the lowest mass
stars tend to be iron poor, with several having [Fe/H]<-0.2, demonstrating that
high metallicity is not required for the formation of short period Jupiter-mass
planets. We show that the average [Fe/H] increases with increasing stellar mass
(for masses below 1.25 solar masses) in both samples, but that the increase is
much more rapid in the stars-with-planet sample. The variation of metallicity
with stellar age also differs between the two samples. We examine possible
selection effects related to variations in the sensitivity of radial velocity
surveys with stellar mass and metallicity, and identify a color cutoff
(B-V>0.48) that contributes to but does not explain the mass-metallicity trend
in the stars-with-planets sample. We use Monte Carlo models to show that adding
an average of 6.5 Earth masses of iron to each star can explain both the
mass-metallicity and the age-metallicity relations of the stars-with-planets
sample. However, for at least one star, HD 38529, there is good evidence that
the bulk metallicity is high. We conclude that the observed metallicities and
metallicity trends are the result of the interaction of three effects;
accretion of about 6 Earth masses of iron rich material, selection effects, and
in some cases, high intrinsic metallicity.Comment: 19 pages 11 figure
Resistive flow in a weakly interacting Bose-Einstein condensate
We report the direct observation of resistive flow through a weak link in a
weakly interacting atomic Bose-Einstein condensate. Two weak links separate our
ring-shaped superfluid atomtronic circuit into two distinct regions, a source
and a drain. Motion of these weak links allows for creation of controlled flow
between the source and the drain. At a critical value of the weak link
velocity, we observe a transition from superfluid flow to superfluid plus
resistive flow. Working in the hydrodynamic limit, we observe a conductivity
that is 4 orders of magnitude larger than previously reported conductivities
for a Bose-Einstein condensate with a tunnel junction. Good agreement with
zero-temperature Gross-Pitaevskii simulations and a phenomenological model
based on phase slips indicate that the creation of excitations plays an
important role in the resulting conductivity. Our measurements of resistive
flow elucidate the microscopic origin of the dissipation and pave the way for
more complex atomtronic devices.Comment: Version published in PR
Off-Diagonal Long-Range Order, Restricted Gauge Transformations, and Aharonov-Bohm Effect in Conductors
The Hamiltonian describing a conductor surrounding an external magnetic field
contains a nonvanishing vector potential in the volume accessible to the
electrons and nuclei of which the conductor is made. That vector potential
cannot be removed by a gauge transformation. Nevertheless, a macroscopic normal
conductor can experience no Aharonov-Bohm effect. That is proved by assuming
only that a normal conductor lacks off-diagonal long-range order (ODLRO). Then
by restricting the Hilbert space to density matrices which lack ODLRO, it is
possible to introduce a restricted gauge transformation that removes the
interaction of the conductor with the vector potential.Comment: Editing errors are corrected. One was slightly misleadin
Probing the evolution of Stark wave packets by a weak half cycle pulse
We probe the dynamic evolution of a Stark wave packet in cesium using weak
half-cycle pulses (HCP's). The state-selective field ionization(SSFI) spectra
taken as a function of HCP delay reveal wave packet dynamics such as Kepler
beats, Stark revivals and fractional revivals. A quantum-mechanical simulation
explains the results as multi-mode interference induced by the HCP.Comment: 4 pages, incl. 3 figures, submitted to PR
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