1,625 research outputs found
Enhancement of the formation of ultracold Rb molecules due to resonant coupling
We have studied the effect of resonant electronic state coupling on the
formation of ultracold ground-state Rb. Ultracold Rb molecules
are formed by photoassociation (PA) to a coupled pair of states,
and , in the region below the
limit. Subsequent radiative decay produces high vibrational levels of the
ground state, . The population distribution of these state
vibrational levels is monitored by resonance-enhanced two-photon ionization
through the state. We find that the populations of vibrational
levels =112116 are far larger than can be accounted for by the
Franck-Condon factors for transitions with
the state treated as a single channel. Further, the
ground-state molecule population exhibits oscillatory behavior as the PA laser
is tuned through a succession of state vibrational levels. Both of
these effects are explained by a new calculation of transition amplitudes that
includes the resonant character of the spin-orbit coupling of the two
states. The resulting enhancement of more deeply bound ground-state molecule
formation will be useful for future experiments on ultracold molecules.Comment: 6 pages, 5 figures; corrected author lis
High Resolution Molecular Gas Maps of M33
New observations of CO (J=1->0) line emission from M33, using the 25 element
BEARS focal plane array at the Nobeyama Radio Observatory 45-m telescope, in
conjunction with existing maps from the BIMA interferometer and the FCRAO 14-m
telescope, give the highest resolution (13'') and most sensitive (RMS ~ 60 mK)
maps to date of the distribution of molecular gas in the central 5.5 kpc of the
galaxy. A new catalog of giant molecular clouds (GMCs) has a completeness limit
of 1.3 X 10^5 M_sun. The fraction of molecular gas found in GMCs is a strong
function of radius in the galaxy, declining from 60% in the center to 20% at
galactocentric radius R_gal ~ 4 kpc. Beyond that radius, GMCs are nearly
absent, although molecular gas exists. Most (90%) of the emission from low mass
clouds is found within 100 pc projected separation of a GMC. In an annulus 2.1<
R_gal <4.1 kpc, GMC masses follow a power law distribution with index -2.1.
Inside that radius, the mass distribution is truncated, and clouds more massive
than 8 X 10^5 M_sun are absent. The cloud mass distribution shows no
significant difference in the grand design spiral arms versus the interarm
region. The CO surface brightness ratio for the arm to interarm regions is 1.5,
typical of other flocculent galaxies.Comment: 14 pages, 14 figures, accepted in ApJ. Some tables poorly typeset in
emulateapj; see source files for raw dat
Phase Behavior of Polyelectrolyte Block Copolymers in Mixed Solvents
We have studied the phase behavior of the poly(n-butyl
acrylate)-b-poly(acrylic acid) block copolymer in a mixture of two miscible
solvents, water and tetrahydrofuran (THF). The techniques used to examine the
different polymers, structures and phases formed in mixed solvents were static
and dynamic light scattering, small-angle neutron scattering, nuclear magnetic
resonance and fluorescence microscopy. By lowering the water/THF mixing ratio
X, the sequence unimers, micron-sized droplets, polymeric micelles was
observed. The transition between unimers and the micron-sized droplets occurred
at X = 0.75, whereas the microstructuration into core-shell polymeric micelles
was effective below X = 0.4. At intermediate mixing ratios, a coexistence
between the micron-sized droplets and the polymeric micelles was observed.
Combining the different aforementioned techniques, it was concluded that the
droplet dispersion resulted from a solvent partitioning that was induced by the
hydrophobic blocks. Comparison of poly(n-butyl acrylate) homopolymers and
poly(n-butyl acrylate)-b-poly(acrylic acid) block copolymers suggested that the
droplets were rich in THF and concentrated in copolymers and that they were
stabilized by the hydrophilic poly(acrylic acid) moieties.Comment: 11 pages, 12 figures, to appear in Macromolecule
Fault-tolerant Cooperative Tasking for Multi-agent Systems
A natural way for cooperative tasking in multi-agent systems is through a
top-down design by decomposing a global task into sub-tasks for each individual
agent such that the accomplishments of these sub-tasks will guarantee the
achievement of the global task. In our previous works [1], [2] we presented
necessary and sufficient conditions on the decomposability of a global task
automaton between cooperative agents. As a follow-up work, this paper deals
with the robustness issues of the proposed top-down design approach with
respect to event failures in the multi-agent systems. The main concern under
event failure is whether a previously decomposable task can still be achieved
collectively by the agents, and if not, we would like to investigate that under
what conditions the global task could be robustly accomplished. This is
actually the fault-tolerance issue of the top-down design, and the results
provide designers with hints on which events are fragile with respect to
failures, and whether redundancies are needed. The main objective of this paper
is to identify necessary and sufficient conditions on failed events under which
a decomposable global task can still be achieved successfully. For such a
purpose, a notion called passivity is introduced to characterize the type of
event failures. The passivity is found to reflect the redundancy of
communication links over shared events, based on which necessary and sufficient
conditions for the reliability of cooperative tasking under event failures are
derived, followed by illustrative examples and remarks for the derived
conditions.Comment: Preprint, Submitted for publicatio
Stellar population and dust extinction in an ultraluminous infrared galaxy at z=1.135
We present the detailed optical to far-infrared observations of SST
J1604+4304, an ULIRG at z = 1.135. Analyzing the stellar absorption lines,
namely, the CaII H & K and Balmer H lines in the optical spectrum, we derive
the upper limits of an age for the stellar population. Given this constraint,
the minimum {chi}^2 method is used to fit the stellar population models to the
observed SED from 0.44 to 5.8um. We find the following properties. The stellar
population has an age 40 - 200 Myr with a metallicity 2.5 Z_{sun}. The
starlight is reddened by E(B-V) = 0.8. The reddening is caused by the
foreground dust screen, indicating that dust is depleted in the starburst site
and the starburst site is surrounded by a dust shell. The infrared (8-1000um)
luminosity is L_{ir} = 1.78 +/- 0.63 * 10^{12} L_{sun}. This is two times
greater than that expected from the observed starlight, suggesting either that
1/2 of the starburst site is completely obscured at UV-optical wavelengths, or
that 1/2 of L_{ir} comes from AGN emission. The inferred dust mass is 2.0 +/-
1.0 * 10^8 M_{sun}. This is sufficient to form a shell surrounding the galaxy
with an optical depth E(B-V) = 0.8. From our best stellar population model - an
instantaneous starburst with an age 40 Myr, we infer the rate of 19
supernovae(SNe) per year. Simply analytical models imply that 2.5 Z_{sun} in
stars was reached when the gas mass reduced to 30% of the galaxy mass. The gas
metallcity is 4.8 Z_{sun} at this point. The gas-to-dust mass ratio is then 120
+/- 73. The inferred dust production rate is 0.24 +/- 0.12 M_{sun} per SN. If
1/2 of L_{ir} comes from AGN emission, the rate is 0.48 +/- 0.24 M_{sun} per
SN. We discuss the evolutionary link of SST J1604+4304 to other galaxy
populations in terms of the stellar masses and the galactic winds.Comment: 11 pages, 9 figures, accepted for publication in MNRA
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