2,124 research outputs found
Probing the interiors of the ice giants: Shock compression of water to 700 GPa and 3.8 g/ccm
Recently there has been tremendous increase in the number of identified
extra-solar planetary systems. Our understanding of their formation is tied to
exoplanet internal structure models, which rely upon equations of state of
light elements and compounds like water. Here we present shock compression data
for water with unprecedented accuracy that shows water equations of state
commonly used in planetary modeling significantly overestimate the
compressibility at conditions relevant to planetary interiors. Furthermore, we
show its behavior at these conditions, including reflectivity and isentropic
response, is well described by a recent first-principles based equation of
state. These findings advocate this water model be used as the standard for
modeling Neptune, Uranus, and "hot Neptune" exoplanets, and should improve our
understanding of these types of planets.Comment: Accepted to Phys. Rev. Lett.; supplementary material attached
including 2 figures and 2 tables; to view attachments, please download and
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Optical Lattice Induced Light Shifts in an Yb Atomic Clock
We present an experimental study of the lattice induced light shifts on the
1S_0-3P_0 optical clock transition (v_clock~518 THz) in neutral ytterbium. The
``magic'' frequency, v_magic, for the 174Yb isotope was determined to be 394
799 475(35)MHz, which leads to a first order light shift uncertainty of 0.38 Hz
on the 518 THz clock transition. Also investigated were the hyperpolarizability
shifts due to the nearby 6s6p 3P_0 - 6s8p 3P_0, 6s8p 3P_2, and 6s5f 3F_2
two-photon resonances at 759.708 nm, 754.23 nm, and 764.95 nm respectively. By
tuning the lattice frequency over the two-photon resonances and measuring the
corresponding clock transition shifts, the hyperpolarizability shift was
estimated to be 170(33) mHz for a linear polarized, 50 uK deep, lattice at the
magic wavelength. In addition, we have confirmed that a circularly polarized
lattice eliminates the J=0 - J=0 two-photon resonance. These results indicate
that the differential polarizability and hyperpolarizability frequency shift
uncertainties in a Yb lattice clock could be held to well below 10^-17.Comment: Accepted to PR
Frequency evaluation of the doubly forbidden transition in bosonic Yb
We report an uncertainty evaluation of an optical lattice clock based on the
transition in the bosonic isotope Yb by use
of magnetically induced spectroscopy. The absolute frequency of the
transition has been determined through comparisons
with optical and microwave standards at NIST. The weighted mean of the
evaluations is (Yb)=518 294 025 309 217.8(0.9) Hz. The uncertainty
due to systematic effects has been reduced to less than 0.8 Hz, which
represents in fractional frequency.Comment: 4 pages, 3 figure -Submitted to PRA Rapid Communication
Mid-Infrared Emission Features in the ISM: Feature-to-Feature Flux Ratios
Using a limited, but representative sample of sources in the ISM of our
Galaxy with published spectra from the Infrared Space Observatory, we analyze
flux ratios between the major mid-IR emission features (EFs) centered around
6.2, 7.7, 8.6 and 11.3 microns, respectively. In a flux ratio-to-flux ratio
plot of EF(6.2)/EF(7.7) as a function of EF(11.3)/EF(7.7), the sample sources
form roughly a -shaped locus which appear to trace, on an overall
basis, the hardness of a local heating radiation field. But some driving
parameters other than the radiation field may also be required for a full
interpretation of this trend. On the other hand, the flux ratio of
EF(8.6)/EF(7.7) shows little variation over the sample sources, except for two
HII regions which have much higher values for this ratio due to an ``EF(8.6\um)
anomaly,'' a phenomenon clearly associated with environments of an intense
far-UV radiation field. If further confirmed on a larger database, these trends
should provide crucial information on how the EF carriers collectively respond
to a changing environment.Comment: 16 pages, 1 figure, 1 table; accepted for publication in ApJ Letter
Anomalous radio emission from dust in the Helix
A byproduct of experiments designed to map the CMB is the recent detection of
a new component of foreground Galactic emission. The anomalous foreground at ~
10--30 GHz, unexplained by traditional emission mechanisms, correlates with
100um dust emission. We report that in the Helix the emission at 31 GHz and
100um are well correlated, and exhibit similar features on sky images, which
are absent in H\beta. Upper limits on the 250 GHz continuum emission in the
Helix rule out cold grains as candidates for the 31 GHz emission, and provide
spectroscopic evidence for an excess at 31 GHz over bremsstrahlung. We estimate
that the 100um-correlated radio emission, presumably due to dust, accounts for
at least 20% of the 31 GHz emission in the Helix. This result strengthens
previous tentative interpretations of diffuse ISM spectra involving a new dust
emission mechanism at radio frequencies. Very small grains have not been
detected in the Helix, which hampers interpreting the new component in terms of
spinning dust. The observed iron depletion in the Helix favors considering the
identity of this new component to be magnetic dipole emission from hot
ferromagnetic grains. The reduced level of free-free continuum we report also
implies an electronic temperature of Te=4600\pm1200K for the free-free emitting
material, which is significantly lower than the temperature of 9500\pm500K
inferred from collisionally-excited lines (abridged).Comment: Accepted for publication in Ap
Determination of limits on disc masses around six pulsars at 15 and 90 microns
We have searched for evidence of emission at 15 microns with ISOCAM and at 90
microns with ISOPHOT from dust orbiting six nearby pulsars, both in binaries
and in isolation, located at distances between about 100 to 1000 pc. No
emission was detected at any of the pulsar positions, and for the nearest
pulsar J0108-1431 the 3 sigma upper limits on the flux density is about 66 mJy
at 15 microns and 22.5 mJy at 90 microns. Upper limits on the masses of
circumpulsar dust are inferred at a given temperature using a simple modelling
of the radiated flux; they are compared to upper limits of orbiting mass
obtained with the dust heating model of Foster & Fisher (1996). These results
suggest that it is unlikely that any of them have sufficiently massive,
circumpulsar discs out of which planets may form in the future.Comment: 7 pages, 2 figures, revised version, accepted for publication in A&
Unidentified Infrared Emission Bands in the Diffuse Interstellar Medium
Using the Mid-Infrared Spectrometer on board the Infrared Telescope in Space
and the low-resolution grating spectrometer (PHT-S) on board the Infrared Space
Observatory, we obtained 820 mid-infrared (5 to 12 m) spectra of the
diffuse interstellar medium (DIM) in the Galactic center, W51, and Carina
Nebula regions. These spectra indicate that the emission is dominated by the
unidentified infrared (UIR) emission bands at 6.2, 7.7, 8.6, and 11.2 m.
The relative band intensities (6.2/7.7 m, 8.6/7.7 m, and 11.2/7.7
m) were derived from these spectra, and no systematic variation in these
ratios was found in our observed regions, in spite of the fact that the
incident radiation intensity differs by a factor of 1500. Comparing our results
with the polycyclic aromatic hydrocarbons (PAHs) model for the UIR band
carriers, PAHs in the DIM have no systematic variation in their size
distribution, their degree of dehydrogenation is independent of the strength of
UV radiation field, and they are mostly ionized. The latter finding is
incompatible with past theoretical studies, in which a large fraction of
neutral PAHs is predicted in this kind of environment. A plausible resolution
of this discrepancy is that the recombination coefficients for electron and
large PAH positive ion are by at least an order of magnitude less than those
adopted in past theoretical studies. Because of the very low population of
neutral state molecules, photoelectric emission from interstellar PAHs is
probably not the dominant source of heating of the diffuse interstellar gas.
The present results imply constant physical and chemical properties of the
carriers of the UIR emission bands in the DIM.Comment: 13 pages, 6 figures. Accepted for publication in Ap
Discrete and fuzzy dynamical genetic programming in the XCSF learning classifier system
A number of representation schemes have been presented for use within
learning classifier systems, ranging from binary encodings to neural networks.
This paper presents results from an investigation into using discrete and fuzzy
dynamical system representations within the XCSF learning classifier system. In
particular, asynchronous random Boolean networks are used to represent the
traditional condition-action production system rules in the discrete case and
asynchronous fuzzy logic networks in the continuous-valued case. It is shown
possible to use self-adaptive, open-ended evolution to design an ensemble of
such dynamical systems within XCSF to solve a number of well-known test
problems
Equilibria-based Probabilistic Model Checking for Concurrent Stochastic Games
Probabilistic model checking for stochastic games enables formal verification
of systems that comprise competing or collaborating entities operating in a
stochastic environment. Despite good progress in the area, existing approaches
focus on zero-sum goals and cannot reason about scenarios where entities are
endowed with different objectives. In this paper, we propose probabilistic
model checking techniques for concurrent stochastic games based on Nash
equilibria. We extend the temporal logic rPATL (probabilistic alternating-time
temporal logic with rewards) to allow reasoning about players with distinct
quantitative goals, which capture either the probability of an event occurring
or a reward measure. We present algorithms to synthesise strategies that are
subgame perfect social welfare optimal Nash equilibria, i.e., where there is no
incentive for any players to unilaterally change their strategy in any state of
the game, whilst the combined probabilities or rewards are maximised. We
implement our techniques in the PRISM-games tool and apply them to several case
studies, including network protocols and robot navigation, showing the benefits
compared to existing approaches
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