4,456 research outputs found
Can Sodium Abundances of A-Type Stars Be Reliably Determined from Na I 5890/5896 Lines?
An extensive non-LTE abundance analysis based on Na I 5890/5896 doublet lines
was carried out for a large unbiased sample of ~120 A-type main-sequence stars
(including 23 Hyades stars) covering a wide v_e sin i range of ~10--300 km/s,
with an aim to examine whether the Na abundances in such A dwarfs can be
reliably established from these strong Na I D lines. The resulting abundances
([Na/H]_{58}), which were obtained by applying the T_eff-dependent
microturbulent velocities of \xi ~2--4 km/s with a peak at T_eff ~ 8000 K
(typical for A stars), turned out generally negative with a large diversity
(from ~-1 to ~0), while showing a sign of v_e sin i-dependence (decreasing
toward higher rotation). However, the reality of this apparently subsolar trend
is very questionable, since these [Na/H]_{58} are systematically lower by
~0.3--0.6 dex than more reliable [Na/H]_{61} (derived from weak Na I 6154/6161
lines for sharp-line stars). Considering the large \xi-sensitivity of the
abundances derived from these saturated Na I D lines, we regard that
[Na/H]_{58} must have been erroneously underestimated, suspecting that the
conventional \xi values are improperly too large at least for such strong
high-forming Na I 5890/5896 lines, presumably due to the depth-dependence of
\xi decreasing with height. The nature of atmospheric turbulent velocity field
in mid-to-late A stars would have to be more investigated before we can
determine reliable sodium abundances from these strong resonance D lines.Comment: 14 pages, 8 figures, accepted for publication in Publ. Astron. Soc.
Japan, Vol. 61, No. 5 (2009
Metastable hard-axis polar state of a spinor Bose-Einstein condensate under a magnetic field gradient
We investigate the stability of a hard-axis polar state in a spin-1
antiferromagnetic Bose-Einstein condensate under a magnetic field gradient,
where the easy-plane spin anisotropy is controlled by a negative quadratic
Zeeman energy . In a uniform magnetic field, the axial polar state is
dynamically unstable and relaxes into the planar polar ground state. However,
under a field gradient , the excited spin state becomes metastable down to
a certain threshold and as decreases below , its intrinsic
dynamical instability is rapidly recalled. The incipient spin excitations in
the relaxation dynamics appear with stripe structures, indicating the
rotational symmetry breaking by the field gradient. We measure the dependences
of on and the sample size, and we find that is highly
sensitive to the field gradient in the vicinity of , exhibiting power-law
behavior of with . Our results
demonstrate the significance of the field gradient effect in the quantum
critical dynamics of spinor condensates.Comment: 8 pages, 7 figure
A relative of Hadwiger's conjecture
Hadwiger's conjecture asserts that if a simple graph has no
minor, then its vertex set can be partitioned into stable sets. This
is still open, but we prove under the same hypotheses that can be
partitioned into sets , such that for , the
subgraph induced on has maximum degree at most a function of . This is
sharp, in that the conclusion becomes false if we ask for a partition into
sets with the same property.Comment: 6 page
Weakly Supervised Semantic Parsing with Execution-based Spurious Program Filtering
The problem of spurious programs is a longstanding challenge when training a
semantic parser from weak supervision. To eliminate such programs that have
wrong semantics but correct denotation, existing methods focus on exploiting
similarities between examples based on domain-specific knowledge. In this
paper, we propose a domain-agnostic filtering mechanism based on program
execution results. Specifically, for each program obtained through the search
process, we first construct a representation that captures the program's
semantics as execution results under various inputs. Then, we run a majority
vote on these representations to identify and filter out programs with
significantly different semantics from the other programs. In particular, our
method is orthogonal to the program search process so that it can easily
augment any of the existing weakly supervised semantic parsing frameworks.
Empirical evaluations on the Natural Language Visual Reasoning and
WikiTableQuestions demonstrate that applying our method to the existing
semantic parsers induces significantly improved performances.Comment: EMNLP 202
A survey on network simulators in three-dimensional wireless ad hoc and sensor networks
© 2016 The Author(s). As steady research in wireless ad hoc and sensor networks is going on, performance evaluation through relevant network simulator becomes indispensable procedure to demonstrate superiority to comparative schemes and suitability in most literatures. Thus, it is very important to establish credibility of simulation results by investigating merits and limitations of each simulator prior to selection. Based on this motivation, in this article, we present a comprehensive survey on current network simulators for new emerging research area, three-dimensional wireless ad hoc and sensor networks which is represented by airborne ad hoc networks and underwater sensor networks by reviewing major existing simulators as well as presenting their main features in several aspects. In addition, we address the outstanding mobility models which are main components in simulation study for self-organizing ad hoc networks. Finally, open research issues and research challenges are discussed and presented
Double resonance of Raman transitions in a degenerate Fermi gas
We measure momentum-resolved Raman spectra of a spin-polarized degenerate
Fermi gas of Yb atoms for a wide range of magnetic fields, where the
atoms are irradiated by a pair of counterpropagating Raman laser beams as in
the conventional spin-orbit coupling scheme. Double resonance of first- and
second-order Raman transitions occurs at a certain magnetic field and the
spectrum exhibits a doublet splitting for high laser intensities. The measured
spectral splitting is quantitatively accounted for by the Autler-Townes effect.
We show that our measurement results are consistent with the spinful band
structure of a Fermi gas in the spatially oscillating effective magnetic field
generated by the Raman laser fields.Comment: 7 pages, 6 figure
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