25,514 research outputs found
Carbon Rich Extremely Metal Poor Stars: Signatures of Population-III AGB stars in Binary Systems
We use the Cambridge stellar evolution code STARS to model the evolution and
nucleosynthesis of zero-metallicity intermediate-mass stars. We investigate the
effect of duplicity on the nucleosynthesis output of these systems and the
potential abundances of the secondaries. The surfaces of zero-metallicity stars
are enriched in CNO elements after second dredge up. During binary interaction,
such as Roche lobe overflow or wind accretion, metals can be released from
these stars and the secondaries enriched in CNO isotopes. We investigate the
formation of the two most metal poor stars known, HE 0107-5240 and HE
1327-2326. The observed carbon and nitrogen abundances of HE 0107-5240 can be
reproduced by accretion of material from the companion-enhanced wind of a seven
solar star after second dredge-up, though oxygen and sodium are underproduced.
We speculate that HE 1327-2326, which is richer in nitrogen and strontium, may
similarly be formed by wind accretion in a later AGB phase after third
dredge-up.Comment: 16 pages, 1 figure, 7 tables, accepted by MNRA
The noise and flow characteristics of inverted-profile coannular jets
A basic understanding of the noise reduction mechanisms in shock-free inverted-velocity-profile coannular jets was studied. Acoustic measurements are first conducted in an anechoic facility to isolate the effects of inverted velocity and inverted temperature for coannular jets having constant total thrust, mass flow rate and exit area. To obtain physical explanations of the measured noise changes, several types of experiments are conducted. These include (1) source location experiments using the polar correlation technique, (2) mean flow surveys using a combination pressure/temperature probe, and (3) detailed mean flow and turbulence measurements using a two-point four-channel laser velocimeter. The results from these experiments are presented and discussed in detail. Finally, the measured variations of coannular jet mixing noise with fan-to-primary velocity ratio and static temperature ratio are interpreted by utilizing the results from the various experimental phases in conjunction with the existing Lockheed single jet noise prediction model
Non-equilibrium fluctuations and mechanochemical couplings of a molecular motor
We investigate theoretically the violations of Einstein and Onsager
relations, and the efficiency for a single processive motor operating far from
equilibrium using an extension of the two-state model introduced by Kafri {\em
et al.} [Biophys. J. {\bf 86}, 3373 (2004)]. With the aid of the Fluctuation
Theorem, we analyze the general features of these violations and this
efficiency and link them to mechanochemical couplings of motors. In particular,
an analysis of the experimental data of kinesin using our framework leads to
interesting predictions that may serve as a guide for future experiments.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let
New variables, the gravitational action, and boosted quasilocal stress-energy-momentum
This paper presents a complete set of quasilocal densities which describe the
stress-energy-momentum content of the gravitational field and which are built
with Ashtekar variables. The densities are defined on a two-surface which
bounds a generic spacelike hypersurface of spacetime. The method used
to derive the set of quasilocal densities is a Hamilton-Jacobi analysis of a
suitable covariant action principle for the Ashtekar variables. As such, the
theory presented here is an Ashtekar-variable reformulation of the metric
theory of quasilocal stress-energy-momentum originally due to Brown and York.
This work also investigates how the quasilocal densities behave under
generalized boosts, i. e. switches of the slice spanning . It is
shown that under such boosts the densities behave in a manner which is similar
to the simple boost law for energy-momentum four-vectors in special relativity.
The developed formalism is used to obtain a collection of two-surface or boost
invariants. With these invariants, one may ``build" several different mass
definitions in general relativity, such as the Hawking expression. Also
discussed in detail in this paper is the canonical action principle as applied
to bounded spacetime regions with ``sharp corners."Comment: Revtex, 41 Pages, 4 figures added. Final version has been revised and
improved quite a bit. To appear in Classical and Quantum Gravit
Spatially resolved spectroscopy of monolayer graphene on SiO2
We have carried out scanning tunneling spectroscopy measurements on
exfoliated monolayer graphene on SiO to probe the correlation between its
electronic and structural properties. Maps of the local density of states are
characterized by electron and hole puddles that arise due to long range
intravalley scattering from intrinsic ripples in graphene and random charged
impurities. At low energy, we observe short range intervalley scattering which
we attribute to lattice defects. Our results demonstrate that the electronic
properties of graphene are influenced by intrinsic ripples, defects and the
underlying SiO substrate.Comment: 6 pages, 7 figures, extended versio
Mapping the Dirac point in gated bilayer graphene
We have performed low temperature scanning tunneling spectroscopy
measurements on exfoliated bilayer graphene on SiO2. By varying the back gate
voltage we observed a linear shift of the Dirac point and an opening of a band
gap due to the perpendicular electric field. In addition to observing a shift
in the Dirac point, we also measured its spatial dependence using spatially
resolved scanning tunneling spectroscopy. The spatial variation of the Dirac
point was not correlated with topographic features and therefore we attribute
its shift to random charged impurities.Comment: 3 pages, 3 figure
Gate Tunable Dissipation and "Superconductor-Insulator" Transition in Carbon Nanotube Josephson Transistors
Dissipation is ubiquitous in quantum systems, and its interplay with
fluctuations is critical to maintaining quantum coherence. We experimentally
investigate the dissipation dynamics in single-walled carbon nanotubes coupled
to superconductors. The voltage-current characteristics display gate-tunable
hysteresis, with sizes that perfectly correlate with the normal state
resistance RN, indicating the junction undergoes a periodic modulation between
underdamped and overdamped regimes. Surprisingly, when a device's Fermi-level
is tuned through a local conductance minimum, we observe a gate-controlled
transition from superconducting-like to insulating-like states, with a
"critical" R_N value of about 8-20 kohm.Comment: Figures revised to improve clarity. Accepted for publication by
Physical Review Letter
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