16,107 research outputs found
High-Precision Spectroscopy with Counter-Propagating Femtosecond Pulses
An experimental realization of high-precision direct frequency comb
spectroscopy using counter-propagating femtosecond pulses on two-photon atomic
transitions is presented. Doppler broadened background signal, hampering
precision spectroscopy with ultrashort pulses, is effectively eliminated with a
simple pulse shaping method. As a result, all four 5S-7S two-photon transitions
in a rubidium vapor are determined with both statistical and systematic
uncertainties below 10, which is an order of magnitude better than
previous experiments on these transitions.Comment: 5 pages, 4 figures. Accepted to PR
Pairing, Ferromagnetism, and Condensation of a normal spin-1 Bose gas
We theoretically study the stability of a normal, spin disordered, homogenous
spin-1 Bose gas against ferromagnetism, pairing, and condensation through a
Random Phase Approximation which includes exchange (RPA-X). Repulsive
spin-independent interactions stabilize the normal state against both
ferromagnetism and pairing, and for typical interaction strengths leads to a
direct transition from an unordered normal state to a fully ordered single
particle condensate. Atoms with much larger spin-dependent interaction may
experience a transition to a ferromagnetic normal state or a paired superfluid,
but, within the RPA-X, there is no instability towards a normal state with
spontaneous nematic order. We analyze the role of the quadratic Zeeman effect
and finite system size.Comment: 4 pages, 3 figures, 1 table. Supplementary materials attache
Trade-offs Between Water Transport Capacity and Drought Resistance in Neotropical Canopy Liana and Tree Species
In tropical forest canopies, it is critical for upper shoots to efficiently provide water to leaves for physiological function while safely preventing loss of hydraulic conductivity due to cavitation during periods of soil water deficit or high evaporative demand. We compared hydraulic physiology of upper canopy trees and lianas in a seasonally dry tropical forest to test whether trade-offs between safety and efficiency of water transport shape differences in hydraulic function between these two major tropical woody growth forms. We found that lianas showed greater maximum stem-specific hydraulic conductivity than trees, but lost hydraulic conductivity at less negative water potentials than trees, resulting in a negative correlation and trade-off between safety and efficiency of water transport. Lianas also exhibited greater diurnal changes in leaf water potential than trees. The magnitude of diurnal water potential change was negatively correlated with sapwood capacitance, indicating that lianas are highly reliant on conducting capability to maintain leaf water status, whereas trees relied more on stored water in stems to maintain leaf water status. Leaf nitrogen concentration was related to maximum leaf-specific hydraulic conductivity only for lianas suggesting that greater water transport capacity is more tied to leaf processes in lianas compared to trees. Our results are consistent with a trade-off between safety and efficiency of water transport and may have implications for increasing liana abundance in neotropical forests
Recommended from our members
Rapid evolution of protein kinase PKR alters sensitivity to viral inhibitors.
Protein kinase PKR (also known as EIF2AK2) is activated during viral infection and phosphorylates the alpha subunit of eukaryotic translation initiation factor 2 (eIF2), leading to inhibition of translation and viral replication. We report fast evolution of the PKR kinase domain in vertebrates, coupled with positive selection of specific sites. Substitution of positively selected residues in human PKR with residues found in related species altered sensitivity to PKR inhibitors from different poxviruses. Species-specific differences in sensitivity to poxviral pseudosubstrate inhibitors were identified between human and mouse PKR, and these differences were traced to positively selected residues near the eIF2alpha binding site. Our findings indicate how an antiviral protein evolved to evade viral inhibition while maintaining its primary function. Moreover, the identified species-specific differences in the susceptibility to viral inhibitors have important implications for studying human infections in nonhuman model systems
Surface-induced heating of cold polar molecules
We study the rotational and vibrational heating of diatomic molecules placed
near a surface at finite temperature on the basis of macroscopic quantum
electrodynamics. The internal molecular evolution is governed by transition
rates that depend on both temperature and position. Analytical and numerical
methods are used to investigate the heating of several relevant molecules near
various surfaces. We determine the critical distances at which the surface
itself becomes the dominant source of heating and we investigate the transition
between the long-range and short-range behaviour of the heating rates. A simple
formula is presented that can be used to estimate the surface-induced heating
rates of other molecules of interest. We also consider how the heating depends
on the thickness and composition of the surface.Comment: 17 pages, 7 figure
Gemini/GMOS photometry of intermediate-age star clusters in the Large Magellanic Cloud
We present Gemini South GMOS g,i photometry of 14 intermediate-age Large
Magellanic Cloud (LMC) star clusters, namely: NGC 2155, 2161, 2162, 2173, 2203,
2209, 2213, 2231, 2249, Hodge 6, SL 244, 505, 674, and 769, as part of a
continuing project to investigate the extended Main Sequence Turnoff (EMSTO)
phenomenon. Extensive artificial star tests were made over the observed field
of view. These tests reveal the observed behaviour of photometric errors with
magnitude and crowding. The cluster stellar density radial profiles were traced
from star counts over the extent of the observed field. We adopt clus- ter
radii and build colour-magnitude diagrams (CMDs) with cluster features clearly
identified. We used the cluster (g,g-i) CMDs to estimate ages from the matching
of theoretical isochrones. The studied LMC clusters are confirmed to be
intermediate-age clusters, which range in age 9.10 < log(t) < 9.60. NGC 2162
and NGC 2249 look like new EMSTO candidates, in addition to NGC 2209, on the
basis of having dual red clumps.Comment: MNRAS, accepte
Agriculture and European Union Enlargement
Potential accession of a number of eastern and central European countries into the European Union (EU) seems destined to lead to further reforms of the Common Agricultural Policy (CAP). The financial costs of absorbing these countries may be extreme. This report documents the modeling framework (European Simulation Model, ESIM) used to analyze the 1992 CAP reform and discusses possible effects of EU enlargement.CAP reform, European Union, EU, European Union enlargement, Agricultural and Food Policy,
SN 1996cr: Confirmation of a Luminous Type IIn Supernova in the Circinus Galaxy
We have recently confirmed SN 1996cr as a late-time type IIn supernova (SN)
via VLT spectroscopy and isolated its explosion date to ~1 yr using archival
optical imaging. We briefly touch upon here the wealth of optical, X-ray, and
radio archival observations available for this enigmatic source. Due to its
relative proximity (3.8 +/-0.6 Mpc), SN 1996cr ranks among the brightest X-ray
and radio SNe ever detected and, as such, may offer powerful insights into the
structure and composition of type IIn SNe. We also find that SN 1996cr is
matched to GRB 4B 960202 at a 2-3 sigma confidence level, making it perhaps the
third GRB to be significantly associated with a type II SN. We speculate on
whether SN 1996cr could be an off-axis or ``failed'' GRB.Comment: 3 pages, 1 figure, poster proceeding for "Supernova 1987A: 20 Years
After: Supernovae and Gamma-Ray Bursters" AIP, New York, eds. S. Immler, K.W.
Weiler, and R. McCra
- âŠ