2,703 research outputs found
Helium and Nitrogen Enrichment in Massive Main Sequence Stars: Mechanisms and Implications for the Origin of WNL Stars
The evolutionary paths taken by massive stars with remain substantially uncertain. They begin their lives as
main sequence (MS) O-stars. Depending on their masses, rotation rates, and
metallicities, they can then encounter a wide range of evolutionary states with
an equally broad set of possible surface compositions and spectral
classifications. We present a new grid of calculations for the evolution of
such stars that covers a broad range in mass, M/M to ,
rotation rate, to , metallicity,
to , and -element enhancement,
to . We show that rotating stars undergo
rotationally-induced dredge-up of nucleosynthetic products, mostly He and N, to
their surfaces while still on the MS. Non-rotating metal-rich stars also reveal
the products of nucleosynthesis on their surfaces because even modest amounts
of mass loss expose their "fossil" convective cores: regions that are no longer
convective, but which were part of the convective core at an early stage in the
star's evolution. Thus surface enhancement of He and N is expected for rotating
stars at all metallicities, and for non-rotating stars if they are relatively
metal-rich. We calculate a stellar atmosphere for a representative model from
our grid, properly accounting for He- and N-enhancement, and show that the
resulting spectrum provides a good match to observed WNL stars, strongly
suggesting that the physical mechanisms we have identified are the ultimate
cause of the WNL phase.Comment: 21 pages, 18 figures, 2 tables, accepted for publication in MNRAS, in
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Surface Electromagnetic Waves with Damping. I. Isotropic Media
Surface-Electromagnetic-Wave Dispersion Curves Are Usually Calculated using a Simple Equation Derived from Maxwell\u27s Equations and Boundary Conditions. When Complex Dielectric Functions Are Used for the Two Media, the Component of the Propagation Vector Along the Surface, Kx, Becomes Infinite as the Frequency Approaches the Surface Polariton Frequency Ωs If Ω is Considered Complex and Kx is Real. on the Other Hand, If Kx is Considered Complex and Ω Real, the Dispersion Curves Bend Back Toward Smaller Kx as Ω Approaches Ωs. We Have Previously Demonstrated that Both Types of Behavior Can Be Obtained from Attenuated-Total-Reflection Measurements of Silver. We Now Extend This Result to Other Materials and Show that Dispersion Curves Alone Present an Inadequate Summary of the Data. © 1976 the American Physical Society
Dispersion Curves for Surface Electromagnetic Waves with Damping
The Recent Observation of Back bending in the Dispersion Curves of Surface Plasmons on Silver Can Be Explained by Use of Fresnel\u27s Equations. in the Presence of Damping, the Results of Attenuated-Total-Reflection Measurements Can Be Displayed as Dispersion Curves Either with or Without Back bending. the Measurements for Silver with Back Bending Are Expected for Experiments in Which the Frequency is Fixed, and the Propagation Constant (Or Angle of Incidence) is Swept. © 1974 the American Physical Society
Surface Electromagnetic Waves with Damping. II. Anisotropic Media
The Technique of Plotting the Attenuated-Total-Reflection (ATR) Reflectance as a Function of Both Frequency and Incident Angle using a Three-Dimensional Plot is Applied to Surface Electromagnetic Waves (SEW) in a Uniaxial Material, MnF2. It is Shown that Dispersion Curves Calculated Without Absorption Do Not Completely Describe the ATR Reflectivity. Experimental Data Confirming the Reflectance Surface Features Are Presented. Also, Additional Minima in the Reflectance Surface Not Associated with SEW Are Discussed. © 1977 the American Physical Society
I.R. Lattice-Vibration Spectra of MnF2
The Room Temperature Reflectivity of MnF2 Has Been Measured in the Far I.r. between 50 and 800 Cm-1. the Reflectivity Spectra Show Four I.r.-Active Modes Which Can Be Identified as Eu and A2u Modes. the Spectra Also Show Structure Which Might Be Related to Multiphonon Effects. the Dielectric Functions Were Determined by Fitting the Reflectivity Data with Classical Oscillator Parameters. © 1974
An optical fiber hydrogen sensor using a palladium-coated ball lens
A self-referenced optical fiber refractometer using a ball lens as a sensor head has been developed and characterized. A 350-μm ball lens created at the tip of a single mode fiber has been coated with a 40-nm optically thin layer of palladium that reacts with hydrogen to form a hydride, which has a lower reflectivity than pure palladium. Optical reflectance measurements from the tip of the ball lens were performed to determine the hydrogen response. The change in reflectivity is proportional to the hydrogen concentration in the range 0% to 1% hydrogen in air with a detection limit down to 10 ppm (1σ) in air. This technique offers a simple sensor head arrangement, with a larger sampling area (~40 times) than a typical single-mode fiber core. A statistical image analysis of a palladium film, with cracks created by accelerated failure, confirms that the anticipated sensor area for a ball lens sensor head has a more predictable reflectivity than that of a bare fiber core
Improving Continuous-variable Quantum Channels with Unitary Averaging
A significant hurdle for quantum information and processing using bosonic
systems are stochastic phase errors, which are likely to occur as the photons
propagate through a channel. We propose and demonstrate a scheme of passive,
linear optical unitary averaging for protecting Gaussian channels. The scheme
requires only linear optical elements and vacuum detectors, and protects
against a loss of purity, squeezing and entanglement. We present numerical
simulations and analytical formula, tailored for currently relevant parameters
with low noise levels, where our approximations perform exceptionally well. We
also show the asymptotic nature of the protocol, highlighting both current and
future relevance.Comment: 6 pages, 7 figures, 1 tabl
Can pre-supernova winds from massive stars enrich the interstellar medium with nitrogen at high redshift?
Understanding the nucleosynthetic origin of nitrogen and the evolution of the
N/O ratio in the interstellar medium is crucial for a comprehensive picture of
galaxy chemical evolution at high-redshift because most observational
metallicity (O/H) estimates are implicitly dependent on the N/O ratio. The
observed N/O at high-redshift shows an overall constancy with O/H, albeit with
a large scatter. We show that these heretofore unexplained features can be
explained by the pre-supernova wind yields from rotating massive stars
(M, ). Our models
naturally produce the observed N/O plateau, as well as the scatter at low O/H.
We find the scatter to arise from varying star formation efficiency. However,
the models that have supernovae dominated yields produce a poor fit to the
observed N/O at low O/H. This peculiar abundance pattern at low O/H suggests
that dwarf galaxies are most likely to be devoid of SNe yields and are
primarily enriched by pre-supernova wind abundances.Comment: 6 pages, 1 figure, published in Proceedings of the International
Astronomical Union No. 366, 2022 titled "The Origin of Outflows in Evolved
Stars
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