18,688 research outputs found
Massive star evolution : rotation, winds, and overshooting vectors in the Mass-Luminosity plane I. A calibrated grid of rotating single star models
We aim to constrain massive star evolution models using the unique testbed
eclipsing binary HD166734 with new grids of MESA stellar evolution models,
adopting calibrated prescriptions of overshooting, mass loss, and rotation. We
introduce a novel tool: the "mass-luminosity plane" or "M-L plane", as an
equivalent to the traditional HR diagram, utilising it to reproduce the testbed
binary HD166734 with newly calibrated MESA stellar evolution models for single
stars. We can only reproduce the Galactic binary system with an enhanced amount
of core overshooting (alpha = 0.5), mass loss, and rotational mixing. We can
utilise the gradient in the M-L plane to constrain the amount of mass loss to
0.5 - 1.5 times the standard Vink et al. 2001 prescriptions, and we can exclude
extreme reduction or multiplication factors. The extent of the vectors in the
M-L plane leads us to conclude that the amount of core overshooting is larger
than is normally adopted in contemporary massive star evolution models. We
furthermore conclude that rotational mixing is mandatory to get the nitrogen
abundance ratios between the primary and secondary components to be correct
(3:1) in our testbed binary system. Our calibrated grid of models, alongside
our new M-L plane approach, present the possibility of a widened main sequence
due to an increased demand for core overshooting. The increased amount of core
overshooting is not only needed to explain the extended main sequence, but the
enhanced overshooting is also needed to explain the location of the
upper-luminosity limit of the red supergiants. Finally, the increased amount of
core overshooting has -- via the compactness parameter -- implications for
supernova explodibility.Comment: Revised version, 14 pages, 19 figures, accepted for publication by
Astronomy & Astrophysic
Coexistence of Antiferromagnetism and Superconductivity in Electron-doped High-Tc Superconductors
We present magnetotransport evidence for antiferromagnetism in films of the
electron-doped cuprates PrCeCuO. Our results show clear
signature of static antiferromagnetism up to optimal doping x=0.15, with a
quantum phase transition close to x=0.16, and a coexistence of static
antiferromagnetism and superconductivity for 0.12x0.15
Quality interoperability within digital libraries: the DL.org perspective
Quality is the most dynamic aspect of DLs, and becomes even more complex with respect to interoperability. This paper formalizes the research motivations and hypotheses on quality interoperability conducted by the Quality Working Group within the EU-funded project DL.org (<a href="http://www.dlorg.eu">http://www.dlorg.eu/</a>). After providing a multi-level interoperability framework – adopted by DL.org - the authors illustrate key-research points and
approaches on the way to the interoperability of DLs quality, grounding them in the DELOS Reference Model. By applying the DELOS Reference Model Quality Concept Map to their interoperability motivating scenario, the authors subsequently present the two main research outcomes of their investigation - the Quality Core Model and the Quality Interoperability Survey
Gapped tunneling spectra in the normal state of PrCeCuO
We present tunneling data in the normal state of the electron doped cuprate
superconductor PrCeCuO for three different values of the doping
. The normal state is obtained by applying a magnetic field greater than the
upper critical field, for . We observe an anomalous normal
state gap near the Fermi level. From our analysis of the tunneling data we
conclude that this is a feature of the normal state density of states. We
discuss possible reasons for the formation of this gap and its implications for
the nature of the charge carriers in the normal and the superconducting states
of cuprate superconductors.Comment: 7 pages ReVTeX, 11 figures files included, submitted to PR
The distribution of extremal points of Gaussian scalar fields
We consider the signed density of the extremal points of (two-dimensional)
scalar fields with a Gaussian distribution. We assign a positive unit charge to
the maxima and minima of the function and a negative one to its saddles. At
first, we compute the average density for a field in half-space with Dirichlet
boundary conditions. Then we calculate the charge-charge correlation function
(without boundary). We apply the general results to random waves and random
surfaces. Furthermore, we find a generating functional for the two-point
function. Its Legendre transform is the integral over the scalar curvature of a
4-dimensional Riemannian manifold.Comment: 22 pages, 8 figures, corrected published versio
Role of oxygen in the electron-doped superconducting cuprates
We report on resistivity and Hall measurements in thin films of the
electron-doped superconducting cuprate PrCeCuO.
Comparisons between x = 0.17 samples subjected to either ion-irradiation or
oxygenation demonstrate that changing the oxygen content has two separable
effects: 1) a doping effect similar to that of cerium, and 2) a disorder
effect. These results are consistent with prior speculations that apical oxygen
removal is necessary to achieve superconductivity in this compound.Comment: 5 pages, 5 figure
The investigation of vertebral injury sustained during aircrew ejection. Phase 2a - Basic science experimental design and investigation of dynamic characteristics of vertebral columns considered as an engineering structure Annual report, 1 Nov. 1966 - 31 Oct. 1967
Dynamic strength studies on human vertebrae for correlation with data on effects of forcible ejection from disabled aircraf
Hall effect in cobalt-doped TiO
We report Hall effect measurements on thin films of cobalt-doped
TiO. Films with low carrier concentrations (10 - 10)
yield a linear behavior in the Hall data while those having higher carrier
concentrations (10 - 10) display anomalous behavior near zero
field. In the entire range of carrier concentration, n-type conduction is
observed. The appearance of the anomalous behavior is accompanied by a possible
structural change from rutile TiO to Ti_[n}O Magneli phase(s)
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