11,960 research outputs found
Li I and K I Scatter in Cool Pleiades Dwarfs
We utilize high-resolution (R~60,000), high S/N (~100) spectroscopy of 17
cool Pleiades dwarfs to examine the confounding star-to-star scatter in the
6707 Li I line strengths in this young cluster. Our Pleiads, selected for their
small projected rotational velocity and modest chromospheric emission, evince
substantial scatter in the linestrengths of 6707 Li I feature that is absent in
the 7699 K I resonance line. The Li I scatter is not correlated with that in
the high-excitation 7774 O I feature, and the magnitude of the former is
greater than the latter despite the larger temperature sensitivity of the O I
feature. These results suggest that systematic errors in linestrength
measurements due to blending, color (or color-based T_eff) errors, or line
formation effects related to an overlying chromosphere are not the principal
source of Li I scatter in our stars. There do exist analytic spot models that
can produce the observed Li scatter without introducing scatter in the K I line
strengths or the color-magnitude diagram. However, these models predict factor
of >3 differences in abundances derived from the subordinate 6104 and resonance
6707 Li I features; we find no difference in the abundances determined from
these two features. These analytic spot models also predict CN line strengths
significantly larger than we observe in our spectra. The simplest explanation
of the Li, K, CN, and photometric data is that there must be a real abundance
component to the Pleiades Li dispersion. We suggest that this real abundance
component is the manifestation of relic differences in erstwhile
pre-main-sequence Li burning caused by effects of surface activity on stellar
structure. We discuss observational predictions of these effects.Comment: 35 pages, 7 figures; accepted by Ap
Report of the Higgs Working Group of the Tevatron Run 2 SUSY/Higgs Workshop
This report presents the theoretical analysis relevant for Higgs physics at
the upgraded Tevatron collider and documents the Higgs Working Group
simulations to estimate the discovery reach in Run 2 for the Standard Model and
MSSM Higgs bosons. Based on a simple detector simulation, we have determined
the integrated luminosity necessary to discover the SM Higgs in the mass range
100-190 GeV. The first phase of the Run 2 Higgs search, with a total integrated
luminosity of 2 fb-1 per detector, will provide a 95% CL exclusion sensitivity
comparable to that expected at the end of the LEP2 run. With 10 fb-1 per
detector, this exclusion will extend up to Higgs masses of 180 GeV, and a
tantalizing 3 sigma effect will be visible if the Higgs mass lies below 125
GeV. With 25 fb-1 of integrated luminosity per detector, evidence for SM Higgs
production at the 3 sigma level is possible for Higgs masses up to 180 GeV.
However, the discovery reach is much less impressive for achieving a 5 sigma
Higgs boson signal. Even with 30 fb-1 per detector, only Higgs bosons with
masses up to about 130 GeV can be detected with 5 sigma significance. These
results can also be re-interpreted in the MSSM framework and yield the required
luminosities to discover at least one Higgs boson of the MSSM Higgs sector.
With 5-10 fb-1 of data per detector, it will be possible to exclude at 95% CL
nearly the entire MSSM Higgs parameter space, whereas 20-30 fb-1 is required to
obtain a 5 sigma Higgs discovery over a significant portion of the parameter
space. Moreover, in one interesting region of the MSSM parameter space (at
large tan(beta)), the associated production of a Higgs boson and a b b-bar pair
is significantly enhanced and provides potential for discovering a non-SM-like
Higgs boson in Run 2.Comment: 185 pages, 124 figures, 55 table
Evidence for alignment of the rotation and velocity vectors in pulsars. II. Further data and emission heights
We have conducted observations of 22 pulsars at frequencies of 0.7, 1.4 and
3.1 GHz and present their polarization profiles. The observations were carried
out for two main purposes. First we compare the orientation of the spin and
velocity vectors to verify the proposed alignment of these vectors by Johnston
et al. (2005). We find, for the 14 pulsars for which we were able to determine
both vectors, that 7 are plausibly aligned, a fraction which is lower than, but
consistent with, earlier measurements. Secondly, we use profiles obtained
simultaneously at widely spaced frequencies to compute the radio emission
heights. We find, similar to other workers in the field, that radiation from
the centre of the profile originates from lower in the magnetosphere than the
radiation from the outer parts of the profile.Comment: Accepted by MNRAS. 14 page
Cygnus A at 99 GHz: Observations of the three principal components and interpretation of the central source
The three principal emission components of Cygnus A were observed at 99 GHz, the highest frequency at which radio measurements of this source have been accomplished. The observations show no definite indication of a high-frequency cutoff in the spectrum of the compact central component, which perhaps may be attributed to an optically thin synchrotron source that peaks at a frequency of several hundred GHz
Detecting Pulsars with Interstellar Scintillation in Variance Images
Pulsars are the only cosmic radio sources known to be sufficiently compact to
show diffractive interstellar scintillations. Images of the variance of radio
signals in both time and frequency can be used to detect pulsars in large-scale
continuum surveys using the next generation of synthesis radio telescopes. This
technique allows a search over the full field of view while avoiding the need
for expensive pixel-by-pixel high time resolution searches. We investigate the
sensitivity of detecting pulsars in variance images. We show that variance
images are most sensitive to pulsars whose scintillation time-scales and
bandwidths are close to the subintegration time and channel bandwidth.
Therefore, in order to maximise the detection of pulsars for a given radio
continuum survey, it is essential to retain a high time and frequency
resolution, allowing us to make variance images sensitive to pulsars with
different scintillation properties. We demonstrate the technique with
Murchision Widefield Array data and show that variance images can indeed lead
to the detection of pulsars by distinguishing them from other radio sources.Comment: 8 papes, 9 figures, accepted for publication in MNRA
Responsive Building Envelopes: Parametic Control of Moveable Facade components
The challenge of developing adaptive, responsive low-energy architecture requires new knowledge about the complex and dynamic interaction between envelope architecture, optimization between competing environmental performance metrics (light, heat and wind indices) and local climate variables. Advances in modeling the geometry of building envelopes and control technologies for adaptive buildings now permit the sophisticated evaluation of alternative envelope configurations for a set of performance criteria. This paper reports on a study of the parametric control of a building envelope based on moveable facade components, acting as a shading device to reduce thermal gain within the building. This is investigated using two alternative tiling strategies, a hexagonal tiling and a pentagonal tiling, considering the component design, supporting structure and control methods
Engineering geology of British rocks and soils : Lias Group
The report begins with an introduction and a detailed modern assessment of the geology of the
Lias Group in terms of both stratigraphy and lithology. The modern lithostratigraphy is placed in
the context of the old, and sometimes more familiar, usage. The next two chapters deal with the
mineralogy of a suite of samples collected for the project, and an assessment of the nature and
influence of weathering based on a detailed analysis of the Lias dataset held in the BGS National
Geotechnical Properties Database. The following chapters cover geohazards associated with the
Lias Group, and a brief overview of the wide variety of industrial applications for which the Lias
is well known. The geotechnical database forms the basis of the penultimate chapter,
geotechnical properties. The contents of the database are analysed, interpreted, presented in
graphical form, and discussed in terms of statistical variation and in the light of likely
engineering behaviour. The engineering geology of the Lias Group is discussed in the final
chapter, borrowing from the preceding chapters. A comprehensive cited reference list and a
bibliography are provided. In addition to the large number of technical data provided to BGS, a
small data set has been generated by BGS laboratories, particularly in areas where the main
database was deficient, and also in connection with associated BGS studies of the swelling and
shrinkage properties of the Lias Group.
The individual items of data making up the database are not attributed. However, the
contribution of a wide range of consultancies, contractors, authorities, and individuals is
acknowledged. It is hoped that this report will provide a source of useful information to a wide
range of engineers, planners, scientists, and other interested parties concerned with Lias Group
materials.
It should be noted that whilst quantitative technical data are included in this report, these should
not be used as a substitute for proper site investigation
A low upper-limit on the lithium isotope ratio in HD140283
We have obtained a high-S/N (900-1100), high-resolving-power (R=95000)
spectrum of the metal-poor subgiant HD 140283 in an effort to measure its
6Li/7Li isotope ratio. From a 1-D atmospheric analysis, we find a value
consistent with zero, 6Li/7Li = 0.001, with an upper limit of 6Li/7Li < 0.026.
This measurement supersedes an earlier detection (0.040 +/- 0.015(1sigma)) by
one of the authors. HD 140283 provides no support for the suggestion that
Population II stars may preserve their 6Li on the portion of the subgiant
branch where 7Li is preserved. However, this star does not defeat the
suggestion either; being at the cool end of subgiant branch of the Spite
plateau, it may be sufficiently cool that 6Li depletion has already set in, or
the star may be sufficiently metal poor that little Galactic production of 6Li
had occurred. Continued investigation of other subgiants is necessary to test
the idea. We also consider the implications of the HD 140283 upper limit in
conjunction with other measurements for models of 6Li production by cosmic rays
from supernovae and structure formation shocks.Comment: 8 pages, 4 figures; accepted for publication in Astronomy and
Astrophysic
Ab-initio spin dynamics applied to nanoparticles: canted magnetism of a finite Co chain along a Pt(111) surface step edge
In order to search for the magnetic ground state of surface nanostructures we
extended first principles adiabatic spin dynamics to the case of fully
relativistic electron scattering. Our method relies on a constrained density
functional theory whereby the evolution of the orientations of the spin-moments
results from a semi-classical Landau-Lifshitz equation. This approach is
applied to a study of the ground state of a finite Co chain placed along a step
edge of a Pt(111) surface. As far as the ground state spin orientation is
concerned we obtain excellent agreement with the experiment. Furthermore we
observe noncollinearity of the atom-resolved spin and orbital moments. In terms
of magnetic force theorem calculations we also demonstrate how a reduction of
symmetry leads to the existence of canted magnetic states.Comment: 4 pages, ReVTeX + 3 figures (Encapsulated Postscript), submitted to
PR
- …