7,561 research outputs found
Temperature effects on the 15-85-micron spectra of olivines and pyroxenes
Far-infrared spectra of laboratory silicates are normally obtained at room
temperature even though the grains responsible for astronomical silicate
emission bands seen at wavelengths >20 micron are likely to be at temperatures
below ~150 K. In order to investigate the effect of temperature on silicate
spectra, we have obtained absorption spectra of powdered forsterite and
olivine, along with two orthoenstatites and diopside clinopyroxene, at 3.5+-0.5
K and at room temperature (295+-2K). To determine the changes in the spectra
the resolution must be increased from 1 to 0.25 cm^-1 at both temperatures
since a reduction in temperature reduces the phonon density, thereby reducing
the width of the infrared peaks. Several bands observed at 295 K split at 3.5
K. At 3.5 K the widths of isolated single bands in olivine, enstatites and
diopside are ~ 90% of their 295 K-widths. However, in forsterite the
3.5-K-widths of the 31-, 49- and 69-micron bands are, respectively, 90%, 45%
and 31% of their 295 K widths. Due to an increase in phonon energy as the
lattice contracts, 3.5-K-singlet peaks occur at shorter wavelengths than do the
corresponding 295-K peaks; the magnitude of the wavelength shift increases from
\~ 0-0.2 micron at 25 micron to ~0.9 micron at 80 micron. Changes in the
relative absorbances of spectral peaks are also observed. The temperature
dependence of lambda_pk and bandwidth shows promise as a means to deduce
characteristic temperatures of mineralogically distinct grain populations. In
addition, the observed changes in band strength with temperature will affect
estimates of grain masses and relative mineral abundances inferred using
room-temperature laboratory data.Comment: 11 pages, 7 figures including figures 3a and 3b. includes latex and
eps files. Accepted by MNRAS on 15th March 200
Measuring the Value of Research: A Generational Accounting Approach
This paper proposes a generational accounting approach to valuating research. Based on the flow of scientific results, a value-added (VA) index is developed that can, in principle, be used to assign a monetary value to any research result and, by aggregation, on entire academic disciplines or sub-disciplines. The VA-index distributes the value of all applications that embody research to the works of research which the applications directly rely on, and further to the works of research of previous generations which the authors of the immediate reference sources have directly or indirectly made use of. The major contribution of the VA index is to provide a measure of the value of research that is comparable across academic disciplines. To illustrate how the generational accounting approach works, I present a VAbased journal rating and a rating of the most influential recent journal articles in the field of economics.Research evaluation, research accounting, journal ranking, citations
Cognitive constraints and island effects
Competence-based theories of island effects play a central role in generative grammar, yet the graded nature of many syntactic islands has never been properly accounted for. Categorical syntactic accounts of island effects have persisted in spite of a wealth of data suggesting that island effects are not categorical in nature and that nonstructural manipulations that leave island structures intact can radically alter judgments of island violations. We argue here, building on work by Paul Deane, Robert Kluender, and others, that processing factors have the potential to account for this otherwise unexplained variation in acceptability judgments.
We report the results of self-paced reading experiments and controlled acceptability studies that explore the relationship between processing costs and judgments of acceptability. In each of the three self-paced reading studies, the data indicate that the processing cost of different types of island violations can be significantly reduced to a degree comparable to that of nonisland filler-gap constructions by manipulating a single nonstructural factor. Moreover, this reduction in processing cost is accompanied by significant improvements in acceptability. This evidence favors the hypothesis that island-violating constructions involve numerous processing pressures that aggregate to drive processing difficulty above a threshold, resulting in unacceptability. We examine the implications of these findings for the grammar of filler-gap dependencies
Late evolution of cataclysmic variables: the loss of AM Her systems
The white dwarf in AM Her systems is strongly magnetic and keeps in
synchronous rotation with the orbit by magnetic coupling to the secondary star.
As the latter evolves through mass loss to a cool, degenerate brown dwarf it
can no longer sustain its own magnetic field and coupling is lost. Angular
momentum accreted then spins up the white dwarf and the system no longer
appears as an AM Her system. Possible consequences are run-away mass transfer
and mass ejection from the system. Some of the unusual cataclysmic variable
systems at low orbital periods may be the outcome of this evolution.Comment: 6 pages, 1 figure, Proceedings of "Cataclysmic Variables", Symposium
in Honour of Brian Warner, Oxford 1999, eds. P.Charles, A.King, O'Donoghue,
to appea
The relation between radio and X-ray luminosity of black hole binaries: affected by inner cool disks?
Observations of the black hole X-ray binaries GX 339-4 and V404 Cygni have
brought evidence of a strong correlation between radio and X-ray emission
during the hard spectral state; however, now more and more sources, the
so-called `outliers', are found with a radio emission noticeably below the
established `standard' relation. Several explanations have already been
considered, but the existence of dual tracks is not yet fully understood.
We suggest that in the hard spectral state re-condensation of gas from the
corona into a cool, weak inner disk can provide additional soft photons for
Comptonization, leading to a higher X-ray luminosity in combination with rather
unchanged radio emission, which presumably traces the mass accretion rate. As
an example, we determined how much additional luminosity due to photons from an
underlying disk would be needed to explain the data from the representative
outlier source H1743-322. From the comparison with calculations of Compton
spectra with and without the photons from an underlying disk, we find that the
required additional X-ray luminosity lies well in the range obtained from
theoretical models of the accretion flow. The radio/X-ray luminosity relation
resulting from Comptonization of additional photons from a weak, cool inner
disk during the hard spectral state can explain the observations of the outlier
sources, especially the data for H1743-322, the source with the most detailed
observations. The existence or non-existence of weak inner disks on the two
tracks might point to a difference in the magnetic fields of the companion
stars. These could affect the effective viscosity and the thermal conductivity,
hence also the re-condensation process.Comment: 7 pages, 2 figures. Accepted for publication in A &
SU Uma stars: Rebrightenings after superoutburst
SU Uma stars after their long superoutbursts often show single or multiple
rebrightenings. We show how this phenomenon can be understood as repeated
reflections of transition waves which mediate changes between the hot and the
cool state of the accretion disk and travel back and forth in the outer disk
region, leaving an inner part permanently hot. This points to a temporarily
increased viscosity, possibly related to the formation of large-scale and
longer persisting magnetic fields by the dynamo operation during the long
superoutburst. The 'mini-rebrightenings' in the early post-outburst light curve
of V585 Lyr discovered by Kato and Osaki in Kepler observations seem to be
understandable as a small limit cycle of low luminosity changes originating
from a wiggle-feature in the thermal equilibrium curve of the cool optically
thick disk.Comment: 9 pages, 6 figures, accepted for publication in PAS
- âŠ