499 research outputs found
Kinetic considerations of the strength of oriented solids
Kinetics of mechanical strength of oriented and stressed solids based on statistical absolute reaction rate theor
Skycorr: A general tool for spectroscopic sky subtraction
Airglow emission lines, which dominate the optical-to-near-IR sky radiation,
show strong, line-dependent variability on various time scales. Therefore, the
subtraction of the sky background in the affected wavelength regime becomes a
problem if plain sky spectra have to be taken at a different time as the
astronomical data. A solution of this issue is the physically motivated scaling
of the airglow lines in the plain sky data to fit the sky lines in the object
spectrum. We have developed a corresponding instrument-independent approach
based on one-dimensional spectra. Our code skycorr separates sky lines and
sky/object continuum by an iterative approach involving a line finder and
airglow line data. The sky lines are grouped according to their expected
variability. The line groups in the sky data are then scaled to fit the sky in
the science data. Required pixel-specific weights for overlapping groups are
taken from a comprehensive airglow model. Deviations in the wavelength
calibration are corrected by fitting Chebyshev polynomials and rebinning via
asymmetric damped sinc kernels. The scaled sky lines and the sky continuum are
subtracted separately. VLT X-Shooter data covering time intervals from two
minutes to about one year were selected to illustrate the performance. Except
for short time intervals of a few minutes, the sky line residuals were several
times weaker than for sky subtraction without fitting. Further tests show that
skycorr performs consistently better than the method of Davies (2007) developed
for VLT SINFONI data.Comment: 17 pages, 18 figures, accepted for publication in A&
Molecfit: A general tool for telluric absorption correction II. Quantitative evaluation on ESO-VLT X-Shooter spectra
Context: Absorption by molecules in the Earth's atmosphere strongly affects
ground-based astronomical observations. The resulting absorption line strength
and shape depend on the highly variable physical state of the atmosphere, i.e.
pressure, temperature, and mixing ratio of the different molecules involved.
Usually, supplementary observations of so-called telluric standard stars (TSS)
are needed to correct for this effect, which is expensive in terms of telescope
time. We have developed the software package molecfit to provide synthetic
transmission spectra based on parameters obtained by fitting narrow ranges of
the observed spectra of scientific objects. These spectra are calculated by
means of the radiative transfer code LBLRTM and an atmospheric model. In this
way, the telluric absorption correction for suitable objects can be performed
without any additional calibration observations of TSS. Aims: We evaluate the
quality of the telluric absorption correction using molecfit with a set of
archival ESO-VLT X-Shooter visible and near-infrared spectra. Methods: Thanks
to the wavelength coverage from the U to the K band, X-Shooter is well suited
to investigate the quality of the telluric absorption correction with respect
to the observing conditions, the instrumental set-up, input parameters of the
code, the signal-to-noise of the input spectrum, and the atmospheric profiles.
These investigations are based on two figures of merit, I_off and I_res, that
describe the systematic offsets and the remaining small-scale residuals of the
corrections. We also compare the quality of the telluric absorption correction
achieved with moelcfit to the classical method based on a telluric standard
star. (Abridged)Comment: Acc. by A&A; Software available via ESO:
http://www.eso.org/sci/software/pipelines/skytools
Time dependent mechanical strength of oriented media
Time dependent mechanical strength of fully oriented solid
Effective emission heights of various OH lines from X‐shooter and SABER observations of a passing quasi‐2‐day wave
Chemiluminescent radiation of the vibrationally and rotationally excited hydroxyl (OH) radical, which dominates the nighttime near-infrared emission of the Earth's atmosphere in wide wavelength regions, is an important tracer of the chemical and dynamical state of the mesopause region between 80 and 100 km. As radiative lifetimes and rate coefficients for collision-related transitions depend on the OH energy level, line-dependent emission profiles are expected. However, except for some height differences for whole bands mostly revealed by satellite-based measurements, there is a lack of data for individual lines. We succeeded in deriving effective emission heights for 298 OH lines thanks to the joint observation of a strong quasi-2-day wave (Q2DW) in eight nights in 2017 with the medium-resolution spectrograph X-shooter at the Very Large Telescope at Cerro Paranal in Chile and the limb-sounding SABER radiometer on the TIMED satellite. Our fitting procedure revealed the most convincing results for a single wave with a period of about 44 hr and a vertical wavelength of about 32 km. The line-dependent as well as altitude-resolved phases of the Q2DW then resulted in effective heights which differ by up to 8 km and tend to increase with increasing vibrational and rotation excitation. The measured dependence of emission heights and wave amplitudes (which were strongest after midnight) on the line parameters implies the presence of a cold thermalized and a hot non-thermalized population for each vibrational level
Climatologies of various OH lines from about 90,000 X‐shooter Spectra
The nocturnal mesopause region of the Earth's atmosphere radiates chemiluminescent emission from various roto-vibrational bands of hydroxyl (OH), which is therefore a good tracer of the chemistry and dynamics at the emission altitudes. Intensity variations can, for example, be caused by the general circulation, gravity waves, tides, planetary waves, and the solar activity. While the basic OH response to the different dynamical influences has been studied quite frequently, detailed comparisons of the various individual lines are still rare. Such studies can improve our understanding of the OH-related variations as each line shows a different emission profile. We have therefore used about 90,000 spectra of the X-shooter spectrograph of the Very Large Telescope at Cerro Paranal in Chile in order to study 10 years of variations of 298 OH lines. The analysis focuses on climatologies of intensity, solar cycle effect (SCE), and residual variability (especially with respect to time scales of hours and about 2 days) for day of year and local time. For a better understanding of the resulting variability patterns and the line-specific differences, we applied decomposition techniques, studied the variability depending on time scale, and calculated correlations. As a result, the mixing of thermalized and nonthermalized OH level populations clearly influences the amplitude of the variations. Moreover, the local times of the variability features shift depending on the effective line emission height, which can mainly be explained by the propagation of the migrating diurnal tide. This behavior also contributes to remarkable differences in the effective SCE
Optical Spectroscopy of IRAS 02091+6333
We present a detailed spectroscopic investigation, spanning four winters, of
the asymptotic giant branch (AGB) star IRAS 02091+6333. Zijlstra & Weinberger
(2002) found a giant wall of dust around this star and modelled this unique
phenomenon. However their work suffered from the quality of the optical
investigations of the central object. Our spectroscopic investigation allowed
us to define the spectral type and the interstellar foreground extinction more
precisely. Accurate multi band photometry was carried out. This provides us
with the possibility to derive the physical parameters of the system. The
measurements presented here suggest a weak irregular photometric variability of
the target, while there is no evidence of a spectroscopic variability over the
last four years.Comment: 5 pages, Latex, 3 tables, 4 figures, Astron. & Astrophys. - in pres
Molecfit: A general tool for telluric absorption correction. I. Method and application to ESO instruments
Context: The interaction of the light from astronomical objects with the
constituents of the Earth's atmosphere leads to the formation of telluric
absorption lines in ground-based collected spectra. Correcting for these lines,
mostly affecting the red and infrared region of the spectrum, usually relies on
observations of specific stars obtained close in time and airmass to the
science targets, therefore using precious observing time. Aims: We present
molecfit, a tool for correcting for telluric absorption lines based on
synthetic modelling of the Earth's atmospheric transmission. Molecfit is
versatile and can be used with data obtained with various ground-based
telescopes and instruments. Methods: Molecfit combines a publicly available
radiative transfer code, a molecular line database, atmospheric profiles, and
various kernels to model the instrument line spread function. The atmospheric
profiles are created by merging a standard atmospheric profile representative
of a given observatory's climate, of local meteorological data, and of
dynamically retrieved altitude profiles for temperature, pressure, and
humidity. We discuss the various ingredients of the method, its applicability,
and its limitations. We also show examples of telluric line correction on
spectra obtained with a suite of ESO Very Large Telescope (VLT) instruments.
Results: Compared to previous similar tools, molecfit takes the best results
for temperature, pressure, and humidity in the atmosphere above the observatory
into account. As a result, the standard deviation of the residuals after
correction of unsaturated telluric lines is frequently better than 2% of the
continuum. Conclusion: Molecfit is able to accurately model and correct for
telluric lines over a broad range of wavelengths and spectral resolutions.
(Abridged)Comment: 18 pages, 13 figures, 5 tables, accepted for publication in Astronomy
and Astrophysic
The Haldane-Rezayi Quantum Hall State and Conformal Field Theory
We propose field theories for the bulk and edge of a quantum Hall state in
the universality class of the Haldane-Rezayi wavefunction. The bulk theory is
associated with the conformal field theory. The topological properties
of the state, such as the quasiparticle braiding statistics and ground state
degeneracy on a torus, may be deduced from this conformal field theory. The
10-fold degeneracy on a torus is explained by the existence of a logarithmic
operator in the theory; this operator corresponds to a novel bulk
excitation in the quantum Hall state. We argue that the edge theory is the
chiral Dirac fermion, which is related in a simple way to the
theory of the bulk. This theory is reformulated as a truncated version of a
doublet of Dirac fermions in which the symmetry -- which corresponds to
the spin-rotational symmetry of the quantum Hall system -- is manifest and
non-local. We make predictions for the current-voltage characteristics for
transport through point contacts.Comment: 37 pages, LaTeX. Some references added, minor changes at the end of
section
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