200 research outputs found
Co-registration and bias corrections of satellite elevation data sets for quantifying glacier thickness change
There are an increasing number of digital elevation models (DEMs) available worldwide for deriving elevation differences over time, including vertical changes on glaciers. Most of these DEMs are heavily post-processed or merged, so that physical error modelling becomes difficult and statistical error modelling is required instead. We propose a three-step methodological framework for assessing and correcting DEMs to quantify glacier elevation changes: (i) remove DEM shifts, (ii) check for elevation-dependent biases, and (iii) check for higher-order, sensor-specific biases. A simple, analytic and robust method to co-register elevation data is presented in regions where stable terrain is either plentiful (case study New Zealand) or limited (case study Svalbard). The method is demonstrated using the three global elevation data sets available to date, SRTM, ICESat and the ASTER GDEM, and with automatically generated DEMs from satellite stereo instruments of ASTER and SPOT5-HRS. After 3-D co-registration, significant biases related to elevation were found in some of the stereoscopic DEMs. Biases related to the satellite acquisition geometry (along/cross track) were detected at two frequencies in the automatically generated ASTER DEMs. The higher frequency bias seems to be related to satellite jitter, most apparent in the back-looking pass of the satellite. The origins of the more significant lower frequency bias is uncertain. ICESat-derived elevations are found to be the most consistent globally available elevation data set available so far. Before performing regional-scale glacier elevation change studies or mosaicking DEMs from multiple individual tiles (e.g. ASTER GDEM), we recommend to co-register all elevation data to ICESat as a global vertical reference system
Optical Properties of Iron Silicates in the Infrared to Millimeter as a Function of Temperatures and Wavelength
The Optical Properties of Astronomical Silicates with Infrared Techniques
(OPASI-T) program utilizes multiple instruments to provide spectral data over a
wide range of temperature and wavelengths. Experimental methods include Vector
Network Analyzer (VNA) and Fourier Transform Spectroscopy (FTS) transmission,
and reflection/scattering measurements. From this data, we can determine the
optical parameters for the index of refraction, \textit{n}, and the absorption
coefficient, \textit{k}. The analysis of the laboratory transmittance data for
each sample type is based upon different mathematical models, which are applied
to each data set according to their degree of coherence. Presented here are
results from iron silicate dust grain analogs, in several sample preparations
and at temperatures ranging from 5--300 K, across the infrared and millimeter
portion of the spectrum (from 2.5--10,000 \mic\ or 4,000--1 \wvn).Comment: Revised manuscript submitted to Ap
Precise DEM extraction from Svalbard using 1936 high oblique imagery
Stretching time series further in the past with the best
possible accuracy is essential to the understanding of climate change impacts
and geomorphological processes evolving on decadal-scale time spans. In the
first half of the twentieth century, large parts of the polar regions were
still unmapped or only superficially so. To create cartographic data, a
number of historic photogrammetric campaigns were conducted using oblique
imagery, which is easier to work with in unmapped environments as collocating
images is an easier task for the human eye given a more familiar viewing
angle and a larger field of view. Even if the data obtained from such
campaigns gave a good baseline for the mapping of the area, the precision and
accuracy are to be considered with caution. Exploiting the possibilities
arising from modern image processing tools and reprocessing the archives to
obtain better data is therefore a task worth the effort. The oblique angle of
view of the data is offering a challenge to classical photogrammetric tools,
but the use of modern structure-from-motion (SfM) photogrammetry offers an
efficient and quantitative way to process these data into terrain models. In
this paper, we propose a good practice method for processing historical
oblique imagery using free and open source software (MicMac and Python) and
illustrate the process using images of the Svalbard archipelago acquired in
1936 by the Norwegian Polar Institute. On these data, our workflow provides
5 m resolution, high-quality elevation data (SD 2 m for moderate terrain)
as well as orthoimages that allow for the reliable quantification of terrain
change when compared to more modern data.</p
On the asymptotic giant branch star origin of peculiar spinel grain OC2
Microscopic presolar grains extracted from primitive meteorites have
extremely anomalous isotopic compositions revealing the stellar origin of these
grains. The composition of presolar spinel grain OC2 is different from that of
all other presolar spinel grains. Large excesses of the heavy Mg isotopes are
present and thus an origin from an intermediate-mass (IM) asymptotic giant
branch (AGB) star was previously proposed for this grain. We discuss the
isotopic compositions of presolar spinel grain OC2 and compare them to
theoretical predictions. We show that the isotopic composition of O, Mg and Al
in OC2 could be the signature of an AGB star of IM and metallicity close to
solar experiencing hot bottom burning, or of an AGB star of low mass (LM) and
low metallicity suffering very efficient cool bottom processing. Large
measurement uncertainty in the Fe isotopic composition prevents us from
discriminating which model better represents the parent star of OC2. However,
the Cr isotopic composition of the grain favors an origin in an IM-AGB star of
metallicity close to solar. Our IM-AGB models produce a self-consistent
solution to match the composition of OC2 within the uncertainties related to
reaction rates. Within this solution we predict that the 16O(p,g)17F and the
17O(p,a)14N reaction rates should be close to their lower and upper limits,
respectively. By finding more grains like OC2 and by precisely measuring their
Fe and Cr isotopic compositions, it may be possible in the future to derive
constraints on massive AGB models from the study of presolar grains.Comment: 10 pages, 8 figures, accepted for publication on Astronomy &
Astrophysic
Measuring the Optical Properties of Astrophysical Dust Analogues: Instrumentation and Methods
Dust is found throughout the universe and plays an important role for a wide range of astrophysical phenomena. In recent years, new infrared facilities have provided powerful new data for understanding these phenomena. However, interpretation of these data is often complicated by a lack of complementary information about the optical properties of astronomically relevant materials. The Optical Properties of Astronomical Silicates with Infrared Techniques (OPASI-T) program at NASA's Goddard Space Flight Center is designed to provide new high-quality laboratory data from which we can derive the optical properties of astrophysical dust analogues. This program makes use of multiple instruments, including new equipment designed and built specifically for this purpose. The suite of instruments allows us to derive optical properties over a wide wavelength range, from the near-infrared through the millimeter, also providing the capability for exploring how these properties depend upon the temperature of the sample. In this paper, we discuss the overall structure of the research program, describe the new instruments that have been developed to meet the science goals, and demonstrate the efficacy of these tools
Reaction rate uncertainties and 26Al in AGB silicon carbide stardust
Stardust is a class of presolar grains each of which presents an ideally
uncontaminated stellar sample. Mainstream silicon carbide (SiC) stardust formed
in the extended envelopes of carbon-rich asymptotic giant branch (AGB) stars
and incorporated the radioactive nucleus 26Al as a trace element. The aim of
this paper is to analyse in detail the effect of nuclear uncertainties, in
particular the large uncertainties of up to four orders of magnitude related to
the 26Al_g+(p,gamma)27Si reaction rate, on the production of 26Al in AGB stars
and compare model predictions to data obtained from laboratory analysis of SiC
stardust grains. Stellar uncertainties are also briefly discussed. We use a
detailed nucleosynthesis postprocessing code to calculate the 26Al/27Al ratios
at the surface of AGB stars of different masses (M = 1.75, 3, and 5 M_sun) and
metallicities (Z = 0.02, 0.012, and 0.008). For the lower limit and recommended
value of the 26Al_g(p,gamma)27Si reaction rate, the predicted 26Al/27Al ratios
replicate the upper values of the range of the 26Al/27Al ratios measured in SiC
grains. For the upper limit of the 26Al_g(p,gamma)27Si reaction rate, instead,
the predicted 26Al/27Al ratios are approximately 100 times lower and lie below
the range observed in SiC grains. When considering models of different masses
and metallicities, the spread of more than an order of magnitude in the
26Al/27Al ratios measured in stellar SiC grains is not reproduced. We propose
two scenarios to explain the spread of the 26Al/27Al ratios observed in
mainstream SiC, depending on the choice of the 26Al_g+p reaction rate. One
involves different times of stardust formation, the other involves extra-mixing
processes. Stronger conclusions will be possible after more information is
available from future nuclear experiments on the 26Al_g+p reaction.Comment: 6 pages, 5 Postscript figures, accepted for publication in Astronomy
and Astrophysic
Infrared spectroscopy of Nova Cassiopeiae 1993 (V705 Cas). IV. A closer look at the dust
Nova Cassiopeiae 1993 (V705 Cas) was an archetypical dust-forming nova. It
displayed a deep minimum in the visual light curve, and spectroscopic evidence
for carbon, hydrocarbon and silicate dust. We report the results of fitting the
infrared spectral energy distribution with the DUSTY code, which we use to
determine the properties and geometry of the emitting dust. The emission is
well described as originating in a thin shell whose dust has a carbon:silicate
ratio of ~2:1 by number (1.26:1 by mass) and a relatively flat size
distribution. The 9.7micron and 18micron silicate features are consistent with
freshly-condensed dust and, while the lower limit to the grain size
distribution is not well constrained, the largest grains have dimensions
\~0.06micron; unless the grains in V705 Cas were anomalously small, the sizes
of grains produced in nova eruptions may previously have been overestimated in
novae with optically thick dust shells. Laboratory work by Grishko & Duley may
provide clues to the apparently unique nature of nova UIR features.Comment: 11 pages, 9 fugure
Sub-Keplerian accretion onto circumstellar disks
Models of the formation, evolution and photoevaporation of circumstellar
disks are an essential ingredient in many theories of the formation of
planetary systems. The ratio of disk mass over stellar mass in the
circumstellar phase of a disk is largely determined by the angular momentum of
the original cloud core from which the system was formed. While full 3D or 2D
axisymmetric hydrodynamical models of accretion onto the disk automatically
treat all aspects of angular momentum, this is not so trivial for 1D and
semi-2D viscous disk models. Since 1D and semi-2D disk models are still very
useful for long-term evolutionary modelling of disks with relatively little
numerical effort, we investigate how the 2D nature of accretion affects the
formation and evolution of the disk in such models. A proper treatment of this
problem requires a correction for the sub-Keplerian velocity at which accretion
takes place. We develop an update of our semi-2D time-dependent disk evolution
model to properly treat the effects of sub-Keplerian accretion. The new model
also accounts for the effects of the vertical extent of the disk on the
accretion streamlines from the envelope. The disks produced with the new method
are smaller than those obtained previously, but their mass is mostly unchanged.
The new disks are a few degrees warmer in the outer parts, so they contain less
solid CO. Otherwise, the results for ices are unaffected. The 2D treatment of
the accretion results in material accreting at larger radii, so a smaller
fraction comes close enough to the star for amorphous silicates to be thermally
annealed into crystalline form. The lower crystalline abundances thus predicted
correspond more closely to observed abundances than did earlier model
predictions. We argue that thermal annealing followed by radial mixing must be
responsible for at least part of the observed crystalline material.Comment: Accepted by A&A; 10 pages, 6 figure
Development and testing of a pyro-driven launcher for harpoon-based comet sample acquisition
The CORSAIR (COmet Rendezvous, Sample Acquisition, Investigation, and Return) mission is a proposal for the fourth NASA New Frontiers program. It belongs to the Comet Surface Sample Return mission theme which focuses on acquiring and returning to Earth a macroscopic sample from the surface of a comet nucleus. CORSAIR uses a harpoon-based Sample Acquisition System (SAS) with the spacecraft hovering several meters above the comet surface. This stand-off strategy overcomes disadvantages of
systems using drills or shovels. Since comets are low gravity objects, these techniques would require anchoring before sampling, which is not necessary here. Moreover, the harpoon-based system allows for acquiring several samples from different locations on the comet maximizing the scientifc output of the mission.
Each SAS assembly consists of a pyro-driven launcher, a Sample Acquisition and Retrieval Projectile (SARP) and a retraction system using a deployable composite boom structure. In order to collect enough cometary material, the launcher has to provide the required kinetic energy to the SARP. Due to high energy densities, pyrotechnically actuated devices ultimately reduce the overall system mass and dimensions. Here, an overview of the development, design and testing of the launcher is given. Furthermore, the launcher theory is introduced explaining the entire reaction chain: initiation -> gas dynamics -> SARP motion
Structural and compositional properties of brown dwarf disks: the case of 2MASS J04442713+2512164
In order to improve our understanding of substellar formation, we have
performed a compositional and structural study of a brown dwarf disk.
We present the result of photometric, spectroscopic and imaging observations
of 2MASS J04442713+2512164, a young brown dwarf (M7.25) member of the Taurus
association. Our dataset, combined with results from the literature, provides a
complete coverage of the spectral energy distribution from the optical to the
millimeter including the first photometric measurement of a brown dwarf disk at
3.7mm, and allows us to perform a detailed analysis of the disk properties.
The target was known to have a disk. High resolution optical spectroscopy
shows that it is intensely accreting, and powers a jet and an outflow. The disk
structure is similar to that observed for more massive TTauri stars. Spectral
decomposition models of Spitzer/IRS spectra suggest that the mid-infrared
emission from the optically thin disk layers is dominated by grains with
intermediate sizes (1.5micron). Crystalline silicates are significantly more
abondant in the outer part and/or deeper layers of the disk, implying very
efficient mixing and/or additional annealing processes. Sub-millimeter and
millimeter data indicate that most of the disk mass is in large grains (>1mm)Comment: 17 pages, 10 figures, 7 tables, accepted for A&
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