235 research outputs found
Site-selective mapping of metastable states using electron-beam induced luminescence microscopy
Metastable states created by electron or hole capture in crystal defects are
widely used in dosimetry and photonic applications. Feldspar, the most abundant
mineral in the Earth crust (>50%), generates metastable states with lifetimes
of millions of years upon exposure to ionizing radiation. Although feldspar is
widely used in dosimetry and geochronometry, the creation of metastable states
and charge transfer across them is poorly understood. Understanding such
phenomena requires next-generation methods based on high-resolution,
site-selective probing of the metastable states. Recent studies using
site-selective techniques such as photoluminescence (PL), and radioluminescence
(RL) at 7 K have revealed that feldspar exhibits two near-infrared (NIR)
emission bands peaking at 880 nm and 955 nm, which are believed to arise from
the principal electron-trapping states.
Here, we map for the first time the electron-trapping states in
potassium-rich feldspar using spectrally-resolved cathodoluminescence
microscopy at a spatial resolution of around 6 to 22 micrometer. Each pixel
probed by a scanning electron microscope provides us a cathodoluminescence
spectrum (SEM-CL) in the range 600-1000 nm, and elemental data from
energy-dispersive x-ray (EDX) spectroscopy. We conclude that the two NIR
emissions are spatially variable and, therefore, originate from different
sites. This conclusion contradicts the existing model that the two emissions
arise from two different excited states of a principal trap. Moreover, we are
able to link the individual NIR emission peaks with the geochemical variations
(K, Na and Fe concentration), and propose a cluster model that explains the
quenching of the NIR emission by Fe4+
A new species of Colostethus (Anura, Dendrobatidae) from French Guiana with a redescription of Colostethus beebei (Noble, 1923) from its type locality
A new species of Colostethus, long mistaken for Colostethus beebei, is described from French Guiana. The new species can be distinguished from congeners by absence of median lingual process, first finger longer than second, third finger not distinctly swollen in males, differences in tadpole morphology, coloration and pattern (e.g. absence of dorsolateral stripe), bioacoustics, and reproductive behavior. A complete redescription of Colostethus beebei plus description of its tadpole and call is provided on the basis of recently collected topotypic specimens. The range of C. beebei is restricted to the Kaieteur plateau, Pakaraima Mountains, Guyana
Water and ammonia abundances in S140 with the Odin satellite
We have used the Odin satellite to obtain strip maps of the ground-state
rotational transitions of ortho-water and ortho-ammonia, as well as CO(5-4) and
13CO(5-4) across the PDR, and H218O in the central position. A physi-chemical
inhomogeneous PDR model was used to compute the temperature and abundance
distributions for water, ammonia and CO. A multi-zone escape probability method
then calculated the level populations and intensity distributions. These
results are compared to a homogeneous model computed with an enhanced version
of the RADEX code. H2O, NH3 and 13CO show emission from an extended PDR with a
narrow line width of ~3 kms. Like CO, the water line profile is dominated by
outflow emission, however, mainly in the red wing. The PDR model suggests that
the water emission mainly arises from the surfaces of optically thick, high
density clumps with n(H2)>10^6 cm^-3 and a clump water abundance, with respect
to H2, of 5x10^-8. The mean water abundance in the PDR is 5x10^-9, and between
~2x10^-8 -- 2x10^-7 in the outflow derived from a simple two-level
approximation. Ammonia is also observed in the extended clumpy PDR, likely from
the same high density and warm clumps as water. The average ammonia abundance
is about the same as for water: 4x10^-9 and 8x10^-9 given by the PDR model and
RADEX, respectively. The similarity of water and ammonia PDR emission is also
seen in the almost identical line profiles observed close to the bright rim.
Around the central position, ammonia also shows some outflow emission although
weaker than water in the red wing. Predictions of the H2O(110-101) and
(111-000) antenna temperatures across the PDR are estimated with our PDR model
for the forthcoming observations with the Herschel Space Observatory.Comment: 13 pages, 14 figures, 10 tables. Accepted for publication in
Astronomy & Astrophysics 14 November 200
Physical Characteristics of a Dark Cloud in an Early Stage of Star Formation toward NGC 7538: an Outer Galaxy Infrared Dark Cloud?
In the inner parts of the Galaxy the Infrared Dark Clouds (IRDCs) are
presently believed to be the progenitors of massive stars and star clusters.
Many of them are predominantly devoid of active star formation and for now they
represent the earliest observed stages of massive star formation. Their Outer
Galaxy counterparts, if present, are not easily identified because of a low or
absent mid-IR background. We characterize the ambient conditions in the Outer
Galaxy IRDC candidate G111.80+0.58, a relatively quiescent molecular core
complex in the vicinity of NGC7538. We conduct molecular line observations on a
number of dense cores and analyze the data in terms of excitation temperature,
column and volume density, mass and stability. The temperatures (15-20K) are
higher than expected from only cosmic ray heating, but comparable to those
found in massive cores. Star forming activity could be present in some cores,
as indicated by the presence of warm gas and YSO candidates. The observed
super-thermal line-widths are typical for star forming regions. The velocity
dispersion is consistent with a turbulent energy cascade over the observed size
scales. We do not find a correlation between the gas temperature and the
line-width. The LTE masses we find are much larger than the thermal Jeans mass
and fragmentation is expected. In that case the observed lines represent the
combined emission of multiple unresolved components. We conclude that
G111.80+0.58 is a molecular core complex with bulk properties very similar to
IRDCs in an early, but not pristine, star forming state. The individual cores
are close to virial equilibrium and some contain sufficient material to form
massive stars and star clusters. The ambient conditions suggest that turbulence
is involved in supporting the cores against gravitational collapse.Comment: Accepted for publication in A&A -- 19 pages, 9 figures -- high
resolution available at
http://www.astro.rug.nl/~frieswyk/Data/Research/OGIRDC/index.htm
Line profiles of water for the photon dominated region and embedded sources in the S140 region
A radiative transfer method for the treatment of molecular lines is
presented. We apply this method to previous SWAS and ISO observations of water
vapor in the source S140 in order to make models to plan for, and to interpret,
HIFI data. Level populations are calculated with the use of a three-dimensional
(multi-zone) escape probability method and with a long characteristic code that
uses Monte Carlo techniques with fixed directions. Homogeneous and
inhomogeneous models are used to compute the differences between water line
profiles across the S140 region. We find that when an outflow or infall
velocity field with a gradient of a few kms^{-1} is adopted, line profiles with
a FWHM of 6 kms^{-1} are found, in agreement with observations. Inhomogeneous
models are favoured to produce a single-peaked line profile. When zooming in on
smaller regions within the PDR, the shapes of the line profiles start to differ
due to the different temperature and density distributions there. The embedded
sources are traced by high excitation lines of, e.g., 3_{21}-2_{21},
3_{03}-2_{12}, 2_{12}-1_{01} and 2_{20}-1_{11}. The computed intensities are
roughly consistent with existing ISO observations. Water emission in a PDR
source like S140 requires a combination of a pure PDR and an embedded source in
order to match the observations. Because of its good angular resolution, HIFI
will be able to distinguish between a dense star forming region or a more
diffuse gas component. It is therefore important for future observing programs
to consider both in their predictions of the emission characteristics of water
in these environments
Rotational Line Emission from Water in Protoplanetary Disks
Circumstellar disks provide the material reservoir for the growth of young
stars and for planet formation. We combine a high-level radiative transfer
program with a thermal-chemical model of a typical T Tauri star disk to
investigate the diagnostic potential of the far-infrared lines of water for
probing disk structure. We discuss the observability of pure rotational H2O
lines with the Herschel Space Observatory, specifically the residual gas where
water is mainly frozen out. We find that measuring both the line profile of the
ground 110-101 ortho-H2O transition and the ratio of this line to the 312-303
and 221-212 line can provide information on the gas phase water between 5-100
AU, but not on the snow line which is expected to occur at smaller radii.Comment: 5 pages, 4 figures. Accepted by ApJ
Radiative transfer models of mid-infrared H2O lines in the Planet-forming Region of Circumstellar Disks
The study of warm molecular gas in the inner regions of protoplanetary disks
is of key importance for the study of planet formation and especially for the
transport of H2O and organic molecules to the surfaces of rocky
planets/satellites. Recent Spitzer observations have shown that the
mid-infrared spectra of protoplanetary disks are covered in emission lines due
to water and other molecules. Here, we present a non-LTE 2D radiative transfer
model of water lines in the 10-36 mum range that can be used to constrain the
abundance structure of water vapor, given an observed spectrum, and show that
an assumption of local thermodynamic equilibrium (LTE) does not accurately
estimate the physical conditions of the water vapor emission zones. By applying
the model to published Spitzer spectra we find that: 1) most water lines are
subthermally excited, 2) the gas-to-dust ratio must be one to two orders of
magnitude higher than the canonical interstellar medium ratio of 100-200, and
3) the gas temperature must be higher than the dust temperature, and 4) the
water vapor abundance in the disk surface must be truncated beyond ~ 1 AU. A
low efficiency of water formation below ~ 300 K may naturally result in a lower
water abundance beyond a certain radius. However, we find that chemistry, may
not be sufficient to produce an abundance drop of many orders of magnitude and
speculate that the depletion may also be caused by vertical turbulent diffusion
of water vapor from the superheated surface to regions below the snow line,
where the water can freeze out and be transported to the midplane as part of
the general dust settling. Such a vertical cold finger effect is likely to be
efficient due to the lack of a replenishment mechanism of large, water-ice
coated dust grains to the disk surface.Comment: 12 pages, accepted for publication in Ap
Modeling water emission from low-mass protostellar envelopes
Within low-mass star formation, water vapor plays a key role in the chemistry
and energy balance of the circumstellar material. The Herschel Space
Observatory will open up the possibility to observe water lines originating
from a wide range of excitation energies.Our aim is to simulate the emission of
rotational water lines from envelopes characteristic of embedded low-mass
protostars. A large number of parameters that influence the water line emission
are explored: luminosity, density,density slope and water abundances.Both dust
and water emission are modelled using full radiative transfer in spherical
symmetry. The temperature profile is calculated for a given density profile.
The H2O level populations and emission profiles are in turn computed with a
non-LTE line code. The results are analyzed to determine the diagnostic value
of different lines, and are compared with existing observations. Lines can be
categorized in: (i) optically thick lines, including ground-state lines, mostly
sensitive to the cold outer part; (ii) highly excited (E_u>200-250 K) optically
thin lines sensitive to the abundance in the hot inner part; and (iii) lines
which vary from optically thick to thin depending on the abundances. Dust
influences the emission of water significantly by becoming optically thick at
the higher frequencies, and by pumping optically thin lines. A good physical
model of a source, including a correct treatment of dust, is a prerequisite to
infer the water abundance structure and possible jumps at the evaporation
temperature from observations. The inner warm (T>100 K) envelope can be probed
byhighly-excited lines, while a combination of excited and spectrally resolved
ground state lines probes the outer envelope. Observations of H218O lines,
although weak, provide even stronger constraints on abundances.Comment: 17 pages with an online appendix of 6 pages. Accepted by A&A. Several
figures are too large for astro-ph. These can be downloaded from
http://www.strw.leidenuniv.nl/~kempen/water.ph
Global ground strike point characteristics in negative downward lightning flashes – Part 2: Algorithm validation
At present the lightning flash density is a key input parameter for assessing the risk of occurrence of a lightning strike in a particular region of interest. Since it is known that flashes tend to have more than one ground termination point on average, the use of ground strike point densities as opposed to flash densities is more appropriate. Lightning location systems (LLSs) do not directly provide ground strike point densities. However, ingesting their observations into an algorithm that groups strokes into respective ground strike points results in the sought-after density value. The aim of this study is to assess the ability of three distinct ground strike point algorithms to correctly determine the observed ground-truth strike points. The output of the algorithms is tested against a large set of ground-truth observations taken from different regions around the world, including Austria, Brazil, France, Spain, South Africa and the United States of America. These observations are linked to the observations made by a local LLS in order to retrieve the necessary parameters of each lightning discharge, which serve as input for the algorithms. Median values of the separation distance between the first stroke in the flash and subsequent ground strike points are found to vary between 1.3 and 2.75 km. It follows that all three of the algorithms perform well, with success rates of up to about 90 % to retrieve the correct type of the strokes in the flash, i.e., whether the stroke creates a new termination point or follows a pre-existing channel. The most important factor that influences the algorithms' performance is the accuracy by which the strokes are located by the LLS. Additionally, it is shown that the strokes' peak current plays an important role, whereby strokes with a larger absolute peak current have a higher probability of being correctly classified compared to the weaker strokes.</p
Water abundances in high-mass protostellar envelopes: Herschel observations with HIFI
We derive the dense core structure and the water abundance in four massive
star-forming regions which may help understand the earliest stages of massive
star formation. We present Herschel-HIFI observations of the para-H2O 1_11-0_00
and 2_02-1_11 and the para-H2-18O 1_11-0_00 transitions. The envelope
contribution to the line profiles is separated from contributions by outflows
and foreground clouds. The envelope contribution is modelled using Monte-Carlo
radiative transfer codes for dust and molecular lines (MC3D and RATRAN), with
the water abundance and the turbulent velocity width as free parameters. While
the outflows are mostly seen in emission in high-J lines, envelopes are seen in
absorption in ground-state lines, which are almost saturated. The derived water
abundances range from 5E-10 to 4E-8 in the outer envelopes. We detect cold
clouds surrounding the protostar envelope, thanks to the very high quality of
the Herschel-HIFI data and the unique ability of water to probe them. Several
foreground clouds are also detected along the line of sight. The low H2O
abundances in massive dense cores are in accordance with the expectation that
high densities and low temperatures lead to freeze-out of water on dust grains.
The spread in abundance values is not clearly linked to physical properties of
the sources.Comment: 8 pages, 5 figures, accepted for publication the 15/07/2010 by
Astronomy&Astrophysics as a letter in the Herschel-HIFI special issu
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