16,339 research outputs found
Pressure dependent isotopic fractionation in the photolysis of formaldehyde-d<sub>2</sub>
The isotope effects in formaldehyde photolysis are the key link between the
δD of methane emissions and the δD of atmospheric in situ
hydrogen production. A few recent studies have suggested that a pressure
dependence in the isotopic fractionation can partly explain enrichment of
deuterium with altitude in the atmosphere. The mechanism and the extent of
this pressure dependency is, however, not adequately described. In the
present work D<sub>2</sub>CO and H<sub>2</sub>CO were photolyzed in a static reaction chamber
at bath gas pressures of 50, 200, 400, 600 and 1000 mbar; these experiments
compliment and extend our earlier work with HDCO vs. H<sub>2</sub>CO. The UV lamps
used for photolysis emit light at wavelengths that primarily dissociate
formaldehyde into molecular products, CO and H<sub>2</sub> or D<sub>2</sub>. The isotope
effect <i>k</i>(H<sub>2</sub>CO)/<i>k</i>(D<sub>2</sub>CO) = 3.16 ± 0.03 at 1000 mbar is in
good agreement with results from previous studies. Similarly to what was
previously shown for <i>k</i>(H<sub>2</sub>CO)/<i>k</i>(HDCO), the isotope effect decreased as
pressure decreased. In addition, a model was constructed using RRKM theory to
calculate the lifetime of excited formaldehyde on the <i>S</i><sub>0</sub> surface, to
investigate its role in the observed pressure dependent photolytic
fractionation of deuterium. The model shows that part of the fractionation is
a result of competition between the isotopologue dependent rates of
unimolecular dissociation and collisional relaxation. We suggest that the
remaining fractionation is due to isotope effects in the rate of the
non-radiative transition from <i>S</i><sub>1</sub> to <i>S</i><sub>0</sub>, which are not considered in the
present model
Novel self-assembled morphologies from isotropic interactions
We present results from particle simulations with isotropic medium range
interactions in two dimensions. At low temperature novel types of aggregated
structures appear. We show that these structures can be explained by
spontaneous symmetry breaking in analytic solutions to an adaptation of the
spherical spin model. We predict the critical particle number where the
symmetry breaking occurs and show that the resulting phase diagram agrees well
with results from particle simulations.Comment: 4 pages, 4 figure
Estimating the marginal maintenance cost of rail infrastructure usage in Sweden; does more data make a difference?
This paper updates knowledge about the marginal cost of railway maintenance. Using a panel dataset comprising 16 years, we test whether more data makes a difference to conclusions. In contrast to previous estimates using a shorter panel, maintenance costs are now demonstrated to exhibit a positive dynamic effect; an increase in maintenance cost during one year indicates the need for more maintenance also the next year. Moreover, the marginal cost from the dynamic model is larger than its static counterpart. We conclude that the use of dynamic models on longer time series may have charging implications in several EU member states, considering that their track access charges are based on econometric studies that use static models and short panel datasets
Stellar properties of z ~ 1 Lyman-break galaxies from ACS slitless grism spectra
Lyman-break galaxies are now regularly found in the high redshift Universe by
searching for the break in the galaxy spectrum caused by the Lyman-limit
redshifted into the optical or even near-IR. At lower redshift, this break is
covered by the GALEX UV channels and small samples of z ~ 1 LBGs have been
presented in the literature. Here we give results from fitting the spectral
energy distributions of a small sub-set of low redshift LBGs and demonstrate
the advantage of including photometric points derived from HST ACS slitless
grism observations. The results show these galaxies to have very young, star
forming populations, while still being massive and dusty. LBGs at low and high
redshift show remarkable similarities in their properties, indicating that the
LBG selection method picks similar galaxies throughout the Universe.Comment: 7 pages, 3 figures, 2 tables, accepted in A&
The tensor part of the Skyrme energy density functional. I. Spherical nuclei
We perform a systematic study of the impact of the J^2 tensor term in the
Skyrme energy functional on properties of spherical nuclei. In the Skyrme
energy functional, the tensor terms originate both from zero-range central and
tensor forces. We build a set of 36 parameterizations, which covers a wide
range of the parameter space of the isoscalar and isovector tensor term
coupling constants, with a fit protocol very similar to that of the successful
SLy parameterizations. We analyze the impact of the tensor terms on a large
variety of observables in spherical mean-field calculations, such as the
spin-orbit splittings and single-particle spectra of doubly-magic nuclei, the
evolution of spin-orbit splittings along chains of semi-magic nuclei, mass
residuals of spherical nuclei, and known anomalies of charge radii. Our main
conclusion is that the currently used central and spin-orbit parts of the
Skyrme energy density functional are not flexible enough to allow for the
presence of large tensor terms.Comment: 38 pages, 36 figures; Minor correction
Localization and clustering in the nuclear Fermi liquid
Using the framework of nuclear energy density functionals we examine the
conditions for single-nucleon localization and formation of cluster structures
in finite nuclei. We propose to characterize localization by the ratio of the
dispersion of single-nucleon wave functions to the average inter-nucleon
distance. This parameter generally increases with mass and describes the
gradual transition from a hybrid phase in light nuclei, characterized by the
spatial localization of individual nucleon states that leads to the formation
of cluster structures, toward the Fermi liquid phase in heavier nuclei. Values
of the localization parameter that correspond to a crystal phase cannot occur
in finite nuclei. Typical length and energy scales in nuclei allow the
formation of liquid drops, clusters, and halo structures.Comment: 6 pages, 3 figure
A filtering approach for PET and PG predictions in a proton treatment planning system
Positron emission tomography (PET) and prompt gamma (PG) detection are promising proton therapy monitoring modalities. Fast calculation of the expected distributions is desirable for comparison to measurements and to develop/train algorithms for automatic treatment error detection. A filtering formalism was used for positron-emitter predictions and adapted to allow for its use for the beamline of any proton therapy centre. A novel approach based on a filtering formalism was developed for the prediction of energy-resolved PG distributions for arbitrary tissues. The method estimates PG yields and their energy spectra in the entire treatment field. Both approaches were implemented in a research version of the RayStation treatment planning system. The method was validated against PET monitoring data and Monte Carlo simulations for four patients treated with scanned proton beams. Longitudinal shifts between profiles from analytical and Monte Carlo calculations were within -1.7 and 0.9 mm, with maximum standard deviation of 0.9 mm and 1.1 mm, for positron-emitters and PG shifts, respectively. Normalized mean absolute errors were within 1.2 and 5.3%. When comparing measured and predicted PET data, the same more complex case yielded an average shift of 3 mm, while all other cases were below absolute average shifts of 1.1 mm. Normalized mean absolute errors were below 7.2% for all cases. A novel solution to predict positron-emitter and PG distributions in a treatment planning system is proposed, enabling calculation times of only a few seconds to minutes for entire patient cases, which is suitable for integration in daily clinical routine
On the dependence between UV luminosity and Lyman-alpha equivalent width in high redshift galaxies
We show that with the simple assumption of no correlation between the
Ly-alpha equivalent width and the UV luminosity of a galaxy, the observed
distribution of high redshift galaxies in an equivalent width - absolute UV
magnitude plane can be reproduced. We further show that there is no dependence
between Ly-alpha equivalent width and Ly-alpha luminosity in a sample of
Ly-alpha emitters. The test was expanded to Lyman-break galaxies and again no
dependence was found. Simultaneously, we show that a recently proposed lack of
large equivalent width, UV bright galaxies (Ando et al. 2006) can be explained
by a simple observational effect, based on too small survey volumes.Comment: 7 pages, 3 figures, 2 tables, accepted in MNRA
PKS 0537-441: extended [O II] emission and a binary QSO?
We present high-resolution imaging and low-resolution spectroscopy of the BL
Lac object PKS 0537-441 (z = 0.893) and its environment. The observations were
designed to clarify, whether the properties of PKS 0537-441 are affected by
gravitational microlensing, or whether PKS 0537-441 and its environment act as
a lensing system itself. Our observations show that neither case seems to be
likely. We did not find a galaxy along the line-of-sight to the BL Lac as
claimed previously, our spectroscopy shows that none of the four closest
companion galaxies is at high redshift. Two of the four nearby companion
galaxies to PKS 0537-441 are within 200 km/s of the systemic velocity of the BL
Lac (z = 0.892 and 0.895, respectively). The third companion galaxy is at
higher redshift (z = 0.947). The fourth companion galaxy shows evidence of Mg
II absorption redwards of its systemic velocity and is perhaps a mini low
ionization BAL QSO at z = 0.885. If the latter can be confirmed, PKS 0537-441
is the first BL Lacertae object being a member of a binary Quasar. We also
detected extended [O II] emission in the off-nuclear spectrum of PKS 05371-441,
which is most likely due to photoionization from the active nucleus.
Alternatively, the extended [O II] emission is due to jet-cloud interaction
with the counterjet of PKS 0537-441. Our clustering analysis indicates that PKS
0537-441 is located in a cluster environment as rich as Abell type 0-1. This is
supported by the detection of four galaxies in the field with similar redshifts
as the BL Lac (Delta z < 0.002). We found serendipitously even more galaxies at
somewhat higher redshifts (z = 0.9-1). Thus, PKS0537-441 might be located in
front of a galaxy cluster at somewhat higher redshift or even be part of a
large-scale structure with an extension towards the BL Lac.Comment: 13 pages, 9 figures, accepted for publication in A&
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