565 research outputs found
Observations of stratospheric source gas profiles during the Arctic winter
An international campaign was performed at ESRANGE rocket base, near Kiruna, Sweden (68 N) from January 4 to February 15 in order to investigate the Chemistry of Ozone in the Polar Stratosphere (CHEOPS). Within the framework of this campaign two sets of large stratospheric air samples were collected by means of a balloon borne cryogenic air sampler. The two balloons were launched on February 1, and February 10, 1988. At present the samples are analyzed in our laboratory for their contents of several long lived trace gases such as CH4, N2O, H2, CO2, CO and the major halocarbons CH3Cl, CFCl3, CF2Cl2, CCl4, CH3CCl3, and C2F3Cl3. The vertical profiles derived from these samples will be presented and compared with previous observations made in February 1987. The data will be discussed in view of the dynamical evolution of the Arctic polar vortex during this winter
Uppermost crustal structure regulates the flow of the Greenland Ice Sheet
The flow of the Greenland Ice Sheet is controlled by subglacial processes and conditions that depend on the geological provenance and temperature of the crust beneath it, neither of which are adequately known. Here we present a seismic velocity model of the uppermost 5 km of the Greenlandic crust. We show that slow velocities in the upper crust tend to be associated with major outlet glaciers along the ice-sheet margin, and elevated geothermal heat flux along the Iceland hotspot track inland. Outlet glaciers particularly susceptible to basal slip over deformable subglacial sediments include Jakobshavn, Helheim and Kangerdlussuaq, while geothermal warming and softening of basal ice may affect the onset of faster ice flow at Petermann Glacier and the Northeast Greenland Ice Stream. Interactions with the solid earth therefore control the past, present and future dynamics of the Greenland Ice Sheet and must be adequately explored and implemented in ice sheet models
Functional approach to the non-mesonic decay of Lambda-hypernuclei
We present an evaluation of the non-mesonic decay widths for
Lambda-hypernuclei (Lambda N --> NN, Lambda NN --> NNN) within the framework of
the polarization propagator method. The full Lambda self-energy is evaluated
microscopically in nuclear matter by using the functional approach, which
supplies a theoretically well grounded approximation scheme for the
classification of the relevant diagrams, according to the prescriptions of the
bosonic loop expansion. We employ average Fermi momenta, suitably adapted to
different mass number regions (medium-light, medium and heavy hypernuclei).
Moreover, we study the dependence of the decay rates on the NN and Lambda-N
short range correlations. With a proper choice of the parameters which control
these correlations in the new approximation scheme, it is possible to reproduce
the experimental decay widths for A > 10 hypernuclei.Comment: 25 pages, 8 figure
Deuteron-deuteron collision at 160 MeV
The experiment was carried out using BINA detector at KVI in Groningen. For
the first time an extensive data analysis of the data collected in back part of
the detector is presented, where a clusterization method is utilized for
angular and energy information. We also present differential cross-sections for
the (dddpn) breakup reaction within \textit{dp} quasi-free
scattering limit and their comparison with first calculations based on Single
Scattering Approximation (SSA) approach.Comment: 6 pages, 4 figures, presented at Jagiellonian Symposium 2015 in
Krakow, PhD wor
Difficulties with Recovering The Masses of Supermassive Black Holes from Stellar Kinematical Data
We investigate the ability of three-integral, axisymmetric, orbit-based
modeling algorithms to recover the parameters defining the gravitational
potential (M/L ratio and black hole mass Mh) in spheroidal stellar systems
using stellar kinematical data. We show that the potential estimation problem
is generically under-determined when applied to long-slit kinematical data of
the kind used in most black hole mass determinations to date. A range of
parameters (M/L, Mh) can provide equally good fits to the data, making it
impossible to assign best-fit values. We illustrate the indeterminacy using a
variety of data sets derived from realistic models as well as published
observations of the galaxy M32. In the case of M32, our reanalysis demonstrates
that data published prior to 2000 are equally consistent with Mh in the range
1.5x10^6-5x10^6 solar masses, with no preferred value in that range. While the
HST/STIS data for this galaxy may overcome the degeneracy in Mh, HST data for
most galaxies do not resolve the black hole's sphere of influence and in these
galaxies the degree of degeneracy allowed by the data may be substantial. We
investigate the effect on the degeneracy of enforcing smoothness
(regularization) constraints. However we find no indication that the true
potential can be recovered simply by enforcing smoothness. For a given
smoothing level, all solutions in the minimum-chisquare valley exhibit similar
levels of noise. These experiments affirm that the indeterminacy is real and
not an artifact associated with non-smooth solutions. (Abridged)Comment: Accepted for publication in The Astrophysical Journal. Changes
include discussion of regularizatio
Fragmentation of exotic oxygen isotopes
Abrasion-ablation models and the empirical EPAX parametrization of projectile fragmentation are described. Their cross section predictions are compared to recent data of the fragmentation of secondary beams of neutron-rich, unstable 19,20,21O isotopes at beam energies near 600 MeV/nucleon as well as data for stable 17,18O beams
The extended tails of Palomar 5: A ten degree arc of globular cluster tidal debris
Using wide-field photometric data from the Sloan Digital Sky Survey (SDSS) we
recently showed that the Galactic globular cluster Palomar 5 is in the process
of being tidally disrupted. Its tidal tails were initially detected in a 2.5
degree wide band along the celestial equator. A new analysis of SDSS data for a
larger field now reveals that the tails of Pal 5 have a much larger spatial
extent and can be traced over an arc of 10 deg across the sky, corresponding to
a projected length of 4 kpc at the distance of the cluster. The number of
former cluster stars found in the tails adds up to about 1.2 times the number
of stars in the cluster. The radial profile of stellar surface density in the
tails follows approximately a power law r^gamma with -1.5 < gamma < -1.2.
The stream of debris from Pal 5 is significantly curved, which demonstrates
its acceleration by the Galactic potential. The cluster is presently near the
apocenter but has repeatedly undergone disk crossings in the inner part of the
Galaxy leading to strong tidal shocks. Our results suggest that the observed
debris originates mostly from mass loss within the last 2 Gyrs. The cluster is
likely to be destroyed after the next disk crossing, which will happen in about
100 Myr. (abridged)Comment: 44 pages, including 14 figures (Figs.1,3 & 14 with decreased
resolution), accepted for publication in the Astronomical Journa
Linking the Metallicity Distribution of Galactic Halo Stars to the Enrichment History of the Universe
We compare the metallicity distribution of Galactic Halo stars with 3D
realizations of hierarchical galaxy formation. Outflows from dwarf galaxies
enrich the intergalactic medium inhomogeneously, at a rate depending on the
local galaxy density. Consequently, the first stars created in small
early-forming galaxies are less metal-rich that the first stars formed in more
massive galaxies which typically form later. As most halo stars are likely to
originate in accreted dwarfs, while disk stars formed out of outflow-enriched
gas, this scenario naturally generates a ``metallicity floor'' for old disk
stars, which we find to be roughly coincident with the higher end of our
predicted metallicity distribution of halo stars, in agreement with
observations. The broad and centrally peaked distribution of halo star
metallicities is well reproduced in our models, with a natural dispersion
depending on the exact accretion history. Our modeling includes the important
``baryonic stripping'' effect of early outflows, which brush away the tenuously
held gas in neighboring pre-virialized density perturbations. This stripping
process does not significantly modify the predicted shape of the halo star
metal distribution but inhibits star-formation and hence the number of accreted
stars, helping to reproduce the observed total Galactic halo luminosity and
also the lack of low-luminosity local dwarf galaxies relative to N-body
predictions.Comment: 5 pages, 1 figure, ApJ Letters, accepte
Measurement of the analyzing powers in pd elastic and pn quasi-elastic scattering at small angles
The analyzing powers in proton-deuteron elastic and proton-neutron
quasi-elastic scattering have been measured at small angles using a polarized
proton beam at the COSY storage ring incident on an unpolarized deuterium
target. The data were taken at 796MeV and five higher energies from 1600MeV to
2400MeV. The analyzing power in pd elastic scattering was studied by detecting
the low energy recoil deuteron in telescopes placed symmetrically in the COSY
plane to the left and right of the beam whereas for pn quasi-elastic scattering
a low energy proton was registered in one of the telescopes in coincidence with
a fast scattered proton measured in the ANKE magnetic spectrometer. Though the
experiment explores new domains, the results are consistent with the limited
published information.Comment: 10 pages with 8 figure
Low-energy cross section of the 7Be(p,g)8B solar fusion reaction from Coulomb dissociation of 8B
Final results from an exclusive measurement of the Coulomb breakup of 8B into
7Be+p at 254 A MeV are reported. Energy-differential Coulomb-breakup cross
sections are analyzed using a potential model of 8B and first-order
perturbation theory. The deduced astrophysical S_17 factors are in good
agreement with the most recent direct 7Be(p,gamma)8B measurements and follow
closely the energy dependence predicted by the cluster-model description of 8B
by Descouvemont. We extract a zero-energy S_17 factor of 20.6 +- 0.8 (stat) +-
1.2 (syst) eV b.Comment: 14 pages including 16 figures, LaTeX, accepted for publication in
Physical Review C. Minor changes in text and layou
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