687 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
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
MG1-688432: A Peculiar Variable System
The short period variable star MG1-688432 has been discovered to exhibit
occasional extremely high energy optical outbursts as high as 10^31 joules.
Outbursts are typically of several hours duration. These events are often
highly structured, resembling sequential associated releases of energy. Twenty
years of time sequence photometry is presented, indicating a basic sinusoidal
light curve of mean period 6.65d, with some phase shifting and long-term
temporal trends in amplitude and mean brightness. Spectroscopy reveals a
peculiar star, best resembling a K3 subgiant that has evolved off the main
sequence moderately red-ward of the giant branch. Spectroscopic and radial
velocity analyses indicate a binary system orbiting its barycenter with an
unseen companion to the K3IV primary. This is not an eclipsing system with the
inclination of the orbit precluding eclipse by the secondary. The system is at
a distance of 1.5kpc and analysis of GAIA observations leads to the conclusion
that the HR diagram position of MG1-688432 is established by an intrinsic
feature of the system, most likely either the stellar evolutionary state of the
observed star or the presence of small (non-gray) dust within the system. Two
mechanisms that might give rise to the system are 1) impacts with tidally
disrupted planetary debris, and 2) magnetically induced chromospheric activity.
An intriguing idea that requires further investigation suggests that the unseen
companion is perhaps a white dwarf star which has encountered a planet and
tidally shredded it to produce a debris and dust veil that modulates the
brightness of the primary.Comment: 74 pages, 36 figures, submitted to Astrophysical Journal Supplemen
Surface structure and stability of the Larsen C ice shelf, Antarctic Peninsula
A structural glaciological description and analysis of surface morphological features of the Larsen C ice shelf, Antarctic Peninsula, is derived from satellite images spanning the period 1963-2007. The data are evaluated in two time ranges: a comparison of a 1963 satellite image photomosaic with a modern digital mosaic compiled using 2003/04 austral summer data; and an image series since 2003 showing recent evolution of the shelf. We map the ice-shelf edge, rift swarms, crevasses and crevasse traces, and linear longitudinal structures (called 'flow stripes' or 'streak lines'). The latter are observed to be continuous over distances of up to 200km from the grounding line to the ice-shelf edge, with little evidence of changes in pattern over that distance. Integrated velocity measurements along a flowline indicate that the shelf has been stable for similar to 560years in the mid-shelf area. Linear longitudinal features may be grouped into 12 units, each related to one or a small group of outlet feeder glaciers to the shelf. We observe that the boundaries between these flow units often mark rift terminations. The boundary zones originate upstream at capes, islands or other suture areas between outlet glaciers. In agreement with previous work, our findings imply that rift terminations within such suture zones indicate that they contain anomalously soft ice. We thus suggest that suture zones within the Larsen C ice shelf, and perhaps within ice shelves more generally, may act to stabilize them by reducing regional stress intensities and thus rates of rift lengthening
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
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
New determination of the mass of the eta meson at COSY-ANKE
A value for the mass of the eta meson has been determined at the COSY-ANKE
facility through the measurement of a set of deuteron laboratory beam momenta
and associated 3He center-of-mass momenta in the d+p -> 3He+X reaction. The eta
was then identified by the missing-mass peak and the production threshold
determined. The individual beam momenta were fixed with a relative precision of
3 x 10^-5 for values around 3 GeV/c by using a polarized deuteron beam and
inducing an artificial depolarizing spin resonance, which occurs at a
well-defined frequency. The final-state momenta in the two-body d+p -> 3He+eta
reaction were investigated in detail by studying the size of the 3He momentum
ellipse with the forward detection system of the ANKE spectrometer. Final
alignment of the spectrometer for this high precision experiment was achieved
through a comprehensive study of the 3He final-state momenta as a function of
the center-of-mass angles, taking advantage of the full geometrical acceptance.
The value obtained for the mass, m(eta)=(547.873 +- 0.005(stat) +- 0.027(syst))
MeV/c^2, is consistent and competitive with other recent measurements, in which
the meson was detected through its decay products.Comment: 11 pages, 11 figures, 3 tables, published versio
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
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