5,640 research outputs found
The tails in the Helix Nebula NGC 7293
We have examined a stream-source model for the production of the cometary
tails observed in the Helix Nebula NGC 7293 in which a transonic or moderately
supersonic stream of ionized gas overruns a source of ionized gas. Hydrodynamic
calculations reveal velocity structures which are in good agreement with the
observational data on tail velocities and are consistent with observations of
the nebular structure. The results also are indicative of a stellar atmosphere
origin for the cometary globules. Tail remnants persist for timescales long
enough for their identification with faint striations visible in the nebula gas
to be plausible.Comment: 7 pages, 6 figures, accepted for publication in A&
Renorming spaces with greedy bases
We study the problem of improving the greedy constant or the democracy
constant of a basis of a Banach space by renorming. We prove that every Banach
space with a greedy basis can be renormed, for a given \vare>0, so that the
basis becomes (1+\vare)-democratic, and hence (2+\vare)-greedy, with
respect to the new norm. If in addition the basis is bidemocratic, then there
is a renorming so that in the new norm the basis is (1+\vare)-greedy. We also
prove that in the latter result the additional assumption of the basis being
bidemocratic can be removed for a large class of bases. Applications include
the Haar systems in , , and in dyadic Hardy space ,
as well as the unit vector basis of Tsirelson space
The evaluation of a shuttle borne lidar experiment to measure the global distribution of aerosols and their effect on the atmospheric heat budget
A shuttle-borne lidar system is described, which will provide basic data about aerosol distributions for developing climatological models. Topics discussed include: (1) present knowledge of the physical characteristics of desert aerosols and the absorption characteristics of atmospheric gas, (2) radiative heating computations, and (3) general circulation models. The characteristics of a shuttle-borne radar are presented along with some laboratory studies which identify schemes that permit the implementation of a high spectral resolution lidar system
The Age of Cluster Galaxies from Continuum Colors
We determine the age of 1,104 early-type galaxies in eight rich clusters ( to ) using a new continuum color technique. We find that
galaxies in clusters divide into two populations, an old population with a mean
age similar to the age of the Universe (12 Gyrs) and a younger population with
a mean age of 9 Gyrs. The older population follows the expected relations for
mass and metallicity that imply a classic monolithic collapse origin. Although
total galaxy metallicity is correlated with galaxy mass, it is uncorrelated
with age. It is impossible, with the current data, to distinguish between a
later epoch of star formation, longer duration of star formation or late bursts
of star formation to explain the difference between the old and young
populations. However, the global properties of this younger population are
correlated with cluster environmental factors, which implies secondary
processes, post-formation epoch, operate on the internal stellar population of
a significant fraction of cluster galaxies. In addition, the mean age of the
oldest galaxies in a cluster are correlated with cluster velocity dispersion
implying that galaxy formation in massive clusters begins at earlier epochs
than less massive clusters.Comment: 35 pages, 10 figures, accepted by Ap
Coefficient Quantization for Frames in Banach Spaces
Let be a fundamental system of a Banach space.
We consider the problem of approximating linear combinations of elements of
this system by linear combinations using quantized coefficients. We will
concentrate on systems which are possibly redundant. Our model for this
situation will be frames in Banach spaces.Comment: 33 page
Traumatic rupture of the descending thoracic aorta
The management of acute traumatic rupture of the descending thoracic aorta at Groote Schuur Hospital between January 1984 and December 1989 is reviewed. Aortic rupture was diagnosed angiographically in 18 of 150 patients (12%), who underwent aortography because this injury was suspected. However. 3 of these patients had false-positive angiograms. The diagnosis was initially missed in 31% of patients, and this contributed to morbidity and mortality. Simple aortic crossclamping (N = 8) was used before September 1988 and 3 patients died - 1 intra-operatively from cardiac arrhythmia and 2 postoperatively, where major peri-operative haemorrhage had occurred. In contrast, partial heparin-less bypass (N = 5) using a centrifugal vortex pump was used after September 1988, and there were no haemorrhagic or paraplegic complications or mortality in this group. This technique is safe and appears to be superior to simple aortic crossclamping in managing this condition
- …