349 research outputs found
The role of light microscopy in aerospace analytical laboratories
Light microscopy has greatly reduced analytical flow time and added new dimensions to laboratory capability. Aerospace analytical laboratories are often confronted with problems involving contamination, wear, or material inhomogeneity. The detection of potential problems and the solution of those that develop necessitate the most sensitive and selective applications of sophisticated analytical techniques and instrumentation. This inevitably involves light microscopy. The microscope can characterize and often identify the cause of a problem in 5-15 minutes with confirmatory tests generally less than one hour. Light microscopy has and will make a very significant contribution to the analytical capabilities of aerospace laboratories
Particle types and sources associated with LDEF
The particulate contamination history of the Long Duration Exposure Facility (LDEF) can be resolved by careful analysis of particle types, the LDEF time line, evidence of the relationship between particles and the surface of the LDEF, and a consideration of probable sources. This work is far from complete but was initiated as part of the characterization of the condition of experimental trays that were returned to principle investigators for their analysis. The work presented in this photo-essay is continuing and will be updated in subsequent reports to NASA and at future technical meetings
Molecular films associated with LDEF
The molecular films deposited on the surface of the Long Duration Exposure Facility (LDEF) originated from the paints and room-temperature-vulcanized (RTV) silicone materials intentionally used on the satellite and not from residual contaminants. The high silicone content of most of the films and the uniformity of the films indicates a homogenization process in the molecular deposition and suggests a chemically most favored composition for the final film. The deposition on interior surfaces and vents indicated multiple bounce trajectories or repeated deposition-reemission cycles. Exterior surface deposits indicated a significant return flux. Ultraviolet light exposure was required to fix the deposited film as is indicated by the distribution of the films on interior surfaces and the thickness of films at the vent locations. Thermal conditions at the time of exposure to ultraviolet light seems to be an important factor in the thickness of the deposit. Sunrise facing (ram direction) surfaces always had the thicker film. These were the coldest surfaces at the time of their exposure to ultraviolet light. The films have a layered structure suggesting cyclic deposition. As many as 34 distinct layers were seen in the films. The cyclic nature of the deposition and the chemical uniformity of the film one layer to the next suggest an early deposition of the films though there is evidence for the deposition of molecular films throughout the nearly six year exposure of the satellite. A final 'spray' of an organic material associated with water soluble salts occurred very late in the mission. This may have been the result of one of the shuttle dump activities
Quantification of contaminants associated with LDEF
The quantification of contaminants on the Long Duration Exposure Facility (LDEF) and associated hardware or tools is addressed. The purpose of this study was to provide a background data base for the evaluation of the surface of the LDEF and the effects of orbital exposure on that surface. This study necessarily discusses the change in the distribution of contaminants on the LDEF with time and environmental exposure. Much of this information may be of value for the improvement of contamination control procedures during ground based operations. The particulate data represents the results of NASA contractor monitoring as well as the results of samples collected and analyzed by the authors. The data from the tapelifts collected in the Space Shuttle Bay at Edwards Air Force Base and KSC are also presented. The amount of molecular film distributed over the surface of the LDEF is estimated based on measurements made at specific locations and extrapolated over the surface area of the LDEF. Some consideration of total amount of volatile-condensible materials available to form the resultant deposit is also presented. All assumptions underlying these estimates are presented along with the rationale for the conclusions. Each section is presented in a subsection for particles and another for molecular films
Migration and generation of contaminants from launch through recovery: LDEF case history
It is possible to recreate the contamination history of the Long Duration Exposure Facility (LDEF) through an analysis of its contaminants and selective samples that were collected from surfaces with better documented exposure histories. This data was then used to compare estimates based on monitoring methods that were selected for the purpose of tracking LDEF's exposure to contaminants. The LDEF experienced much more contamination than would have been assumed based on the monitors. Work is still in progress but much of what was learned so far is already being used in the selection of materials and in the design of systems for space. Now experiments are being prepared for flight to resolve questions created by the discoveries on the LDEF. A summary of what was learned about LDEF contaminants over the first year since recovery and deintegration is presented. Over 35 specific conclusions in 5 contamination related categories are listed
VLA OH and H I Zeeman Observations of the NGC 6334 Complex
We present OH and H I Zeeman observations of the NGC 6334 complex taken with
the Very Large Array. The OH absorption profiles associated with the complex
are relatively narrow (del-v_FWHM ~ 3 km s^1) and single-peaked over most of
the sources. The H I absorption profiles contain several blended velocity
components. One of the compact continuum sources in the complex (source A) has
a bipolar morphology. The OH absorption profiles toward this source display a
gradient in velocity from the northern continuum lobe to the southern continuum
lobe; this velocity gradient likely indicates a bipolar outflow of molecular
gas from the central regions to the northern and southern lobes. Magnetic
fields of the order of 200 microG have been detected toward three discrete
continuum sources in the complex. Virial estimates suggest that the detected
magnetic fields in these sources are of the same order as the critical magnetic
fields required to support the molecular clouds associated with the sources
against gravitational collapse.Comment: 14 pages, 9 postscript figures, accepted for publication in the
Astrophysical Journal (ApJ), tentatively scheduled for vol. 533, Apr. 20,
2000; also available at
http://www.pa.uky.edu/~sarma/RESEARCH/aps_research.htm
A search for interstellar molecules in the spectra of highly reddened stars
A total of ten stars were observed with cameras of the International Ultraviolet Explorer (IUE) in both high and low dispersion. One star, X Persei (HD 24534, 6.0 BE), was analyzed in detail. Ultraviolet observations of the column densities of CO match those derived from the radio to within a factor of 4, with the difference probably due to the larger beam size of the radio measurement and the assumption of a thermal population in the rotational levels of CO. Upper limits are given to the log column densities for OH, HCl, and CH2 of 14.0, 12.3 and 12.8. The carbon abundance was found to be about solar with a possible depletion of about a factor of 2. With precautions concerning both noise and correct background, the IUE can be used for studies of interstellar molecules
Collapse of Turbulent Cores and Reconnection Diffusion
For a molecular cloud clump to form stars some transport of magnetic flux is
required from the denser, inner regions to the outer regions of the cloud,
otherwise this can prevent the collapse. Fast magnetic reconnection which takes
place in the presence of turbulence can induce a process of reconnection
diffusion (RD). Extending earlier numerical studies of reconnection diffusion
in cylindrical clouds, we consider more realistic clouds with spherical
gravitational potentials and also account for the effects of the gas
self-gravity. We demonstrate that within our setup RD is efficient. We have
also identified the conditions under which RD becomes strong enough to make an
initially subcritical cloud clump supercritical and induce its collapse. Our
results indicate that the formation of a supercritical core is regulated by a
complex interplay between gravity, self-gravity, the magnetic field strength
and nearly transonic and trans-Alfv\'enic turbulence, confirming that RD is
able to remove magnetic flux from collapsing clumps, but only a few of them
become nearly critical or supercritical, sub-Alfv\'enic cores, which is
consistent with the observations. Besides, we have found that the supercritical
cores built up in our simulations develop a predominantly helical magnetic
field geometry which is also consistent with observations. Finally, we have
evaluated the effective values of the turbulent reconnection diffusion
coefficient and found that they are much larger than the numerical diffusion,
especially for initially trans-Alfv\'enic clouds, ensuring that the detected
magnetic flux removal is due to to the action of the RD rather than to
numerical diffusivity.Comment: 24 pages, 18 figures, accepted for publication in the Ap
Statistical analysis of the mass-to-flux ratio in turbulent cores: effects of magnetic field reversals and dynamo amplification
We study the mass-to-flux ratio (M/\Phi) of clumps and cores in simulations
of supersonic, magnetohydrodynamical turbulence for different initial magnetic
field strengths. We investigate whether the (M/\Phi)-ratio of core and
envelope, R = (M/\Phi)_{core}/(M/\Phi)_{envelope} can be used to distinguish
between theories of ambipolar diffusion and turbulence-regulated star
formation. We analyse R for different Lines-of-Sight (LoS) in various sub-cubes
of our simulation box. We find that, 1) the average and median values of |R|
for different times and initial magnetic field strengths are typically greater,
but close to unity, 2) the average and median values of |R| saturate at average
values of |R| ~ 1 for smaller magnetic fields, 3) values of |R| < 1 for small
magnetic fields in the envelope are caused by field reversals when turbulence
twists the field lines such that field components in different directions
average out. Finally, we propose two mechanisms for generating values |R| ~< 1
for the weak and strong magnetic field limit in the context of a turbulent
model. First, in the weak field limit, the small-scale turbulent dynamo leads
to a significantly increased flux in the core and we find |R| ~< 1. Second, in
the strong field limit, field reversals in the envelope also lead to values |R|
~< 1. These reversals are less likely to occur in the core region where the
velocity field is more coherent and the internal velocity dispersion is
typically subsonic.Comment: 12 pages, 8 figures, accepted for publication in MNRA
From the warm magnetized atomic medium to molecular clouds
{It has recently been proposed that giant molecular complexes form at the
sites where streams of diffuse warm atomic gas collide at transonic
velocities.} {We study the global statistics of molecular clouds formed by
large scale colliding flows of warm neutral atomic interstellar gas under ideal
MHD conditions. The flows deliver material as well as kinetic energy and
trigger thermal instability leading eventually to gravitational collapse.} {We
perform adaptive mesh refinement MHD simulations which, for the first time in
this context, treat self-consistently cooling and self-gravity.} {The clouds
formed in the simulations develop a highly inhomogeneous density and
temperature structure, with cold dense filaments and clumps condensing from
converging flows of warm atomic gas. In the clouds, the column density
probability density distribution (PDF) peaks at \sim 2 \times 10^{21} \psc
and decays rapidly at higher values; the magnetic intensity correlates weakly
with density from to 10^4 \pcc, and then varies roughly as
for higher densities.} {The global statistical properties of such
molecular clouds are reasonably consistent with observational determinations.
Our numerical simulations suggest that molecular clouds formed by the
moderately supersonic collision of warm atomic gas streams.}Comment: submitted to A&
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