1,980 research outputs found
Self-Consistent Analysis of OH-Zeeman Observations: Too Much Noise about Noise
We had recently re-analyzed in a self-consistent way OH-Zeeman observations
in four molecular-cloud envelopes and we had shown that, contrary to claims by
Crutcher et al., there is no evidence that the mass-to-flux ratio decreases
from the envelopes to the cores of these clouds. The key difference between our
data analysis and the earlier one by Crutcher et al. is the relaxation of the
overly restrictive assumption made by Crutcher et al, that the magnetic field
strength is independent of position in each of the four envelopes. In a more
recent paper, Crutcher et al. (1) claim that our analysis is not
self-consistent, in that it misses a cosine factor, and (2) present new
arguments to support their contention that the magnetic-field strength is
indeed independent of position in each of the four envelopes. We show that the
claim of the missing cosine factor is false, that the new arguments contain
even more serious problems than the Crutcher et al. original data analysis, and
we present new observational evidence, independent of the OH-Zeeman data, that
suggests significant variations in the magnetic-field strength in the four
cloud envelopes.Comment: 8 pages, 3 figures, MNRAS in pres
Testing Magnetic Star Formation Theory
We report here observations of the Zeeman effect in the 18-cm lines of OH in
the envelope regions surrounding four molecular cloud cores toward which
detections of B(LOS) have been achieved in the same lines, and evaluate the
ratio of mass to magnetic flux, M/Phi, between the cloud core and envelope.
This relative M/Phi measurement reduces uncertainties in previous studies, such
as the angle between B and the line of sight and the value of [OH/H]. Our
result is that for all four clouds, the ratios R of the core to the envelope
values of M/Phi are less than 1. Stated another way, the ratios R' of the core
to the total cloud M/Phi are less than 1. The extreme case or idealized (no
turbulence) ambipolar diffusion theory of core formation requires the ratio of
the central to total M/Phi to be approximately equal to the inverse of the
original subcritical M/Phi, or R' > 1. The probability that all four of our
clouds have R' > 1 is 3 x 10^{-7}; our results are therefore significantly in
contradiction with the hypothesis that these four cores were formed by
ambipolar diffuson. Highly super-Alfvenic turbulent simulations yield a wide
range of relative M/Phi, but favor a ratio R < 1, as we observe. Our experiment
is limited to four clouds, and we can only directly test the predictions of the
extreme-case "idealized" models of ambipolar-diffusion driven star formation
that have a regular magnetic field morphology. Nonetheless, our experimental
results are not consistent with the "idealized" strong field, ambipolar
diffusion theory of star formation.Comment: 30 pages, 6 figures; paper revised after journal review, now accepted
by Ap
Self-Consistent Analysis of OH Zeeman Observations
Crutcher, Hakobian, and Troland (2009) used OH Zeeman observations of four
nearby molecular dark clouds to show that the ratio of mass to magnetic flux
was smaller in the ~0.1 pc cores than in the ~1 pc envelopes, in contradiction
to the prediction of ambipolar diffusion driven core formation. A crucial
assumption was that the magnetic field direction is nearly the same in the
envelope and core regions of each cloud. Mouschovias and Tassis (2009) have
argued that the data are not consistent with this assumption, and presented a
new analysis that changes the conclusions of the study. Here we show that the
data are in fact consistent with the nearly uniform field direction assumption;
hence, the original study is internally self-consistent and the conclusions are
valid under the assumptions that were made. We also show that the Mouschovias
and Tassis model of magnetic fields in cloud envelopes is inconsistent with
their own analysis of the data. However, the data do not rule out a more
complex field configuration that future observations may discern.Comment: 3 pages, 1 figure, accepted for publication by MNRAS Letter
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
Materials SIG quantification and characterization of surface contaminants
When LDEF entered orbit its cleanliness was approximately a MIL-STD-1246B Level 2000C. Its burden of contaminants included particles from every part of its history including a relatively small contribution from the shuttle bay itself. Although this satellite was far from what is normally considered clean in the aerospace industry, contaminating events in orbit and from processing after recovery were easily detected. The molecular contaminants carried into orbit were dwarfed by the heavy deposition of UV polymerized films from outgassing urethane paints and silicone based materials. Impacts by relatively small objects in orbit could create particulate contaminants that easily dominated the particle counts within a centimeter of the impact site. During the recovery activities LDEF was 'sprayed' with a liquid high in organics and water soluble salts. With reentry turbulence, vibration, and gravitational loading particulate contaminants were redistributed about LDEF and the shuttle bay
Contamination on LDEF: Sources, distribution, and history
An introduction to contamination effects observed on the Long Duration Exposure Facility (LDEF) is presented. The activities reported are part of Boeing's obligation to the LDEF Materials Special Investigation Group. The contamination films and particles had minimal influence on the thermal performance of the LDEF. Some specific areas did have large changes in optical properties. Films also interfered with recession rate determination by reacting with the oxygen or physically shielding underlying material. Generally, contaminant films lessen the measured recession rate relative to 'clean' surfaces. On orbit generation of particles may be an issue for sensitive optics. Deposition on lenses may lead to artifacts on photographic images or cause sensors to respond inappropriately. Particles in the line of sight of sensors can cause stray light to be scattered into sensors. Particles also represent a hazard for mechanisms in that they can physically block and/or increase friction or wear on moving surfaces. LDEF carried a rather complex mixture of samples and support hardware into orbit. The experiments were assembled under a variety of conditions and time constraints and stored for up to five years before launch. The structure itself was so large that it could not be baked after the interior was painted with chemglaze Z-306 polyurethane based black paint. Any analysis of the effects of molecular and particulate contamination must account for a complex array of sources, wide variation in processes over time, and extreme variation in environment from ground to launch to flight. Surface conditions at certain locations on LDEF were established by outgassing of molecular species from particular materials onto adjacent surfaces, followed by alteration of those species due to exposure to atomic oxygen and/or solar radiation
First Interstellar HCO Maser
A previously unseen maser in the J = 1 - 0 transition of HCO has been
detected by the Combined Array for Millimeter-wave Astronomy (CARMA). A
sub-arcsecond map was produced of the 2 arcmin region around DR21(OH),
which has had previous detections of OH and methanol masers. This new object
has remained undetected until now due to its extremely compact size. The object
has a brightness temperature of 2500 K and a FWHM linewidth of 0.497 km
s, both of which suggest non-thermal line emission consistent with an
unsaturated maser. This object coincides in position and velocity with the
methanol maser named DR21(OH)-1 by \citet{plambeck90}. No compact HCO
emission was present in the CARMA data towards the other methanol masers
described in that paper. These new results support the theory introduced in
\citet{plambeck90} that these masers likely arise from strong outflows
interacting with low mass, high density pockets of molecular gas. This is
further supported by recent observations of a CO outflow by \citet{zapata12}
that traces the outflow edges and confirms that the maser position lies along
the edge of the outflow where interaction with molecular tracers can occur.Comment: 4 pages, 3 figures, accepted by ApJL on September 10, 201
Assessing the Feasibility of Cosmic-Ray Acceleration by Magnetic Turbulence at the Galactic Center
The presence of relativistic particles at the center of our galaxy is
evidenced by the diffuse TeV emission detected from the inner
of the Galaxy. Although it is not yet entirely clear whether the origin of the
TeV photons is due to hadronic or leptonic interactions, the tight correlation
of the intensity distribution with the distribution of molecular gas along the
Galactic ridge strongly points to a pionic-decay process involving relativistic
protons. In earlier work, we concluded that point-source candidates, such as
the supermassive black hole Sagittarius A* (identified with the HESS source
J1745-290), or the pulsar wind nebulae dispersed along the Galactic plane,
could not account for the observed diffuse TeV emission from this region.
Motivated by this result, we consider here the feasibility that the cosmic rays
populating the Galactic Center (GC) region are accelerated in situ by magnetic
turbulence. Our results indicate that even in a highly conductive environment,
this mechanism is efficient enough to energize protons within the intercloud
medium to the \ga TeV energies required to produce the HESS emission.Comment: Accepted for publication in Ap
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
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