1,002 research outputs found
VLBI for Gravity Probe B. VII. The Evolution of the Radio Structure of IM Pegasi
We present measurements of the total radio flux density as well as
very-long-baseline interferometry (VLBI) images of the star, IM Pegasi, which
was used as the guide star for the NASA/Stanford relativity mission Gravity
Probe B. We obtained flux densities and images from 35 sessions of observations
at 8.4 GHz (wavelength = 3.6 cm) between 1997 January and 2005 July. The
observations were accurately phase-referenced to several extragalactic
reference sources, and we present the images in a star-centered frame, aligned
by the position of the star as derived from our fits to its orbital motion,
parallax, and proper motion. Both the flux density and the morphology of IM Peg
are variable. For most sessions, the emission region has a single-peaked
structure, but 25% of the time, we observed a two-peaked (and on one occasion
perhaps a three-peaked) structure. On average, the emission region is elongated
by 1.4 +- 0.4 mas (FWHM), with the average direction of elongation being close
to that of the sky projection of the orbit normal. The average length of the
emission region is approximately equal to the diameter of the primary star. No
significant correlation with the orbital phase is found for either the flux
density or the direction of elongation, and no preference for any particular
longitude on the star is shown by the emission region.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
Lessons Learned from Recent Failure and Incident Investigations of Composite Structures
During the past few decades, NASA Langley Research Center (LaRC) has supported several large-scale failure and incident investigations and numerous requests for engineering consultations. Although various extenuating circumstances contributed to each of these incidents, in all cases, the failure resulted from accumulation and/or propagation of damage that reduced the load carrying capability of the structure to a level below that which was needed to sustain structural loads. A brief overview of various failure and incident investigations supported by LaRC, including some of the computational and experimental methodologies that have been applied, is presented. An important outcome of many of these failure and incident investigations is the development of an improved understanding of not only the state-of-the-art in experimental and analytical methods but also the state-of-the-art in the design and manufacturing processes that may contribute to such failures. In order to provide insight into such large-scale investigations, a series of lessons learned were captured. Awareness of these lessons learned is highly beneficial to engineers involved in similar investigations. Therefore, it is prudent that the lessons learned are disseminated such that they can be built upon in other investigations and in ensuing research and development activities
Lignocellulose-Degrading Microbial Communities in Landfill Sites Represent a Repository of Unexplored Biomass- Degrading Diversity Emma
The microbial conversion of lignocellulosic biomass for biofuel production represents a renewable alternative to fossil fuels. However, the discovery of new microbial enzymes with high activity is critical for improving biomass conversion processes. While attempts to identify superior lignocellulose-degrading enzymes have focused predominantly on the animal gut, biomass-degrading communities in landfill sites represent an unexplored resource of hydrolytic enzymes for biomass conversion. Here, to address the paucity of information on biomass-degrading microbial diversity beyond the gastrointestinal tract, cellulose (cotton) “baits” were incubated in landfill leachate microcosms to enrich the landfill cellulolytic microbial community for taxonomic and functional characterization. Metagenome and 16S rRNA gene amplicon sequencing demonstrated the dominance of Firmicutes, Bacteroidetes, Spirochaetes, and Fibrobacteres in the landfill cellulolytic community. Functional metagenome analysis revealed 8,371 carbohydrate active enzymes (CAZymes) belonging to 244 CAZyme families. In addition to observing biomass-degrading enzymes of anaerobic bacterial “cellulosome” systems of members of the Firmicutes, we report the first detection of the Fibrobacter cellulase system and the Bacteroidetes polysaccharide utilization locus (PUL) in landfill sites. These data provide evidence for the presence of multiple mechanisms of biomass degradation in the landfill microbiome and highlight the extraordinary functional diversity of landfill microorganisms as a rich source of biomass-degrading enzymes of potential biotechnological significance
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VLBI for Gravity Probe B. VII. The Evolution of the Radio Structure of IM Pegasi
We present measurements of the total radio flux density as well as very-long-baseline interferometry (VLBI) images of the star, IM Pegasi, which was used as the guide star for the NASA/Stanford relativity mission Gravity Probe B. We obtained flux densities and images from 35 sessions of observations at 8.4 GHz (wavelength = 3.6 cm) between 1997 January and 2005 July. The observations were accurately phase-referenced to several extragalactic reference sources, and we present the images in a star-centered frame, aligned by the position of the star as derived from our fits to its orbital motion, parallax, and proper motion. Both the flux density and the morphology of IM Peg are variable. For most sessions, the emission region has a single-peaked structure, but 25% of the time, we observed a two-peaked (and on one occasion perhaps a three-peaked) structure. On average, the emission region is elongated by 1.4 +- 0.4 mas (FWHM), with the average direction of elongation being close to that of the sky projection of the orbit normal. The average length of the emission region is approximately equal to the diameter of the primary star. No significant correlation with the orbital phase is found for either the flux density or the direction of elongation, and no preference for any particular longitude on the star is shown by the emission region.Astronom
Constraining Parity Violation in Gravity with Measurements of Neutron-Star Moments of Inertia
Neutron stars are sensitive laboratories for testing general relativity,
especially when considering deviations where velocities are relativistic and
gravitational fields are strong. One such deviation is described by dynamical,
Chern-Simons modified gravity, where the Einstein-Hilbert action is modified
through the addition of the gravitational parity-violating Pontryagin density
coupled to a field. This four-dimensional effective theory arises naturally
both in perturbative and non-perturbative string theory, loop quantum gravity,
and generic effective field theory expansions. We calculate here Chern-Simons
modifications to the properties and gravitational fields of slowly spinning
neutron stars. We find that the Chern-Simons correction affects only the
gravitomagnetic sector of the metric to leading order, thus introducing
modifications to the moment of inertia but not to the mass-radius relation. We
show that an observational determination of the moment of inertia to an
accuracy of 10%, as is expected from near-future observations of the double
pulsar, will place a constraint on the Chern-Simons coupling constant of
\xi^{1/4} < 5 km, which is at least three-orders of magnitude stronger than the
previous strongest bound.Comment: 14 pages, 6 figures, replaced with version accepted for publication
in Phys. Rev.
VLBI for Gravity Probe B. V. Proper Motion and Parallax of the Guide Star, IM Pegasi
We present the principal astrometric results of the very-long-baseline
interferometry (VLBI) program undertaken in support of the Gravity Probe B
(GP-B) relativity mission. VLBI observations of the GP-B guide star, the RS CVn
binary IM Pegasi (HR 8703), yielded positions at 35 epochs between 1997 and
2005. We discuss the statistical assumptions behind these results and our
methods for estimating the systematic errors. We find the proper motion of IM
Peg in an extragalactic reference frame closely related to the International
Celestial Reference Frame 2 (ICRF2) to be -20.83 +- 0.03 +- 0.09 mas/yr in
right ascension and -27.27 +- 0.03 +- 0.09 mas/yr in declination. For each
component the first uncertainty is the statistical standard error and the
second is the total standard error (SE) including plausible systematic errors.
We also obtain a parallax of 10.37 +- 0.07 mas (distance: 96.4 +- 0.7 pc), for
which there is no evidence of any significant contribution of systematic error.
Our parameter estimates for the ~25-day-period orbital motion of the stellar
radio emission have SEs corresponding to ~0.10 mas on the sky in each
coordinate. The total SE of our estimate of IM Peg's proper motion is ~30%
smaller than the accuracy goal set by the GP-B project before launch: 0.14
mas/yr for each coordinate of IM Peg's proper motion. Our results ensure that
the uncertainty in IM Peg's proper motion makes only a very small contribution
to the uncertainty of the GP-B relativity tests.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
A Binary Millisecond Pulsar in Globular Cluster NGC6544
We report the detection of a new 3.06 ms binary pulsar in the globular
cluster NGC6544 using a Fourier-domain ``acceleration'' search. With an implied
companion mass of ~0.01 solar masses and an orbital period of only P_b~1.7
hours, it displays very similar orbital properties to many pulsars which are
eclipsed by their companion winds. The orbital period is the second shortest of
known binary pulsars after 47 Tuc R. The measured flux density of 1.3 +/- 0.4
mJy at 1332 MHz indicates that the pulsar is almost certainly the known
steep-spectrum point source near the core of NGC6544.Comment: Accepted by ApJ Letters on 11 October 2000, 5 page
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