5,999 research outputs found
The possibility of a metal insulator transition in antidot arrays induced by an external driving
It is shown that a family of models associated with the kicked Harper model
is relevant for cyclotron resonance experiments in an antidot array. For this
purpose a simplified model for electronic motion in a related model system in
presence of a magnetic field and an AC electric field is developed. In the
limit of strong magnetic field it reduces to a model similar to the kicked
Harper model. This model is studied numerically and is found to be extremely
sensitive to the strength of the electric field. In particular, as the strength
of the electric field is varied a metal -- insulator transition may be found.
The experimental conditions required for this transition are discussed.Comment: 6 files: kharp.tex, fig1.ps fig2.ps fi3.ps fig4.ps fig5.p
Radio continuum observations of local star-forming galaxies using the Caltech Continuum Backend on the Green Bank Telescope
We observed radio continuum emission in 27 local (D < 70 Mpc) star-forming
galaxies with the Robert C. Byrd Green Bank Telescope between 26 GHz and 40 GHz
using the Caltech Continuum Backend. We obtained detections for 22 of these
galaxies at all four sub-bands and four more marginal detections by taking the
average flux across the entire bandwidth. This is the first detection (full or
marginal) at these frequencies for 22 of these galaxies. We fit spectral energy
distributions (SEDs) for all of the four-sub-band detections. For 14 of the
galaxies, SEDs were best fit by a combination of thermal free-free and
nonthermal synchrotron components. Eight galaxies with four-sub-band detections
had steep spectra that were only fit by a single nonthermal component. Using
these fits, we calculated supernova rates, total number of equivalent O stars,
and star formation rates within each ~23 arcsecond beam. For unresolved
galaxies, these physical properties characterize the galaxies' recent star
formation on a global scale. We confirm that the radio-far-infrared correlation
holds for the unresolved galaxies' total 33 GHz flux regardless of their
thermal fractions, though the scatter on this correlation is larger than that
at 1.4 GHz. In addition, we found that for the unresolved galaxies, there is an
inverse relationship between the ratio of 33 GHz flux to total far-infrared
flux and the steepness of the galaxy's spectral index between 1.4 GHz and 33
GHz. This relationship could be an indicator of the timescale of the observed
episode of star formation.Comment: 36 pages, 9 figures; accepted for publication in ApJ. First and
second author affiliation updated to reflect departmental name chang
Modal Test of the NASA Mobile Launcher at Kennedy Space Center
The NASA Mobile Launcher (ML), located at Kennedy Space Center (KSC), has recently been modified to support the launch of the new NASA Space Launch System (SLS). The ML is a massive structureconsisting of a 345-foot tall tower attached to a two-story base, weighing approximately 10.5 million poundsthat will secure the SLS vehicle as it rolls to the launch pad on a Crawler Transporter, as well as provide a launch platform at the pad. The ML will also provide the boundary condition for an upcoming SLS Integrated Modal Test (IMT). To help correlate the ML math models prior to this modal test, and allow focus to remain on updating SLS vehicle models during the IMT, a ML-only experimental modal test was performed in June 2019. Excitation of the tower and platform was provided by five uniquely-designed test fixtures, each enclosing a hydraulic shaker, capable of exerting thousands of pounds of force into the structure. For modes not that were not sufficiently excited by the test fixture shakers, a specially-designed mobile drop tower provided impact excitation at additional locations of interest. The response of the ML was measured with a total of 361 accelerometers. Following the random vibration, sine sweep vibration, and modal impact testing, frequency response functions were calculated and modes were extracted for three different configurations of the ML in 0 Hz to 12 Hz frequency range. This paper will provide a case study in performing modal tests on large structures by discussing the Mobile Launcher, the test strategy, an overview of the test results, and recommendations for meeting a tight test schedule for a large-scale modal test
Advising Student-Athletes for Success: Predicting the Academic Success and Persistence of Collegiate Student-Athletes
Stakeholders at institutions across the United States are continuously looking for ways to improve the academic success and retention of students. We used logistical regression in an examination of noncognitive, cognitive, and demographic factors as predictors of academic success and retention of Division I first-year student-athletes. The results indicated that high school GPA is the best predictor for academic success. The Transition to College Inventory index, self-confidence, institutional commitment, and independent activity focus can be used in the prediction of academic success. Retention was most accurately predicted by students\u27 first-year cumulative GPA. University advisors can use the results of this study to enhance the resources designed to improve the academic performance and persistence of student-athletes
Untangling the Recombination Line Emission from HII Regions with Multiple Velocity Components
HII regions are the ionized spheres surrounding high-mass stars. They are
ideal targets for tracing Galactic structure because they are predominantly
found in spiral arms and have high luminosities at infrared and radio
wavelengths. In the Green Bank Telescope HII Region Discovery Survey (GBT HRDS)
we found that >30% of first Galactic quadrant HII regions have multiple
hydrogen radio recombination line (RRL) velocities, which makes determining
their Galactic locations and physical properties impossible. Here we make
additional GBT RRL observations to determine the discrete HII region velocity
for all 117 multiple-velocity sources within 18deg. < l < 65deg. The
multiple-velocity sources are concentrated in the zone 22deg. < l < 32deg.,
coinciding with the largest regions of massive star formation, which implies
that the diffuse emission is caused by leaked ionizing photons. We combine our
observations with analyses of the electron temperature, molecular gas, and
carbon recombination lines to determine the source velocities for 103 discrete
H II regions (88% of the sample). With the source velocities known, we resolve
the kinematic distance ambiguity for 47 regions, and thus determine their
heliocentric distances.Comment: 44 pages, 5 figures, 16 pages of tables; Accepted by ApJ
Advising Student-Athletes for Success: Predicting the Academic Success and Persistence of Collegiate Student-Athletes
Stakeholders at institutions across the United States are continuously looking for ways to improve the academic success and retention of students. We used logistical regression in an examination of noncognitive, cognitive, and demographic factors as predictors of academic success and retention of Division I first-year student-athletes. The results indicated that high school GPA is the best predictor for academic success. The Transition to College Inventory index, self-confidence, institutional commitment, and independent activity focus can be used in the prediction of academic success. Retention was most accurately predicted by students\u27 first-year cumulative GPA. University advisors can use the results of this study to enhance the resources designed to improve the academic performance and persistence of student-athletes
The 67 Hz Feature in the Black Hole Candidate GRS 1915+105 as a Possible ``Diskoseismic'' Mode
The Rossi X-ray Timing Explorer (RXTE) has made feasible for the first time
the search for high-frequency (~ 100 Hz) periodic features in black hole
candidate (BHC) systems. Such a feature, with a 67 Hz frequency, recently has
been discovered in the BHC GRS 1915+105 (Morgan, Remillard, & Greiner). This
feature is weak (rms variability ~0.3%-1.6%), stable in frequency (to within ~2
Hz) despite appreciable luminosity fluctuations, and narrow (quality factor Q ~
20). Several of these properties are what one expects for a ``diskoseismic''
g-mode in an accretion disk about a 10.6 M_sun (nonrotating) - 36.3 M_sun
(maximally rotating) black hole (if we are observing the fundamental mode
frequency). We explore this possibility by considering the expected luminosity
modulation, as well as possible excitation and growth mechanisms---including
turbulent excitation, damping, and ``negative'' radiation damping. We conclude
that a diskoseismic interpretation of the observations is viable.Comment: 4 Pages, Latex (emulateapj.sty included), to Appear in ApJ Letters,
Vol. 477, Final Version with Updated Reference
Attenuated Codon Optimality Contributes to Neural-Specific mRNA Decay in Drosophila.
Tissue-specific mRNA stability is important for cell fate and physiology, but the mechanisms involved are not fully understood. We found that zygotic mRNA stability in Drosophila correlates with codon content: optimal codons are enriched in stable transcripts associated with metabolic functions like translation, while non-optimal codons are enriched in unstable transcripts, including those associated with neural development. Bioinformatic analyses and reporter assays revealed that similar codons stabilize or destabilize mRNAs in the nervous system and other tissues, but the link between codon content and stability is attenuated in the nervous system. We confirmed that optimal codons are decoded by abundant tRNAs while non-optimal codons are decoded by less abundant tRNAs in embryos and in the nervous system. We conclude that codon optimality is a general determinant of zygotic mRNA stability, and attenuation of codon optimality allows trans-acting factors to exert greater influence over mRNA decay in the nervous system
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