3,373 research outputs found
Accurate molecular energies by extrapolation of atomic energies using an analytic quantum mechanical model
Using a new analytic quantum mechanical method based on Slater's Xalpha
method, we show that a fairly accurate estimate of the total energy of a
molecule can be obtained from the exact energies of its constituent atoms. The
mean absolute error in the total energies thus determined for the G2 set of 56
molecules is about 16 kcal/mol, comparable to or better than some popular pure
and hybrid density functional models.Comment: 5 pages, REVTE
Methods for evaluating the performance of volume phase holographic gratings for the VIRUS spectrograph array
The Visible Integral Field Replicable Unit Spectrograph (VIRUS) is an array
of at least 150 copies of a simple, fiber-fed integral field spectrograph that
will be deployed on the Hobby-Eberly Telescope (HET) to carry out the HET Dark
Energy Experiment (HETDEX). Each spectrograph contains a volume phase
holographic grating as its dispersing element that is used in first order for
350 nm to 550 nm. We discuss the test methods used to evaluate the performance
of the prototype gratings, which have aided in modifying the fabrication
prescription for achieving the specified batch diffraction efficiency required
for HETDEX. In particular, we discuss tests in which we measure the diffraction
efficiency at the nominal grating angle of incidence in VIRUS for all orders
accessible to our test bench that are allowed by the grating equation. For
select gratings, these tests have allowed us to account for > 90% of the
incident light for wavelengths within the spectral coverage of VIRUS. The
remaining light that is unaccounted for is likely being diffracted into
reflective orders or being absorbed or scattered within the grating layer (for
bluer wavelengths especially, the latter term may dominate the others).
Finally, we discuss an apparatus that will be used to quickly verify the first
order diffraction efficiency specification for the batch of at least 150 VIRUS
production gratings.Comment: 18 pages, 11 figures. To be published in Proc. SPIE, 2012,
"Ground-Based and Airborne Instrumentation for Astronomy IV", 8446-20
Reducing the Risks: Reflections on Bridging Home and School Communication
Recent scholarship on literacy development has focused on studying young at-risk learners (Allen and Mason, 1989; Clay, 1982; Taylor and Dorsey-Gaines, 1988; Swap, 1990; Teale and Sulzby, 1986). As kindergarten and first grade teachers we worried about many of our students whose families were not in the cultural mainstream and whose literacy backgrounds appeared different from those of our more successful children. As we thought about how we might better teach our children we began to consider how we could improve our communication with the children\u27s parents to begin to build a partner ship between home and school literacy experiences. We wanted to be supportive and invitational with the parents. We hoped to provide the parents with information which they could use in helping their children interact with print, and, importantly, we wanted to learn from the parents. We valued their input and welcomed information that they could provide which would allow us to build our program to sup port the home. We wished to begin to build a two way bridge that would connect home and school literacy practice
Evidence from K2 for rapid rotation in the descendant of an intermediate-mass star
Using patterns in the oscillation frequencies of a white dwarf observed by
K2, we have measured the fastest rotation rate, 1.13(02) hr, of any isolated
pulsating white dwarf known to date. Balmer-line fits to follow-up spectroscopy
from the SOAR telescope show that the star (SDSSJ0837+1856, EPIC 211914185) is
a 13,590(340) K, 0.87(03) solar-mass white dwarf. This is the highest mass
measured for any pulsating white dwarf with known rotation, suggesting a
possible link between high mass and fast rotation. If it is the product of
single-star evolution, its progenitor was a roughly 4.0 solar-mass
main-sequence B star; we know very little about the angular momentum evolution
of such intermediate-mass stars. We explore the possibility that this rapidly
rotating white dwarf is the byproduct of a binary merger, which we conclude is
unlikely given the pulsation periods observed.Comment: 5 pages, 4 figure, 1 table; accepted for publication in The
Astrophysical Journal Letter
Fourier analyses of commensurability oscillations in Fibonacci lateral superlattices
Magnetotransport measurements have been performed on Fibonacci lateral
superlattices (FLSLs) -- two-dimensional electron gases subjected to a weak
potential modulation arranged in the Fibonacci sequence, LSLLSLS..., with
L/S=tau (the golden ratio). Complicated commensurability oscillation (CO) is
observed, which can be accounted for as a superposition of a series of COs each
arising from a sinusoidal modulation representing the characteristic length
scale of one of the self-similar generations in the Fibonacci sequence.
Individual CO components can be separated out from the magnetoresistance trace
by performing a numerical Fourier band-pass filter. From the analysis of the
amplitude of a single-component CO thus extracted, the magnitude of the
corresponding Fourier component in the potential modulation can be evaluated.
By examining all the Fourier contents observed in the magnetoresistance trace,
the profile of the modulated potential seen by the electrons can be
reconstructed with some remaining ambiguity about the interrelation of the
phase between different components.Comment: 11 pages, 10 figures, added references in Introduction, minor
revision
Zener transitions between dissipative Bloch bands. II: Current Response at Finite Temperature
We extend, to include the effects of finite temperature, our earlier study of
the interband dynamics of electrons with Markoffian dephasing under the
influence of uniform static electric fields. We use a simple two-band
tight-binding model and study the electric current response as a function of
field strength and the model parameters. In addition to the Esaki-Tsu peak,
near where the Bloch frequency equals the damping rate, we find current peaks
near the Zener resonances, at equally spaced values of the inverse electric
field. These become more prominenent and numerous with increasing bandwidth (in
units of the temperature, with other parameters fixed). As expected, they
broaden with increasing damping (dephasing).Comment: 5 pages, LateX, plus 5 postscript figure
Physical Interaction Between VIVID and White Collar Complex Regulates Photoadaptation in Neurospora
Photoadaptation, the ability to attenuate a light response on prolonged light exposure while remaining sensitive to escalating changes in light intensity, is essential for organisms to decipher time information appropriately, yet the underlying molecular mechanisms are poorly understood. In Neurospora crassa, VIVID (VVD), a small LOV domain containing blue-light photoreceptor protein, affects photoadaptation for most if not all light-responsive genes. We report that there is a physical interaction between VVD and the white collar complex (WCC), the primary blue-light photoreceptor and the transcription factor complex that initiates light-regulated transcriptional responses in Neurospora. Using two previously characterized VVD mutants, we show that the level of interaction is correlated with the level of WCC repression in constant light and that even light-insensitive VVD is sufficient partly to regulate photoadaptation in vivo. We provide evidence that a functional GFP-VVD fusion protein accumulates in the nucleus on light induction but that nuclear localization of VVD does not require light. Constitutively expressed VVD alone is sufficient to change the dynamics of photoadaptation. Thus, our results demonstrate a direct molecular connection between two of the most essential light signaling components in Neurospora, VVD and WCC, illuminating a previously uncharacterized process for light-sensitive eukaryotic cells
White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Linewidths: Kepler Observations of 27 Pulsating DA White Dwarfs Through K2 Campaign 8
We present photometry and spectroscopy for 27 pulsating hydrogen-atmosphere
white dwarfs (DAVs, a.k.a. ZZ Ceti stars) observed by the Kepler space
telescope up to K2 Campaign 8, an extensive compilation of observations with
unprecedented duration (>75 days) and duty cycle (>90%). The space-based
photometry reveals pulsation properties previously inaccessible to ground-based
observations. We observe a sharp dichotomy in oscillation mode linewidths at
roughly 800 s, such that white dwarf pulsations with periods exceeding 800 s
have substantially broader mode linewidths, more reminiscent of a damped
harmonic oscillator than a heat-driven pulsator. Extended Kepler coverage also
permits extensive mode identification: We identify the spherical degree of 61
out of 154 unique radial orders, providing direct constraints of the rotation
period for 20 of these 27 DAVs, more than doubling the number of white dwarfs
with rotation periods determined via asteroseismology. We also obtain
spectroscopy from 4m-class telescopes for all DAVs with Kepler photometry.
Using these homogeneously analyzed spectra we estimate the overall mass of all
27 DAVs, which allows us to measure white dwarf rotation as a function of mass,
constraining the endpoints of angular momentum in low- and intermediate-mass
stars. We find that 0.51-to-0.73-solar-mass white dwarfs, which evolved from
1.7-to-3.0-solar-mass ZAMS progenitors, have a mean rotation period of 35 hr
with a standard deviation of 28 hr, with notable exceptions for higher-mass
white dwarfs. Finally, we announce an online repository for our Kepler data and
follow-up spectroscopy, which we collect at http://www.k2wd.org.Comment: 33 pages, 31 figures, 5 tables; accepted for publication in ApJS. All
raw and reduced data are collected at http://www.k2wd.or
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