18,929 research outputs found
Design, cost, and advanced technology applications for a military trainer aircraft
The potential impact is examined of advanced aerodynamic and propulsive technologies in terms of operating and acquisition costs on conceptual mission and performance requirements for a future undergraduate jet pilot trainer aircraft
Fluorine in the solar neighborhood - is it all produced in AGB-stars?
The origin of 'cosmic' fluorine is uncertain, but there are three proposed
production sites/mechanisms: AGB stars, nucleosynthesis in Type II
supernovae, and/or the winds of Wolf-Rayet stars. The relative importance of
these production sites has not been established even for the solar
neighborhood, leading to uncertainties in stellar evolution models of these
stars as well as uncertainties in the chemical evolution models of stellar
populations.
We determine the fluorine and oxygen abundances in seven bright, nearby
giants with well-determined stellar parameters. We use the 2.3 m
vibrational-rotational HF line and explore a pure rotational HF line at 12.2
m. The latter has never been used before for an abundance analysis. To be
able to do this we have calculated a line list for pure rotational HF lines. We
find that the abundances derived from the two diagnostics agree.
Our derived abundances are well reproduced by chemical evolution models only
including fluorine production in AGB-stars and therefore we draw the conclusion
that this might be the main production site of fluorine in the solar
neighborhood. Furthermore, we highlight the advantages of using the 12 m
HF lines to determine the possible contribution of the -process to the
fluorine budget at low metallicities where the difference between models
including and excluding this process is dramatic
Mg I emission lines at 12 and 18 micrometer in K giants
The solar Mg I emission lines at 12 micrometer have already been observed and
analyzed well. Previous modeling attempts for other stars have, however, been
made only for Procyon and two cool evolved stars, with unsatisfactory results
for the latter. We present high-resolution observational spectra for the K
giants Pollux, Arcturus, and Aldebaran, which show strong Mg I emission lines
at 12 micrometer as compared to the Sun. We also present the first observed
stellar emission lines from Mg I at 18 micrometer and from Al I, Si I, and
presumably Ca I at 12 micrometer. To produce synthetic line spectra, we employ
standard non-LTE modeling for trace elements in cool stellar photospheres. We
compute model atmospheres with the MARCS code, apply a comprehensive magnesium
model atom, and use the radiative transfer code MULTI to solve for the
magnesium occupation numbers in statistical equilibrium. We successfully
reproduce the observed Mg I emission lines simultaneously in the giants and in
the Sun, but show how the computed line profiles depend critically on atomic
input data and how the inclusion of energy levels with n > 9 and collisions
with neutral hydrogen are necessary to obtain reasonable fits.Comment: 9 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
TEXES Observations of M Supergiants: Dynamics and Thermodynamics of Wind Acceleration
We have detected [Fe II] 17.94 um and 24.52 um emission from a sample of M
supergiants using TEXES on the IRTF. These low opacity emission lines are
resolved at R = 50, 000 and provide new diagnostics of the dynamics and
thermodynamics of the stellar wind acceleration zone. The [Fe II] lines, from
the first excited term, are sensitive to the warm plasma where energy is
deposited into the extended atmosphere to form the chromosphere and wind
outflow. These diagnostics complement previous KAO and ISO observations which
were sensitive to the cooler and more extended circumstellar envelopes. The
turbulent velocities, Vturb is about 12 to 13 km/s, observed in the [Fe II]
forbidden lines are found to be a common property of our sample, and are less
than that derived from the hotter chromospheric C II] 2325 Angstrom lines
observed in alpha Ori, where Vturb is about 17 to 19 km/s. For the first time,
we have dynamically resolved the motions of the dominant cool atmospheric
component discovered in alpha Ori from multi-wavelength radio interferometry by
Lim et al. (1998). Surprisingly, the emission centroids are quite Gaussian and
at rest with respect to the M supergiants. These constraints combined with
model calculations of the infrared emission line fluxes for alpha Ori imply
that the warm material has a low outflow velocity and is located close to the
star. We have also detected narrow [Fe I] 24.04 um emission that confirms that
Fe II is the dominant ionization state in alpha Ori's extended atmosphere.Comment: 79 pages including 10 figures and 2 appendices. Accepted by Ap
Method of studying the Bogoliubov-de Gennes equations for the superconducting vortex lattice state
In this paper, we present a method to construct the eigenspace of the
normal-state electrons moving in a 2D square lattice in presence of a
perpendicular uniform magnetic field which imposes (quasi)-periodic boundary
conditions for the wave functions in the magnetic unit cell. An exact unitary
transformations are put forward to correlate the discrete eigenvectors of the
2D electrons with those of the Harper's equation. The cyclic-tridiagonal matrix
associated with the Harper's equation is then tridiagonalized by another
unitary transformation. The obtained eigenbasis is utilized to expand the
Bogoliubov-de Gennes equations for the superconducting vortex lattice state,
which showing the merit of our method in studying the large-sized system. To
test our method, we have applied our results to study the vortex lattice state
of an s-wave superconductor.Comment: 8 pages; 3 figure
The āCopy and Pasteā Function, Patterns of Learner Activity and Enhancing Cognitive Tools
This research analyses video data to examine changes between levels of process of textbased content of learners using two versions of the āCopy and Pasteā function, an unmodified control version and an experimental version with an embedded interaction strategy designed to prompt learners to process more content effectively. Observed learner activity types were categorized as involving Low, Medium or High levels of cognitive processing and the pattern of learner switching between these levels is examined. The conclusions drawn from this analysis indicate that learners not only need mediating artifacts that prompt them to use appropriate processing strategies but that they also need to be made aware of; i) the level of cognitive processing in which they are engaged and ii) more importantly to be prompted to switch activities to those involving higher levels of cognitive processing when they have been conducting low level processing for extended periods
Chromatic transit light curves of disintegrating rocky planets
Context. Kepler observations have revealed a class of short period
exoplanets, of which Kepler-1520 b is the prototype, which have comet-like dust
tails thought to be the result of small, rocky planets losing mass. The shape
and chromaticity of the transits constrain the properties of the dust particles
originating from the planet's surface, offering a unique opportunity to probe
the composition and geophysics of rocky exoplanets.
Aims. We aim to approximate the average Kepler long-cadence light curve of
Kepler-1520 b and investigate how the optical thickness and transit
cross-section of a general dust tail can affect the observed wavelength
dependence and depth of transit light curves.
Methods. We developed a new 3D model that ejects sublimating particles from
the planet surface to build up a dust tail, assuming it to be optically thin,
and used 3D radiative transfer computations that fully treat scattering using
the distribution of hollow spheres (DHS) method, to generate transit light
curves between 0.45 and 2.5 m.
Results. We show that the transit depth is wavelength independent for
optically thick tails, potentially explaining why only some observations
indicate a wavelength dependence. From the 3D nature of our simulated tails, we
show that their transit cross-sections are related to the component of particle
ejection velocity perpendicular to the planet's orbital plane and use this to
derive a minimum ejection velocity of 1.2 kms. To fit the average
transit depth of Kepler-1520 b of 0.87%, we require a high dust mas-loss rate
of 7 80 M Gyr which implies planet lifetimes that may be
inconsistent with the observed sample. Therefore, these mass-loss rates should
be considered to be upper limits.Comment: 22 pages, 22 figures, accepted for publication in A&
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