714 research outputs found
Two-dimensional model studies of the effect of supersonic aircraft operations on the stratospheric ozone content
For a fleet of 250 aircraft, the change in the ozone column is predicted to be very close to zero; in fact, the ozone overburden may actually increase as a result of show that above 25 to 30 km the ozone abundance decreases via catalytic destruction, but at lower heights it increases, mainly as a result of coupling with odd hydrogen species. Water vapor released in the engine exhaust is predicted to cause ozone decreases; for the hypothetical engines used in the study, the total column ozone changes due to water vapor emission largely offset the predicted ozone increases due to NOx emission. The actual effect of water vapor may be less than calculated because present models do not include thermal feedback. Feedback refers to the cooling effect of additional water vapor that would tend to slow the NOx reactions which destroy ozone
Theoretical Spectra of Terrestrial Exoplanet Surfaces
We investigate spectra of airless rocky exoplanets with a theoretical
framework that self-consistently treats reflection and thermal emission. We
find that a silicate surface on an exoplanet is spectroscopically detectable
via prominent Si-O features in the thermal emission bands of 7 - 13 \mu m and
15 - 25 \mu m. The variation of brightness temperature due to the silicate
features can be up to 20 K for an airless Earth analog, and the silicate
features are wide enough to be distinguished from atmospheric features with
relatively high-resolution spectra. The surface characterization thus provides
a method to unambiguously identify a rocky exoplanet. Furthermore,
identification of specific rocky surface types is possible with the planet's
reflectance spectrum in near-infrared broad bands. A key parameter to observe
is the difference between K band and J band geometric albedos (A_g (K)-A_g
(J)): A_g (K)-A_g (J) > 0.2 indicates that more than half of the planet's
surface has abundant mafic minerals, such as olivine and pyroxene, in other
words primary crust from a magma ocean or high-temperature lavas; A_g (K)-A_g
(J) < -0.09 indicates that more than half of the planet's surface is covered or
partially covered by water ice or hydrated silicates, implying extant or past
water on its surface. Also, surface water ice can be specifically distinguished
by an H-band geometric albedo lower than the J-band geometric albedo. The
surface features can be distinguished from possible atmospheric features with
molecule identification of atmospheric species by transmission spectroscopy. We
therefore propose that mid-infrared spectroscopy of exoplanets may detect rocky
surfaces, and near-infrared spectrophotometry may identify ultramafic surfaces,
hydrated surfaces and water ice.Comment: Accepted for publication on the Ap
Tests of a multichannel photometer based on silicon diode detectors
A breadboard photometer was constructed that demonstrates a precision of 2 times 10 to the 4th power in the laboratory and scintillation-limited performance when used with an 0.5 m aperture telescope. Because the detectors and preamps are not cooled, only stars with m sub v approx. less than 4 are bright enough to allow the photometer to attain a precision of 1 times 10 to the 3rd power for three minute observations with an 0.5 m aperature telescope. Cooling the telescope should allow much fainter stars to be observed. Increasing the aperture of the telescope will allow higher precision and the observation of fainter stars
Environmental effects of SPS: The middle atmosphere
The heavy lift launch vehicle associated with the solar power satellite (SPS) would deposit in the upper atmosphere exhaust and reentry products which could modify the composition of the stratosphere, mesosphere, and lower ionosphere. In order to assess such effects, atmospheric model simulations were performed, especially considering a geographic zone centered at the launch and reentry latitudes
Solar-like oscillations in a metal-poor globular cluster with the HST
We present analyses of variability in the red giant stars in the metal-poor
globular cluster NGC6397, based on data obtained with the Hubble Space
Telescope. We use an non-standard data reduction approach to turn a 23-day
observing run originally aimed at imaging the white dwarf population, into
time-series photometry of the cluster's highly saturated red giant stars. With
this technique we obtain noise levels in the final power spectra down to 50
parts per million, which allows us to search for low amplitude solar-like
oscillations. We compare the observed excess power seen in the power spectra
with estimates of the typical frequency range, frequency spacing and amplitude
from scaling the solar oscillations. We see evidence that the detected
variability is consistent with solar-like oscillations in at least one and
perhaps up to four stars. With metallicities two orders of magnitude lower than
of the Sun, these stars present so far the best evidence of solar-like
oscillations in such a low metallicity environment.Comment: 7 pages, 6 figures, accepted by Ap
The Kepler Pixel Response Function
Kepler seeks to detect sequences of transits of Earth-size exoplanets
orbiting Solar-like stars. Such transit signals are on the order of 100 ppm.
The high photometric precision demanded by Kepler requires detailed knowledge
of how the Kepler pixels respond to starlight during a nominal observation.
This information is provided by the Kepler pixel response function (PRF),
defined as the composite of Kepler's optical point spread function, integrated
spacecraft pointing jitter during a nominal cadence and other systematic
effects. To provide sub-pixel resolution, the PRF is represented as a
piecewise-continuous polynomial on a sub-pixel mesh. This continuous
representation allows the prediction of a star's flux value on any pixel given
the star's pixel position. The advantages and difficulties of this polynomial
representation are discussed, including characterization of spatial variation
in the PRF and the smoothing of discontinuities between sub-pixel polynomial
patches. On-orbit super-resolution measurements of the PRF across the Kepler
field of view are described. Two uses of the PRF are presented: the selection
of pixels for each star that maximizes the photometric signal to noise ratio
for that star, and PRF-fitted centroids which provide robust and accurate
stellar positions on the CCD, primarily used for attitude and plate scale
tracking. Good knowledge of the PRF has been a critical component for the
successful collection of high-precision photometry by Kepler.Comment: 10 pages, 5 figures, accepted by ApJ Letters. Version accepted for
publication
Detection of Potential Transit Signals in the First Three Quarters of Kepler Mission Data
We present the results of a search for potential transit signals in the first
three quarters of photometry data acquired by the Kepler Mission. The targets
of the search include 151,722 stars which were observed over the full interval
and an additional 19,132 stars which were observed for only 1 or 2 quarters.
From this set of targets we find a total of 5,392 detections which meet the
Kepler detection criteria: those criteria are periodicity of the signal, an
acceptable signal-to-noise ratio, and a composition test which rejects spurious
detections which contain non-physical combinations of events. The detected
signals are dominated by events with relatively low signal-to-noise ratio and
by events with relatively short periods. The distribution of estimated transit
depths appears to peak in the range between 40 and 100 parts per million, with
a few detections down to fewer than 10 parts per million. The detected signals
are compared to a set of known transit events in the Kepler field of view which
were derived by a different method using a longer data interval; the comparison
shows that the current search correctly identified 88.1% of the known events. A
tabulation of the detected transit signals, examples which illustrate the
analysis and detection process, a discussion of future plans and open,
potentially fruitful, areas of further research are included
The NASA Ames Research Center one- and two-dimensional stratospheric models. Part 2: The two-dimensional model
The two-dimensional model of stratospheric constituents is presented in detail. The derivation of pertinent transport parameters and the numerical solution of the species continuity equations, including a technique for treating the stiff differential equations that represent the chemical kinetic terms, and appropriate methods for simulating the diurnal variations of the solar zenith angle and species concentrations are discussed. Predicted distributions of tracer constituents (ozone, carbon 14, nitric acid) are compared with observed distributions
Photometric Variability in Kepler Target Stars: The Sun Among Stars -- A First Look
The Kepler mission provides an exciting opportunity to study the lightcurves
of stars with unprecedented precision and continuity of coverage. This is the
first look at a large sample of stars with photometric data of a quality that
has heretofore been only available for our Sun. It provides the first
opportunity to compare the irradiance variations of our Sun to a large cohort
of stars ranging from vary similar to rather different stellar properties, at a
wide variety of ages. Although Kepler data is in an early phase of maturity,
and we only analyze the first month of coverage, it is sufficient to garner the
first meaningful measurements of our Sun's variability in the context of a
large cohort of main sequence stars in the solar neighborhood. We find that
nearly half of the full sample is more active than the active Sun, although
most of them are not more than twice as active. The active fraction is closer
to a third for the stars most similar to the Sun, and rises to well more than
half for stars cooler than mid K spectral types.Comment: 13 pages, 4 figures, accepted to ApJ Letter
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