611 research outputs found
Method of identifying clusters representing statistical dependencies in multivariate data
Approach is first to cluster and then to compute spatial boundaries for resulting clusters. Next step is to compute, from set of Monte Carlo samples obtained from scrambled data, estimates of probabilities of obtaining at least as many points within boundaries as were actually observed in original data
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
Red-giant stars in eccentric binaries
The unparalleled photometric data obtained by NASA’s Kepler Space Telescope has led to improved understanding of red-giant stars and binary stars. We discuss the characterization of known eccentric system, containing a solar-like oscillating red-giant primary component. We also report several new binary systems that are candidates for hosting an oscillating companion. A powerful approach to study binary stars is to combine asteroseimic techniques with light curve fitting. Seismology allows us to deduce the properties of red giants. In addition, by modeling the ellipsoidal modulations we can constrain the parameters of the binary system. An valuable independent source are ground-bases, high-resolution spectrographs
Minimizing follow-up for space-based transit surveys using full lightcurve analysis
One of the biggest challenges facing large transit surveys is the elimination
of false-positives from the vast number of transit candidates. We investigate
to what extent information from the lightcurves can identify blend scenarios
and eliminate them as planet candidates, to significantly decrease the amount
of follow-up observing time required to identify the true exoplanet systems. If
a lightcurve has a sufficiently high signal-to-noise ratio, a distinction can
be made between the lightcurve of a stellar binary blended with a third star
and the lightcurve of a transiting exoplanet system. We perform simulations to
determine what signal-to-noise level is required to make the distinction
between blended and non-blended systems as function of transit depth and impact
parameter. Subsequently we test our method on real data from the first IRa01
field observed by the CoRoT satellite, concentrating on the 51 candidates
already identified by the CoRoT team. About 70% of the planet candidates in the
CoRoT IRa01 field are best fit with an impact parameter of b>0.85, while less
than 15% are expected in this range considering random orbital inclinations. By
applying a cut at b<0.85, meaning that ~15% of the potential planet population
would be missed, the candidate sample decreases from 41 to 11. The lightcurves
of 6 of those are best fit with such low host star densities that the
planet-to-star size ratii imply unrealistic planet radii of R>2RJup. Two of the
five remaining systems, CoRoT1b and CoRoT4b, have been identified as planets by
the CoRoT team, for which the lightcurves alone rule out blended light at 14%
(2sigma) and 31% (2sigma). We propose to use this method on the Kepler database
to study the fraction of real planets and to potentially increase the
efficiency of follow-up.Comment: 13 pages, 11 figures, 2 tables. Accepted for publication in A&
SOPHIE velocimetry of Kepler transit candidates VII. A false-positive rate of 35% for Kepler close-in giant exoplanet candidates
The false-positive probability (FPP) of Kepler transiting candidates is a key
value for statistical studies of candidate properties. A previous investigation
of the stellar population in the Kepler field has provided an estimate for the
FPP of less than 5% for most of the candidates. We report here the results of
our radial velocity observations on a sample of 46 Kepler candidates with a
transit depth greater than 0.4%, orbital period less than 25 days and host star
brighter than Kepler magnitude 14.7. We used the SOPHIE spectrograph mounted on
the 1.93-m telescope at the Observatoire de Haute-Provence to establish the
nature of the transiting candidates. In this sample, we found five undiluted
eclipsing binaries, two brown dwarfs, six diluted eclipsing binaries, and nine
new transiting planets that complement the 11 already published planets. The
remaining 13 candidates were not followed-up or remain unsolved due to photon
noise limitation or lack of observations. From these results we computed the
FPP for Kepler close-in giant candidates to be 34.8% \pm 6.5%. We aimed to
investigate the variation of the FPP for giant candidates with the longer
orbital periods and found that it should be constant for orbital periods
between 10 and 200 days. This significant disagrees with the previous
estimates. We discuss the reasons for this discrepancy and the possible
extension of this work toward smaller planet candidates. Finally, taking the
false-positive rate into account, we refined the occurrence rate of hot
jupiters from the Kepler data.Comment: Accepted in A&A. 16 pages including 4 online material pages. 6
figures and 1 tabl
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
Transit Timing Observations from Kepler: VI. Potentially interesting candidate systems from Fourier-based statistical tests
We analyze the deviations of transit times from a linear ephemeris for the
Kepler Objects of Interest (KOI) through Quarter six (Q6) of science data. We
conduct two statistical tests for all KOIs and a related statistical test for
all pairs of KOIs in multi-transiting systems. These tests identify several
systems which show potentially interesting transit timing variations (TTVs).
Strong TTV systems have been valuable for the confirmation of planets and their
mass measurements. Many of the systems identified in this study should prove
fruitful for detailed TTV studies.Comment: 32 pages, 6 of text and one long table, Accepted to Ap
The Vulcan Photometer: A Dedicated Photometer for Extrasolar Planet Searches
A small CCD photometer dedicated to the detection of extrasolar planets has been developed and put into operation at Mount Hamilton, California. It simultaneously monitors 6000 stars brighter than 13th magnitude in its 49 deg2 field of view. Observations are conducted all night every clear night of the year. A single field is monitored at a cadence of eight images per hour for a period of about 3 months. When the data are folded for the purpose of discovering low-amplitude transits, transit amplitudes of 1% are readily detected. This precision is sufficient to find Jovian-size planets orbiting solar-like stars, which have signal amplitudes from 1% to 2% depending on the inflation of the planet’s atmosphere and the size of the star. An investigation of possible noise sources indicates that neither star field crowding, scintillation noise, nor photon shot noise are the major noise sources for stars brighter than visual magnitude 11.6. Over one hundred variable stars have been found in each star field. About 50 of these stars are eclipsing binary stars, several with transit amplitudes of only a few percent. Three stars that showed only primary transits were examined with high-precision spectroscopy. Two were found to be nearly identical stars in binary pairs orbiting at double the photometric period. Spectroscopic observations showed the third star to be a high mass ratio single-lined binary. On 1999 November 22 the transit of a planet orbiting HD 209458 was observed and the predicted amplitude and immersion times were confirmed. These observations show that the photometer and the data reduction and analysis algorithms have the necessary precision to find companions with the expected area ratio for Jovian-size planets orbiting solar-like stars
Asteroseismology of red giants from the first four months of Kepler data: Fundamental parameters
Clear power excess in a frequency range typical for solar-type oscillations
in red giants has been detected in more than 1000 stars, which have been
observed during the first 138 days of the science operation of the NASA Kepler
satellite. This sample includes stars in a wide mass and radius range with
spectral types G and K, extending in luminosity from the bottom of the giant
branch up to high-luminous red giants. The high-precision asteroseismic
observations with Kepler provide a perfect source for testing stellar structure
and evolutionary models, as well as investigating the stellar population in our
Galaxy. We fit a global model to the observed frequency spectra, which allows
us to accurately estimate the granulation background signal and the global
oscillation parameters, such as the frequency of maximum oscillation power. We
find regular patterns of radial and non-radial oscillation modes and use a new
technique to automatically identify the mode degree and the characteristic
frequency separations between consecutive modes of the same spherical degree.
In most cases, we can also measure the small separation. The seismic parameters
are used to estimate stellar masses and radii and to place the stars in an H-R
diagram by using an extensive grid of stellar models that covers a wide
parameter range. Using Bayesian techniques throughout our analysis allows us to
determine reliable uncertainties for all parameters. We provide accurate
seismic parameters and their uncertainties for a large sample of red giants and
determine their asteroseismic fundamental parameters. We investigate the
influence of the stars' metallicities on their positions in the H-R diagram. We
study the red-giant populations in the red clump and bump and compare them to a
synthetic population and find a mass and metallicity gradient in the red clump
and clear evidence of a secondary-clump population.Comment: accepted for publication in Astronomy and Astrophysics, 14 pages, 13
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