1,518 research outputs found
Kepler Mission Stellar and Instrument Noise Properties Revisited
An earlier study of the Kepler Mission noise properties on time scales of
primary relevance to detection of exoplanet transits found that higher than
expected noise followed to a large extent from the stars, rather than
instrument or data analysis performance. The earlier study over the first six
quarters of Kepler data is extended to the full four years ultimately
comprising the mission. Efforts to improve the pipeline data analysis have been
successful in reducing noise levels modestly as evidenced by smaller values
derived from the current data products. The new analyses of noise properties on
transit time scales show significant changes in the component attributed to
instrument and data analysis, with essentially no change in the inferred
stellar noise. We also extend the analyses to time scales of several days,
instead of several hours to better sample stellar noise that follows from
magnetic activity. On the longer time scale there is a shift in stellar noise
for solar-type stars to smaller values in comparison to solar values.Comment: 10 pages, 8 figures, accepted by A
Detection of an Extrasolar Planet Atmosphere
We report high precision spectrophotometric observations of four planetary
transits of HD 209458, in the region of the sodium resonance doublet at 589.3
nm. We find that the photometric dimming during transit in a bandpass centered
on the sodium feature is deeper by (2.32 +/- 0.57) x 10^{-4} relative to
simultaneous observations of the transit in adjacent bands. We interpret this
additional dimming as absorption from sodium in the planetary atmosphere, as
recently predicted from several theoretical modeling efforts. Our model for a
cloudless planetary atmosphere with a solar abundance of sodium in atomic form
predicts more sodium absorption than we observe. There are several
possibilities that may account for this reduced amplitude, including reaction
of atomic sodium into molecular gases and/or condensates, photoionization of
sodium by the stellar flux, a low primordial abundance of sodium, or the
presence of clouds high in the atmosphere.Comment: 26 pages, 8 figures, accepted by ApJ 2001 November 1
Kepler Mission Stellar and Instrument Noise Properties
Kepler Mission results are rapidly contributing to fundamentally new
discoveries in both the exoplanet and asteroseismology fields. The data
returned from Kepler are unique in terms of the number of stars observed,
precision of photometry for time series observations, and the temporal extent
of high duty cycle observations. As the first mission to provide extensive time
series measurements on thousands of stars over months to years at a level
hitherto possible only for the Sun, the results from Kepler will vastly
increase our knowledge of stellar variability for quiet solar-type stars. Here
we report on the stellar noise inferred on the timescale of a few hours of most
interest for detection of exoplanets via transits. By design the data from
moderately bright Kepler stars are expected to have roughly comparable levels
of noise intrinsic to the stars and arising from a combination of fundamental
limitations such as Poisson statistics and any instrument noise. The noise
levels attained by Kepler on-orbit exceed by some 50% the target levels for
solar-type, quiet stars. We provide a decomposition of observed noise for an
ensemble of 12th magnitude stars arising from fundamental terms (Poisson and
readout noise), added noise due to the instrument and that intrinsic to the
stars. The largest factor in the modestly higher than anticipated noise follows
from intrinsic stellar noise. We show that using stellar parameters from
galactic stellar synthesis models, and projections to stellar rotation,
activity and hence noise levels reproduces the primary intrinsic stellar noise
features.Comment: Accepted by ApJ; 26 pages, 20 figure
Multilamellar bodies as potential scattering particles in human age-related nuclear cataracts
Using Stellar Limb-Darkening to Refine the Properties of HD 209458b
We use multi-band photometry to refine estimates for the planetary radius and
orbital inclination of the transiting planet system HD 209458. We gathered 1066
spectra over four distinct transits with the STIS spectrometer on the Hubble
Space Telescope using two gratings with a resolution R=1500 and a combined
wavelength range of 290-1030 nm. We divide the spectra into ten
spectrophotometric bandpasses, five for each grating, of equal wavelength span
within each grating, and fit a transit curve over all bandpasses
simultaneously. In our fit we use theoretical values for the stellar
limb-darkening to further constrain the planetary radius. We find that the
radius of HD 209458b is 1.320 +/- 0.025 R_Jup, which is a factor of two more
precise than current estimates. We also obtain improved estimates for the
orbital period P and time of center of transit T_C. Although in principle the
photon-limited precision of the STIS data should allow us to measure the timing
of individual transits to a precision of 2-7 s, we find that systematic
instrumental offsets in the measured flux from one orbit of the spacecraft to
the next degrade these measurements to a typical precision of +/- 14 s. Within
this level of error, we find no significant variations in the timing of the
eight events examined in this work.Comment: final accepted version of pape
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