10,400 research outputs found
PKI Interoperability: Still an Issue? A Solution in the X. 509 Realm
There exist many obstacles that slow the global adoption of public key infrastructure (PKI) technology. The PKI interoperability problem, being poorly understood, is one of the most confusing. In this paper, we clarify the PKI interoperability issue by exploring both the juridical and technical domains. We demonstrate the origin of the PKI interoperability problem by determining its root causes, the latter being legal, organizational and technical differences between countries, which mean that relying parties have no one to rely on. We explain how difficult it is to harmonize them. Finally, we propose to handle the interoperability problem from the trust management point of view, by introducing the role of a trust broker which is in charge of helping relying parties make informed decisions about X.509 certificates
A Smaller Radius for the Transiting Exoplanet WASP-10b
We present photometry of WASP-10 during the transit of its short-period
Jovian planet. We employed the novel PSF-shaping capabilities the OPTIC camera
mounted on the UH 2.2m telescope to achieve a photometric precision of 4.7e-4
per 1.3 min sample. With this new light curve, in conjunction with stellar
evolutionary models, we improve on existing measurements of the planetary,
stellar and orbital parameters. We find a stellar radius Rstar = 0.698 +/-
0.012 Rsun and a planetary radius Rp = 1.080 +/- 0.020 Rjup. The quoted errors
do not include any possible systematic errors in the stellar evolutionary
models. Our measurement improves the precision of the planet's radius by a
factor of 4, and revises the previous estimate downward by 16% (2.5sigma, where
sigma is the quadrature sum of the respective confidence limits). Our measured
radius of WASP-10b is consistent with previously published theoretical radii
for irradiated Jovian planets.Comment: 4 pages, 2 tables, 2 figures, table 1 available upon reques
The Detectability of Transit Depth Variations due to Exoplanetary Oblateness and Spin Precession
Knowledge of an exoplanet's oblateness and obliquity would give clues about
its formation and internal structure. In principle, a light curve of a
transiting planet bears information about the planet's shape, but previous work
has shown that the oblateness-induced signal will be extremely difficult to
detect. Here we investigate the potentially larger signals due to planetary
spin precession. The most readily detectable effects are transit depth
variations (TV) in a sequence of light curves. For a planet as oblate
as Jupiter or Saturn, the transit depth will undergo fractional variations of
order 1%. The most promising systems are those with orbital periods of
approximately 15--30 days, which is short enough for the precession period to
be less than about 40 years, and long enough to avoid spin-down due to tidal
friction. The detectability of the TV signal would be enhanced by moons
(which would decrease the precession period) or planetary rings (which would
increase the amplitude). The Kepler mission should find several planets for
which precession-induced TV signals will be detectable. Due to modeling
degeneracies, Kepler photometry would yield only a lower bound on oblateness.
The degeneracy could be lifted by observing the oblateness-induced asymmetry in
at least one transit light curve, or by making assumptions about the planetary
interior.Comment: Accepted for publication in The Astrophysical Journa
Line-profile tomography of exoplanet transits I: The Doppler shadow of HD 189733b
We present a direct method for isolating the component of the starlight
blocked by a planet as it transits its host star, and apply it to spectra of
the bright transiting planet HD 189733b. We model the global shape of the
stellar cross-correlation function as the convolution of a limb-darkened
rotation profile and a gaussian representing the Doppler core of the average
photospheric line profile. The light blocked by the planet during the transit
is a gaussian of the same intrinsic width, whose trajectory across the line
profile yields a precise measure of the misalignment angle and an independent
measure of v sin I. We show that even when v sin I is less than the width of
the intrinsic line profile, the travelling Doppler "shadow" cast by the planet
creates an identifiable distortion in the line profiles which is amenable to
direct modelling. Direct measurement of the trajectory of the missing starlight
yields self-consistent measures of the projected stellar rotation rate, the
intrinsic width of the mean local photospheric line profile, the projected
spin-orbit misalignment angle, and the system's centre-of-mass velocity.
Combined with the photometric rotation period, the results give a geometrical
measure of the stellar radius which agrees closely with values obtained from
high-precision transit photometry if a small amount of differential rotation is
present in the stellar photosphere.Comment: 8 pages, 5 figures, 2 tables; accepted by MNRA
Parameter Estimation from Time-Series Data with Correlated Errors: A Wavelet-Based Method and its Application to Transit Light Curves
We consider the problem of fitting a parametric model to time-series data
that are afflicted by correlated noise. The noise is represented by a sum of
two stationary Gaussian processes: one that is uncorrelated in time, and
another that has a power spectral density varying as . We present
an accurate and fast [O(N)] algorithm for parameter estimation based on
computing the likelihood in a wavelet basis. The method is illustrated and
tested using simulated time-series photometry of exoplanetary transits, with
particular attention to estimating the midtransit time. We compare our method
to two other methods that have been used in the literature, the time-averaging
method and the residual-permutation method. For noise processes that obey our
assumptions, the algorithm presented here gives more accurate results for
midtransit times and truer estimates of their uncertainties.Comment: Accepted in ApJ. Illustrative code may be found at
http://www.mit.edu/~carterja/code/ . 17 page
A Prograde, Low-Inclination Orbit for the Very Hot Jupiter WASP-3b
We present new spectroscopic and photometric observations of the transiting
exoplanetary system WASP-3. Spectra obtained during two separate transits
exhibit the Rossiter-McLaughlin (RM) effect and allow us to estimate the
sky-projected angle between the planetary orbital axis and the stellar rotation
axis, lambda = 3.3^{+2.5}_{-4.4} degrees. This alignment between the axes
suggests that WASP-3b has a low orbital inclination relative to the equatorial
plane of its parent star. During our first night of spectroscopic measurements,
we observed an unexpected redshift briefly exceeding the expected sum of the
orbital and RM velocities by 140 m/s. This anomaly could represent the
occultation of material erupting from the stellar photosphere, although it is
more likely to be an artifact caused by moonlight scattered into the
spectrograph.Comment: 23 pages, 4 figures, Accepted for publication in The Astrophysical
Journal, Replacement includes revised citation
Evaluation of a series hybird thrust bearing at DN values to three million. 1: Analysis and design
The analysis and design are presented of a hybrid bearing consisting of a 150-mm ball bearing and a centrifugally actuated, conical, fluid film bearing fitting an envelope with an outer radius of 86.4 mm (3.4 in.) and an inner radius of 71 mm (2.8 in.). The bearing analysis, combined with available torque data on ball bearings, indicates that an effective speed split between the ball and fluid-film bearings of 50 percent may be expected during operation at 20,000 rpm and under an axial load of 17,800 newtons (4000 lbs.). This speed split can result in a ten-fold increase in the life of the ball bearing when compared to a simple ball bearing system operating under similar conditions
Assessment of geophysical flows for zero-gravity simulation
The results of research relating to the feasibility of using a low gravity environment to model geophysical flows are presented. Atmospheric and solid earth flows are considered. Possible experiments and their required apparatus are suggested
A High Stellar Obliquity in the WASP-7 Exoplanetary System
We measure a tilt of 86+-6 deg between the sky projections of the rotation
axis of the WASP-7 star, and the orbital axis of its close-in giant planet.
This measurement is based on observations of the Rossiter-McLaughlin (RM)
effect with the Planet Finder Spectrograph on the Magellan II telescope. The
result conforms with the previously noted pattern among hot-Jupiter hosts,
namely, that the hosts lacking thick convective envelopes have high
obliquities. Because the planet's trajectory crosses a wide range of stellar
latitudes, observations of the RM effect can in principle reveal the stellar
differential rotation profile; however, with the present data the signal of
differential rotation could not be detected. The host star is found to exhibit
radial-velocity noise (``stellar jitter') with an amplitude of ~30m/s over a
timescale of days.Comment: ApJ accepted, 9 pages, 9 figure
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