2,976 research outputs found
Blue, white, and red ocean planets - Simulations of orbital variations in flux and polarization colors
An exoplanet's habitability will depend strongly on the presence of liquid
water. Flux and/or polarization measurements of starlight that is reflected by
exoplanets could help to identify exo-oceans. We investigate which broadband
spectral features in flux and polarization phase functions of reflected
starlight uniquely identify exo-oceans. We compute total fluxes F and polarized
fluxes Q of starlight reflected by cloud-free and (partly) cloudy exoplanets,
for wavelengths from 350 to 865 nm. The ocean surface has waves composed of
Fresnel reflecting wave facets and whitecaps, and scattering within the water
body is included. Total flux F, polarized flux Q, and degree of polarization P
of ocean planets change color from blue, through white, to red at phase angles
alpha ranging from 134-108 deg for F, and from 123-157 deg for Q, with cloud
coverage fraction fc increasing from 0.0 to 1.0 for F, and to 0.98 for Q. The
color change in P only occurs for fc ranging from 0.03-0.98, with the color
crossing angle alpha ranging from 88-161 deg. The total flux F of a cloudy,
zero surface albedo planet can also change color, and for fc=0.0, an ocean
planet's F will not change color for surface pressures ps > 8 bars. Polarized
flux Q of a zero surface albedo planet does not change color for any fc. The
color change of P of starlight reflected by an exoplanet, from blue, through
white, to red with increasing alpha above 88 deg, appears to identify a
(partly) cloudy exo-ocean. The color change of polarized flux Q with increasing
alpha above 123 deg appears to uniquely identify an exo-ocean, independent of
surface pressure or cloud fraction. At the color changing phase angle, the
angular distance between a star and its planet is much larger than at the phase
angle where the glint appears in reflected light. The color change in
polarization thus offers better prospects for detecting exo-oceans.Comment: Accepted for publication in Astron. Astrophys; multicolumn versio
The influence of forward-scattered light in transmission measurements of (exo)planetary atmospheres
[Abridged] The transmission of light through a planetary atmosphere can be
studied as a function of altitude and wavelength using stellar or solar
occultations, giving often unique constraints on the atmospheric composition.
For exoplanets, a transit yields a limb-integrated, wavelength-dependent
transmission spectrum of an atmosphere. When scattering haze and/or cloud
particles are present in the planetary atmosphere, the amount of transmitted
flux not only depends on the total optical thickness of the slant light path
that is probed, but also on the amount of forward-scattering by the scattering
particles. Here, we present results of calculations with a three-dimensional
Monte Carlo code that simulates the transmitted flux during occultations or
transits. For isotropically scattering particles, like gas molecules, the
transmitted flux appears to be well-described by the total atmospheric optical
thickness. Strongly forward-scattering particles, however, such as commonly
found in atmospheres of Solar System planets, can increase the transmitted flux
significantly. For exoplanets, such added flux can decrease the apparent radius
of the planet by several scale heights, which is comparable to predicted and
measured features in exoplanet transit spectra. We performed detailed
calculations for Titan's atmosphere between 2.0 and 2.8 micron and show that
haze and gas abundances will be underestimated by about 8% if
forward-scattering is ignored in the retrievals. At shorter wavelengths, errors
in the gas and haze abundances and in the spectral slope of the haze particles
can be several tens of percent, also for other Solar System planetary
atmospheres. We also find that the contribution of forward-scattering can be
fairly well described by modelling the atmosphere as a plane-parallel slab.Comment: Icarus, accepted for publicatio
Looking for the rainbow on exoplanets covered by liquid and icy water clouds
Looking for the primary rainbow in starlight that is reflected by exoplanets
appears to be a promising method to search for liquid water clouds in
exoplanetary atmospheres. Ice water clouds, that consist of water crystals
instead of water droplets, could potentially mask the rainbow feature in the
planetary signal by covering liquid water clouds. Here, we investigate the
strength of the rainbow feature for exoplanets that have liquid and icy water
clouds in their atmosphere, and calculate the rainbow feature for a realistic
cloud coverage of Earth. We calculate flux and polarization signals of
starlight that is reflected by horizontally and vertically inhomogeneous
Earth--like exoplanets, covered by patchy clouds consisting of liquid water
droplets or water ice crystals. The planetary surfaces are black. On a planet
with a significant coverage of liquid water clouds only, the total flux signal
shows a weak rainbow feature. Any coverage of the liquid water clouds by ice
clouds, however, dampens the rainbow feature in the total flux, and thus the
discovery of liquid water in the atmosphere. On the other hand, detecting the
primary rainbow in the polarization signal of exoplanets appears to be a
powerful tool for detecting liquid water in exoplanetary atmospheres, even when
these clouds are partially covered by ice clouds. In particular, liquid water
clouds covering as little as 10%-20% of the planetary surface, with more than
half of these covered by ice clouds, still create a polarized rainbow feature
in the planetary signal. Indeed, calculations of flux and polarization signals
of an exoplanet with a realistic Earth--like cloud coverage, show a strong
polarized rainbow feature.Comment: accepted for publication in Astronomy & Astrophysic
Reaching the hydrodynamic regime in a Bose-Einstein condensate by suppression of avalanche
We report the realization of a Bose-Einstein condensate (BEC) in the
hydrodynamic regime. The hydrodynamic regime is reached by evaporative cooling
at a relative low density suppressing the effect of avalanches. With the
suppression of avalanches a BEC containing 120.10^6 atoms is produced. The
collisional opacity can be tuned from the collisionless regime to a collisional
opacity of more than 3 by compressing the trap after condensation. In the
collisional opaque regime a significant heating of the cloud at time scales
shorter than half of the radial trap period is measured. This is direct proof
that the BEC is hydrodynamic.Comment: Article submitted for Phys. Rev. Letters, 6 figure
Motivations and experiences of UK students studying abroad
This report summarises the findings of research aimed at improving understanding of the motivations behind the international diploma mobility of UK student
Scattering matrices and expansion coefficients of Martian analogue palagonite particles
We present measurements of ratios of elements of the scattering matrix of
Martian analogue palagonite particles for scattering angles ranging from 3 to
174 degrees and a wavelength of 632.8 nm. To facilitate the use of these
measurements in radiative transfer calculations we have devised a method that
enables us to obtain, from these measurements, a normalized synthetic
scattering matrix covering the complete scattering angle range from 0 to 180
degrees. Our method is based on employing the coefficients of the expansions of
scattering matrix elements into generalized spherical functions. The synthetic
scattering matrix elements and/or the expansion coefficients obtained in this
way, can be used to include multiple scattering by these irregularly shaped
particles in (polarized) radiative transfer calculations, such as calculations
of sunlight that is scattered in the dusty Martian atmosphere.Comment: 34 pages 7 figures 1 tabl
Large atom number Bose-Einstein condensate of sodium
We describe the setup to create a large Bose-Einstein condensate containing
more than 120x10^6 atoms. In the experiment a thermal beam is slowed by a
Zeeman slower and captured in a dark-spot magneto-optical trap (MOT). A typical
dark-spot MOT in our experiments contains 2.0x10^10 atoms with a temperature of
320 microK and a density of about 1.0x10^11 atoms/cm^3. The sample is spin
polarized in a high magnetic field, before the atoms are loaded in the magnetic
trap. Spin polarizing in a high magnetic field results in an increase in the
transfer efficiency by a factor of 2 compared to experiments without spin
polarizing. In the magnetic trap the cloud is cooled to degeneracy in 50 s by
evaporative cooling. To suppress the 3-body losses at the end of the
evaporation the magnetic trap is decompressed in the axial direction.Comment: 11 pages, 12 figures, submitted to Review Of Scientific Instrument
Obfuscation for Cryptographic Purposes
An obfuscation of a function F should satisfy two requirements: firstly, using it should be possible to evaluate F; secondly, should not reveal anything about F that cannot be learnt from oracle access to F. Several definitions for obfuscation exist. However, most of them are either too weak for or incompatible with cryptographic applications, or have been shown impossible to achieve, or both.
We give a new definition of obfuscation and argue for its reasonability and usefulness. In particular, we show that it is strong enough for cryptographic applications, yet we show that it has the potential for interesting positive results. We illustrat
- …