275 research outputs found
Binning is sinning: morphological light-curve distortions due to finite integration time
We explore how finite integration times or equivalently temporal binning
induces morphological distortions to the transit light-curve. These
distortions, if uncorrected for, lead to the retrieval of erroneous system
parameters and may even lead to some planetary candidates being rejected as
ostensibly unphysical. We provide analytic expressions for estimating the
disturbance to the various light-curve parameters as a function of the
integration time. These effects are particularly crucial in light of the
long-cadence photometry often used for discovering new exoplanets by, for
example, Convection Rotation and Planetary Transits (COROT) and the Kepler
Mission (8.5 and 30 min). One of the dominant effects of long integration times
is a systematic underestimation of the light-curve-derived stellar density,
which has significant ramifications for transit surveys. We present a
discussion of numerical integration techniques to compensate for the effects
and produce expressions to quickly estimate the errors of such techniques, as a
function of integration time and numerical resolution. This allows for an
economic choice of resolution before attempting fits of long-cadence
light-curves. We provide a comparison of the short- and long-cadence
light-curves of TrES-2b and show that the retrieved transit parameters are
consistent using the techniques discussed here.Comment: Long delayed upload of the MNRAS accepted version, 10 pages, 3
figure
Theoretical Transmission Spectra During Extrasolar Giant Planet Transits
The recent transit observation of HD 209458 b - an extrasolar planet orbiting
a sun-like star - confirmed that it is a gas giant and determined that its
orbital inclination is 85 degrees. This inclination makes possible
investigations of the planet atmosphere. In this paper we discuss the planet
transmission spectra during a transit. The basic tenet of the method is that
the planet atmosphere absorption features will be superimposed on the stellar
flux as the stellar flux passes through the planet atmosphere above the limb.
The ratio of the planet's transparent atmosphere area to the star area is
small, approximately 10^{-3} to 10^{-4}; for this method to work very strong
planet spectral features are necessary. We use our models of close-in
extrasolar giant planets to estimate promising absorption signatures: the
alkali metal lines, in particular the Na I and K I resonance doublets, and the
He I - triplet line at 1083.0 nm. If successful, observations
will constrain the line-of-sight temperature, pressure, and density. The most
important point is that observations will constrain the cloud depth, which in
turn will distinguish between different atmosphere models. We also discuss the
potential of this method for EGPs at different orbital distances and orbiting
non-solar-type stars.Comment: revised to agree with accepted paper, ApJ, in press. 12 page
Evolution of "51Peg b-like" Planets
About one-quarter of the extrasolar giant planets discovered so far have
orbital distances smaller than 0.1 AU. These ``51Peg b-like'' planets can now
be directly characterized, as shown by the planet transiting in front the star
HD209458. We review the processes that affect their evolution.
We apply our work to the case of HD209458b, whose radius has been recently
measured. We argue that its radius can be reproduced only when the deep
atmosphere is assumed to be unrealistically hot. When using more realistic
atmospheric temperatures, an energy source appears to be missing in order to
explain HD209458b's large size. The most likely source of energy available is
not in the planet's spin or orbit, but in the intense radiation received from
the parent star. We show that the radius of HD209458b can be reproduced if a
small fraction (~1%) of the stellar flux is transformed into kinetic energy in
the planetary atmosphere and subsequently converted to thermal energy by
dynamical processes at pressures of tens of bars.Comment: 11 pages including 9 figures. A&A, in press. Also available at
http://www.obs-nice.fr/guillot/pegasi-planets
Nightside Pollution of Exoplanet Transit Depths
Out of the known transiting extrasolar planets, the majority are gas giants
orbiting their host star at close proximity. Both theoretical and observational
studies support the hypothesis that such bodies emit significant amounts of
flux relative to the host star, increasing towards infrared wavelengths. For
the dayside of the exoplanet, this phenomenon typically permits detectable
secondary eclipses at such wavelengths, which may be used to infer atmospheric
composition. In this paper, we explore the effects of emission from the
nightside of the exoplanet on the primary transit lightcurve, which is
essentially a self-blend. Allowing for nightside emission, an exoplanet's
transit depth is no longer exclusively a function of the ratio-of-radii. The
nightside of an exoplanet is emitting flux and the contrast to the star's
emission is of the order of ~10^(-3) for hot-Jupiters. Consequently, we show
that the transit depth in the mid-infrared will be attenuated due to flux
contribution from the nightside emission by ~10^(-4). We show how this effect
can be compensated for in the case where exoplanet phase curves have been
measured, in particular for HD 189733b. For other systems, it may be possible
to make a first-order correction by using temperature estimates of the planet.
Unless the effect is accounted for, transmission spectra will also be polluted
by nightside emission and we estimate that a Spitzer broadband spectrum on a
bright target is altered at the 1-sigma level. Using archived Spitzer
measurements, we show that the effect respectively increases the 8.0um and
24.0um transit depths by 1-sigma and 0.5-sigma per transit for HD 189733b.
Consequently, we estimate that this would be 5-10 sigma effect for near-future
JWST observations.Comment: Accepted in MNRA
Atmospheric Circulation and Tides of "51Peg b-like" Planets
We examine the properties of the atmospheres of extrasolar giant planets at
orbital distances smaller than 0.1 AU from their stars. We show that these
``51Peg b-like'' planets are rapidly synchronized by tidal interactions, but
that small departures from synchronous rotation can occur because of
fluid-dynamical torques within these planets. Previous radiative-transfer and
evolution models of such planets assume a homogeneous atmosphere. Nevertheless,
we show using simple arguments that, at the photosphere, the day-night
temperature difference and characteristic wind speeds may reach ~500 K and ~2
km/s, respectively. Substantial departures from chemical equilibrium are
expected. The cloud coverage depends sensitively on the dynamics; clouds could
exist predominantly either on the dayside or nightside, depending on the
circulation regime. Radiative-transfer models that assume homogeneous
conditions are therefore inadequate in describing the atmospheric properties of
51Peg b-like planets. We present preliminary three-dimensional, nonlinear
simulations of the atmospheric circulation of HD209458b that indicate plausible
patterns for the circulation and generally agree with our simpler estimates.
Furthermore, we show that kinetic energy production in the atmosphere can lead
to the deposition of substantial energy in the interior, with crucial
consequences for the evolution of these planets. Future measurements of
reflected and thermally-emitted radiation from these planets will help test our
ideas.Comment: 14 pages, 8 figures. A&A, in press. Also available at
http://www.obs-nice.fr/guillot/pegasi-planets
A Time-Dependent Radiative Model of HD209458b
We present a time-dependent radiative model of the atmosphere of HD209458b
and investigate its thermal structure and chemical composition. In a first
step, the stellar heating profile and radiative timescales were calculated
under planet-averaged insolation conditions. We find that 99.99% of the
incoming stellar flux has been absorbed before reaching the 7 bar level.
Stellar photons cannot therefore penetrate deeply enough to explain the large
radius of the planet. We derive a radiative time constant which increases with
depth and reaches about 8 hr at 0.1 bar and 2.3 days at 1 bar. Time-dependent
temperature profiles were also calculated, in the limit of a zonal wind that is
independent on height (i.e. solid-body rotation) and constant absorption
coefficients. We predict day-night variations of the effective temperature of
\~600 K, for an equatorial rotation rate of 1 km/s, in good agreement with the
predictions by Showman &Guillot (2002). This rotation rate yields day-to-night
temperature variations in excess of 600 K above the 0.1-bar level. These
variations rapidly decrease with depth below the 1-bar level and become
negligible below the ~5--bar level for rotation rates of at least 0.5 km/s. At
high altitudes (mbar pressures or less), the night temperatures are low enough
to allow sodium to condense into Na2S. Synthetic transit spectra of the visible
Na doublet show a much weaker sodium absorption on the morning limb than on the
evening limb. The calculated dimming of the sodium feature during planetary
transites agrees with the value reported by Charbonneau et al. (2002).Comment: 9 pages, 8 figures, replaced with the revised versio
Phase light curves for extrasolar Jupiters and Saturns
We predict how a remote observer would see the brightness variations of giant
planets similar to Jupiter and Saturn as they orbit their central stars. We
model the geometry of Jupiter, Saturn and Saturn's rings for varying orbital
and viewing parameters. Scattering properties for the planets and rings at
wavelenghts 0.6-0.7 microns follow Pioneer and Voyager observations, namely,
planets are forward scattering and rings are backward scattering. Images of the
planet with or without rings are simulated and used to calculate the
disk-averaged luminosity varying along the orbit, that is, a light curve is
generated. We find that the different scattering properties of Jupiter and
Saturn (without rings) make a substantial difference in the shape of their
light curves. Saturn-size rings increase the apparent luminosity of the planet
by a factor of 2-3 for a wide range of geometries. Rings produce asymmetric
light curves that are distinct from the light curve of the planet without
rings. If radial velocity data are available for the planet, the effect of the
ring on the light curve can be distinguished from effects due to orbital
eccentricity. Non-ringed planets on eccentric orbits produce light curves with
maxima shifted relative to the position of the maximum planet's phase. Given
radial velocity data, the amount of the shift restricts the planet's unknown
orbital inclination and therefore its mass. Combination of radial velocity data
and a light curve for a non-ringed planet on an eccentric orbit can also be
used to constrain the surface scattering properties of the planet. To summarize
our results for the detectability of exoplanets in reflected light, we present
a chart of light curve amplitudes of non-ringed planets for different
eccentricities, inclinations, and the viewing azimuthal angles of the observer.Comment: 40 pages, 13 figures, submitted to Ap.
A new powerful method for probing the atmospheres of transiting exoplanets
Although atmospheric transmission spectroscopy of HD209458b with the Hubble
Space Telescope has been very successful, attempts to detect its atmospheric
absorption features using ground-based telescopes have so far been fruitless.
Here we present a new method for probing the atmospheres of transiting
exoplanets which may be more suitable for ground-based observations, making use
of the Rossiter effect. During a transit, an exoplanet sequentially blocks off
light from the approaching and receding parts of the rotating star, causing an
artificial radial velocity wobble. The amplitude of this signal is directly
proportional to the effective size of the transiting object, and the wavelength
dependence of this effect can reveal atmospheric absorption features, in a
similar way as with transmission spectroscopy. The advantage of this method
over conventional atmospheric transmission spectroscopy is that it does not
rely on accurate photometric comparisons of observations on and off transit,
but instead depends on the relative velocity shifts of individual stellar
absorption lines within the same on-transit spectra. We used an archival
VLT/UVES data set to apply this method to HD209458. The amplitude of the
Rossiter effect is shown to be 1.7+-1.2 m/sec higher in the Sodium D lines than
in the weighted average of all other absorption lines in the observed
wavelength range, corresponding to an increment of 4.3+-3% (1.4 sigma). The
uncertainty in this measurement compares to a photometric accuracy of 5e-4 for
conventional atmospheric transmission spectroscopy, more than an order of
magnitude higher than previous attempts using ground-based telescopes.
Observations specifically designed for this method could increase the accuracy
further by a factor 2-3.Comment: LaTex, 5 pages, 4 figs; submitted to MNRAS Letter
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