94 research outputs found
Radiative braking in the extended exosphere of GJ436b
The recent detection of a giant exosphere surrounding the warm Neptune GJ436
b has shed new light on the evaporation of close-in planets, revealing that
moderately irradiated, low-mass exoplanets could make exceptional targets for
studying this mechanism and its impact on the exoplanet population. Three
HST/STIS observations were performed in the Lyman- line of GJ436 at
different epochs, showing repeatable transits with large depths and extended
durations. Here, we study the role played by stellar radiation pressure on the
structure of the exosphere and its transmission spectrum. We found that the
neutral hydrogen atoms in the exosphere of GJ436 b are not swept away by
radiation pressure as shown to be the case for evaporating hot Jupiters.
Instead, the low radiation pressure from the M-dwarf host star only brakes the
gravitational fall of the escaping hydrogen toward the star and allows its
dispersion within a large volume around the planet, yielding radial velocities
up to about -120 km s that match the observations. We performed
numerical simulations with the EVaporating Exoplanets code (EVE) to study the
influence of the escape rate, the planetary wind velocity, and the stellar
photoionization. While these parameters are instrumental in shaping the
exosphere and yield simulation results in general agreement with the
observations, the spectra observed at the different epochs show specific,
time-variable features that require additional physics.Comment: 10 pages, 5 figure
A Spitzer Search for Water in the Transiting Exoplanet HD189733b
We present Spitzer Space Telescope observations of the extrasolar planet
HD189733b primary transit, obtained simultaneously at 3.6 and 5.8 microns with
the Infrared Array Camera. The system parameters, including planetary radius,
stellar radius, and impact parameter are derived from fits to the transit light
curves at both wavelengths. We measure two consistent planet-to-star radius
ratios, (Rp/Rs)[3.6m] = 0.1560 +/- 0.0008(stat) +/- 0.0002(syst) and
(Rp/Rs)[5.8m] = 0.1541 +/- 0.0009(stat) +/- 0.0009(syst), which include
both the random and systematic errors in the transit baseline. Although planet
radii are determined at 1%-accuracy, if all uncertainties are taken into
account the resulting error bars are still too large to allow for the detection
of atmospheric constituants like water vapour. This illustrates the need to
observe multiple transits with the longest possible out-of-transit baseline, in
order to achieve the precision required by transmission spectroscopy of giant
extrasolar planets.Comment: Accepted in The Astrophysical Journal Letter
A Spitzer Search for Water in the Transiting Exoplanet HD189733b
We present Spitzer Space Telescope observations of the extrasolar planet
HD189733b primary transit, obtained simultaneously at 3.6 and 5.8 microns with
the Infrared Array Camera. The system parameters, including planetary radius,
stellar radius, and impact parameter are derived from fits to the transit light
curves at both wavelengths. We measure two consistent planet-to-star radius
ratios, (Rp/Rs)[3.6m] = 0.1560 +/- 0.0008(stat) +/- 0.0002(syst) and
(Rp/Rs)[5.8m] = 0.1541 +/- 0.0009(stat) +/- 0.0009(syst), which include
both the random and systematic errors in the transit baseline. Although planet
radii are determined at 1%-accuracy, if all uncertainties are taken into
account the resulting error bars are still too large to allow for the detection
of atmospheric constituants like water vapour. This illustrates the need to
observe multiple transits with the longest possible out-of-transit baseline, in
order to achieve the precision required by transmission spectroscopy of giant
extrasolar planets.Comment: Accepted in The Astrophysical Journal Letter
Temperature-Pressure Profile of the hot Jupiter HD 189733b from HST Sodium Observations: Detection of Upper Atmospheric Heating
We present transmission spectra of the hot Jupiter HD 189733b taken with the
Space Telescope Imaging Spectrograph aboard HST. The spectra cover the
wavelength range 5808-6380 Ang with a resolving power of R=5000. We detect
absorption from the NaI doublet within the exoplanet's atmosphere at the 9
sigma confidence level within a 5 Ang band (absorption depth 0.09 +/- 0.01%)
and use the data to measure the doublet's spectral absorption profile. We
detect only the narrow cores of the doublet. The narrowness of the feature
could be due to an obscuring high-altitude haze of an unknown composition or a
significantly sub-solar NaI abundance hiding the line wings beneath a H2
Rayleigh signature. We compare the spectral absorption profile over 5.5 scale
heights with model spectral absorption profiles and constrain the temperature
at different atmospheric regions, allowing us to construct a vertical
temperature profile. We identify two temperature regimes; a 1280 +/- 240 K
region derived from the NaI doublet line wings corresponding to altitudes below
~ 500 km, and a 2800 +/- 400 K region derived from the NaI doublet line cores
corresponding to altitudes from ~ 500-4000 km. The zero altitude is defined by
the white-light radius of Rp/Rstar=0.15628 +/- 0.00009. The temperature rises
with altitude, which is likely evidence of a thermosphere. The absolute
pressure scale depends on the species responsible for the Rayleigh signature
and its abundance. We discuss a plausible scenario for this species, a
high-altitude silicate haze, and the atmospheric temperature-pressure profile
that results. In this case, the high altitude temperature rise for HD 189733b
occurs at pressures of 10^-5 to 10^-8 bar
Rayleigh scattering by H2 in the extrasolar planet HD 209458b
Copyright © ESO 2008Transiting planets, such as HD 209458b, offer a unique opportunity to scrutinize the planetary atmospheric content. Although molecular hydrogen is expected to be the main atmospheric constituent, H2
remains uncovered because of the lack of strong transition
from near-ultraviolet to near-infrared. Here we analyse the absorption spectrum of HD 209458b obtained by Sing et al. (2008a, ApJ,
submitted) which provides a measurement of the absorption depth in the 3000â6200 Ă
wavelength range. We show that the rise in absorption depth at short wavelengths can be interpreted as Rayleigh scattering within the atmosphere of HD 209458b. Since Rayleigh
scattering traces the entire atmosphere, this detection enables a direct determination of the pressure-altitude relationship, which is required to determine the absolute fraction of other elements such as sodium. At the zero altitude defined by the absorption depth
of 1.453%, which corresponds to a planetary radius of 0.1205 times the stellar radius, we find a pressure of 33±5 mbar. Using the
variation of the Rayleigh scattering cross-section as a function of wavelength, we determine the temperature to be 2200 ± 260 K at
33 mbar pressure
HST/STIS Optical Transit Transmission Spectra of the hot-Jupiter HD209458b
We present the transmission spectra of the hot-Jupiter HD209458b taken with
the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. Our
analysis combines data at two resolutions and applies a complete pixel-by-pixel
limb-darkening correction to fully reveal the spectral line shapes of
atmospheric absorption features. Terrestrial-based Na I and H I contamination
are identified which mask the strong exoplanetary absorption signature in the
Na core, which we find reaches total absorption levels of ~0.11% in a 4.4 Ang
band. The Na spectral line profile is characterized by a wide absorption
profile at the lowest absorption depths, and a sharp transition to a narrow
absorption profile at higher absorption values. The transmission spectra also
shows the presence of an additional absorber at ~6,250 Ang, observed at both
medium and low resolutions. We performed various limb-darkening tests,
including using high precision limb-darkening measurements of the sun to
characterize a general trend of Atlas models to slightly overestimate the
amount of limb-darkening at all wavelengths, likely due to the limitations of
the model's one-dimensional nature. We conclude that, despite these
limitations, Atlas models can still successfully model limb-darkening in high
signal-to-noise transits of solar-type stars, like HD209458, to a high level of
precision over the entire optical regime (3,000-10,000 Ang) at transit phases
between 2nd and 3rd contact.Comment: 18 pages, 11 figures, Accepted to Ap
A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b
Exoplanets orbiting close to their parent stars could lose some fraction of
their atmospheres because of the extreme irradiation. Atmospheric mass loss
primarily affects low-mass exoplanets, leading to suggest that hot rocky
planets might have begun as Neptune-like, but subsequently lost all of their
atmospheres; however, no confident measurements have hitherto been available.
The signature of this loss could be observed in the ultraviolet spectrum, when
the planet and its escaping atmosphere transit the star, giving rise to deeper
and longer transit signatures than in the optical spectrum. Here we report that
in the ultraviolet the Neptune-mass exoplanet GJ 436b (also known as Gliese
436b) has transit depths of 56.3 +/- 3.5% (1 sigma), far beyond the 0.69%
optical transit depth. The ultraviolet transits repeatedly start ~2 h before,
and end >3 h after the ~1 h optical transit, which is substantially different
from one previous claim (based on an inaccurate ephemeris). We infer from this
that the planet is surrounded and trailed by a large exospheric cloud composed
mainly of hydrogen atoms. We estimate a mass-loss rate in the range of
~10^8-10^9 g/s, which today is far too small to deplete the atmosphere of a
Neptune-like planet in the lifetime of the parent star, but would have been
much greater in the past.Comment: Published in Nature on 25 June 2015. Preprint is 28 pages, 12
figures, 2 table
On the Possible Properties of Small and Cold Extrasolar Planets: Is OGLE-2005-BLG-390Lb Entirely Frozen?
Extrasolar planets as light as a few Earths are now being detected. Such
planets are likely not gas or ice giants. Here, we present a study on the
possible properties of the small and cold extrasolar planets, applied to the
case of the recently discovered planet OGLE-2005-BLG-390Lb (Beaulieu et al.
2006). This planet (5.5[+5.5/-2.7] Earth masses) orbits
2.6[+1.5/-0.6]-astronomical units away from an old M-type star of the Galactic
Bulge. The planet should be entirely frozen given the low surface temperature
(35 to 47 K). However, depending on the rock-to-ice mass ratio in the planet,
the radiogenic heating could be sufficient to make the existence of liquid
water within an icy crust possible. This possibility is estimated as a function
of the planetary mass and the illumination received from the parent star, both
being strongly related by the observational constraints. The results are
presented for water-poor and water-rich planets. We find that no oceans can be
present in any cases at 9-10 Gyr, a typical age for a star of the Bulge.
However, we find that, in the past when the planet was < 5-billion-years old,
liquid water was likely present below an icy surface. Nevertheless, the planet
is now likely to be entirely frozen.Comment: Accepted for publication in Ap
TiO and VO broad band absorption features in the optical spectrum of the atmosphere of the hot-Jupiter HD 209458b
Copyright
© ESO 2008Aims. The presence of titanium oxide (TiO) and vanadium oxide (VO) gas phase species is searched for in the atmosphere of the hot Jupiter HD 209458b .
Methods. We compared a model for the planet's transmitted spectrum to multi-wavelength eclipse-depth measurements (from 3000 to 10â000 Ă
) using archived HST-STIS time series spectra. We make use of these observations to search for spectral signatures from extra absorbers in the planet atmosphere between 6000 and 8000 Ă
.
Results. Along with sodium depletion and Rayleigh scattering recently published for this exoplanet atmosphere, an extra absorber of uncertain origin, redward of the sodium lines, is present in the atmosphere of the planet. Furthermore, this planet has a stratosphere experiencing a thermal inversion caused by the capture of optical stellar flux by absorbers at altitude. Recent models have predicted that the presence of TiO and VO in the atmosphere of HD 209458b âmay be responsible for this temperature inversion. Although no specific TiO and VO spectral band head signatures have been identified unambiguously in the observed spectrum, we suggest here that the opacities of those molecules are possible candidates to explain the remaining continuous broad band absorption observed between 6200 and 8000 Ă
. To match the data reasonably well, the abundances of TiO and VO molecules are evaluated from ten to one thousand times below solar. This upper limit result is in agreement with expected variations with altitude due to depletion effects such as condensation
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