152 research outputs found
Evidence against a strong thermal inversion in HD 209458 b from high-dispersion spectroscopy
Broadband secondary-eclipse measurements of hot Jupiters have indicated the
existence of atmospheric thermal inversions, but their presence is difficult to
determine from broadband measurements because of degeneracies between molecular
abundances and temperature structure. We apply high-resolution (R = 100 000)
infrared spectroscopy to probe the temperature-pressure profile of HD 209458 b.
This bright, transiting hot-Jupiter has long been considered the gold standard
for a hot Jupiter with an inversion layer, but this has been challenged in
recent publications. We observed the thermal dayside emission of HD 209458 b
with CRIRES / VLT during three nights, targeting the carbon monoxide band at
2.3 microns. Thermal inversions give rise to emission features, which means
that detecting emission lines in the planetary spectrum, as opposed to
absorption lines, would be direct evidence of a region in which the temperature
increases with altitude.
We do not detect any significant absorption or emission of CO in the dayside
spectrum of HD 209458 b, although cross-correlation with template spectra
either with CO absorption lines or with weak emission at the core of the lines
show a low-significance correlation signal with a signal-to-noise ratio of 3 -
3.5. Models with strong CO emission lines show a weak anti-correlation with
similar or lower significance levels. Furthermore, we found no evidence of
absorption or emission from H2O at these wavelengths.
The non-detection of CO in the dayside spectrum of HD 209458 b is interesting
in light of a previous CO detection in the transmission spectrum. That there is
no signal indicates that HD 209458 b either has a nearly isothermal atmosphere
or that the signal is heavily muted. Assuming a clear atmosphere, we can rule
out a full-disc dayside inversion layer in the pressure range 1 bar to 1 mbar.Comment: 11 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Finding extraterrestrial life using ground-based high-resolution spectroscopy
Exoplanet observations promise one day to unveil the presence of
extraterrestrial life. Atmospheric compounds in strong chemical disequilibrium
would point to large-scale biological activity just as oxygen and methane do in
the Earth's atmosphere. The cancellation of both the Terrestrial Planet Finder
and Darwin missions means that it is unlikely that a dedicated space telescope
to search for biomarker gases in exoplanet atmospheres will be launched within
the next 25 years. Here we show that ground-based telescopes provide a strong
alternative for finding biomarkers in exoplanet atmospheres through transit
observations. Recent results on hot Jupiters show the enormous potential of
high-dispersion spectroscopy to separate the extraterrestrial and telluric
signals making use of the Doppler shift of the planet. The transmission signal
of oxygen from an Earth-twin orbiting a small red dwarf star is only a factor 3
smaller than that of carbon monoxide recently detected in the hot Jupiter tau
Bootis b, albeit such a star will be orders of magnitude fainter. We show that
if Earth-like planets are common, the planned extremely large telescopes can
detect oxygen within a few dozen transits. Ultimately, large arrays of
dedicated flux collector telescopes equipped with high-dispersion spectrographs
can provide the large collecting area needed to perform a statistical study of
life-bearing planets in the solar neighborhood.Comment: 22 pages, 3 figures; published in Ap
The orbital motion, absolute mass, and high-altitude winds of exoplanet HD209458b
For extrasolar planets discovered using the radial velocity method, the
spectral characterization of the host star leads to a mass-estimate of the star
and subsequently of the orbiting planet. In contrast, if also the orbital
velocity of the planet would be known, the masses of both star and planet could
be determined directly using Newton's law of gravity, just as in the case of
stellar double-line eclipsing binaries. Here we report on the detection of the
orbital velocity of extrasolar planet HD209458b. High dispersion ground-based
spectroscopy during a transit of this planet reveals absorption lines from
carbon monoxide produced in the planet atmosphere, which shift significantly in
wavelength due to the change in the radial component of the planet orbital
velocity. These observations result in a mass determination of the star and
planet of 1.00+-0.22 Msun and 0.64+-0.09 Mjup respectively. A ~2 km/sec
blueshift of the carbon monoxide signal with respect to the systemic velocity
of the host star suggests the presence of a strong wind flowing from the
irradiated dayside to the non-irradiated nightside of the planet within the
0.01-0.1 mbar atmospheric pressure range probed by these observations. The
strength of the carbon monoxide signal suggests a CO mixing ratio of 1-3x10-3
in this planet's upper atmosphere.Comment: 11 Pages main article and 6 pages suppl. information: A final, edited
version appears in the 24 May 2010 issue of Natur
Search for water in a super-Earth atmosphere: High-resolution optical spectroscopy of 55 Cancri e
We present the analysis of high-resolution optical spectra of four transits
of 55Cnc e, a low-density, super-Earth that orbits a nearby Sun-like star in
under 18 hours. The inferred bulk density of the planet implies a substantial
envelope, which, according to mass-radius relationships, could be either a
low-mass extended or a high-mass compact atmosphere. Our observations
investigate the latter scenario, with water as the dominant species. We take
advantage of the Doppler cross-correlation technique, high-spectral resolution
and the large wavelength coverage of our observations to search for the
signature of thousands of optical water absorption lines. Using our
observations with HDS on the Subaru telescope and ESPaDOnS on the
Canada-France-Hawaii Telescope, we are able to place a 3-sigma lower limit of
10 g/mol on the mean-molecular weight of 55Cnc e's water-rich (volume mixing
ratio >10%), optically-thin atmosphere, which corresponds to an atmospheric
scale-height of ~80 km. Our study marks the first high-spectral resolution
search for water in a super-Earth atmosphere and demonstrates that it is
possible to recover known water-vapour absorption signals, in a nearby
super-Earth atmosphere, using high-resolution transit spectroscopy with current
ground-based instruments.Comment: Accepted for publication in ApJ 12 pages, 9 figures. Email:
[email protected]; [email protected]; [email protected];
[email protected]; [email protected]
Detection of carbon monoxide in the high-resolution day-side spectrum of the exoplanet HD 189733b
[Abridged] After many attempts over more than a decade, high-resolution
spectroscopy has recently delivered its first detections of molecular
absorption in exoplanet atmospheres, both in transmission and thermal emission
spectra. Targeting the combined signal from individual lines in molecular
bands, these measurements use variations in the planet radial velocity to
disentangle the planet signal from telluric and stellar contaminants. In this
paper we apply high resolution spectroscopy to probe molecular absorption in
the day-side spectrum of the bright transiting hot Jupiter HD 189733b. We
observed HD 189733b with the CRIRES high-resolution near-infrared spectograph
on the Very Large Telescope during three nights. We detect a 5-sigma absorption
signal from CO at a contrast level of ~4.5e-4 with respect to the stellar
continuum, revealing the planet orbital radial velocity at 154+4/-3 km s-1.
This allows us to solve for the planet and stellar mass in a similar way as for
stellar eclipsing binaries, resulting in Ms= 0.846+0.068/-0.049 Msun and Mp=
1.162+0.058/-0.039 MJup. No significant absorption is detected from H2O, CO2 or
CH4 and we determined upper limits on their line contrasts here. The detection
of CO in the day-side spectrum of HD 189733b can be made consistent with the
haze layer proposed to explain the optical to near-infrared transmission
spectrum if the layer is optically thin at the normal incidence angles probed
by our observations, or if the CO abundance is high enough for the CO
absorption to originate from above the haze. Our non-detection of CO2 at 2.0
micron is not inconsistent with the deep CO2 absorption from low resolution
NICMOS secondary eclipse data in the same wavelength range. If genuine, the
absorption would be so strong that it blanks out any planet light completely in
this wavelength range, leaving no high-resolution signal to be measured.Comment: A&A, accepted for publication. Fig.1 reduced in qualit
Combining high-dispersion spectroscopy (HDS) with high contrast imaging (HCI): Probing rocky planets around our nearest neighbors
Aims: In this work, we discuss a way to combine High Dispersion Spectroscopy
and High Contrast Imaging (HDS+HCI). For a planet located at a resolvable
angular distance from its host star, the starlight can be reduced up to several
orders of magnitude using adaptive optics and/or coronography. In addition, the
remaining starlight can be filtered out using high-dispersion spectroscopy,
utilizing the significantly different (or Doppler shifted) high-dispersion
spectra of the planet and star. In this way, HDS+HCI can in principle reach
contrast limits of ~1e-5 x 1e-5, although in practice this will be limited by
photon noise and/or sky-background.
Methods: We present simulations of HDS+HCI observations with the E-ELT, both
probing thermal emission from a planet at infrared wavelengths, and starlight
reflected off a planet atmosphere at optical wavelengths. For the infrared
simulations we use the baseline parameters of the E-ELT and METIS instrument,
with the latter combining extreme adaptive optics with an R=100,000 IFS. We
include realistic models of the adaptive optics performance and atmospheric
transmission and emission. For the optical simulation we also assume R=100,000
IFS with adaptive optics capabilities at the E-ELT.
Results: One night of HDS+HCI observations with the E-ELT at 4.8 um (d_lambda
= 0.07 um) can detect a planet orbiting alpha Cen A with a radius of R=1.5
R_earth and a twin-Earth thermal spectrum of T_eq=300 K at a signal-to-noise
(S/N) of 5. In the optical, with a Strehl ratio performance of 0.3, reflected
light from an Earth-size planet in the habitable zone of Proxima Centauri can
be detected at a S/N of 10 in the same time frame. Recently, first HDS+HCI
observations have shown the potential of this technique by determining the
spin-rotation of the young massive exoplanet beta Pictoris b. [abridged]Comment: 9 pages, A&A in press: A movie of the simulation can be found at
http://www.strw.leidenuniv.nl/~snellen/simulation.mpe
Search for Rayleigh scattering in the atmosphere of GJ1214b
We investigate the atmosphere of GJ1214b, a transiting super-Earth planet
with a low mean density, by measuring its transit depth as a function of
wavelength in the blue optical portion of the spectrum. It is thought that this
planet is either a mini-Neptune, consisting of a rocky core with a thick,
hydrogen-rich atmosphere, or a planet with a composition dominated by water.
Most observations favor a water-dominated atmosphere with a small scale-height,
however, some observations indicate that GJ1214b could have an extended
atmosphere with a cloud layer muting the molecular features. In an atmosphere
with a large scale-height, Rayleigh scattering at blue wavelengths is likely to
cause a measurable increase in the apparent size of the planet towards the
blue. We observed the transit of GJ1214b in the B-band with the FOcal Reducing
Spectrograph (FORS) at the Very Large Telescope (VLT) and in the g-band with
both ACAM on the William Hershel Telescope (WHT) and the Wide Field Camera
(WFC) at the Isaac Newton Telescope (INT). We find a planet-to-star radius
ratio in the B-band of 0.1162+/-0.0017, and in the g-band 0.1180+/-0.0009 and
0.1174+/-0.0017 for the WHT & INT observations respectively. These optical data
do not show significant deviations from previous measurements at longer
wavelengths. In fact, a flat transmission spectrum across all wavelengths best
describes the combined observations. When atmospheric models are considered a
small scale-height water-dominated model fits the data best.Comment: Accepted for publication in Ap
HCN ice in Titan's high-altitude southern polar cloud
Titan's middle atmosphere is currently experiencing a rapid change of season
after northern spring arrived in 2009. A large cloud was observed for the first
time above Titan's southern pole in May 2012, at an altitude of 300 km. This
altitude previously showed a temperature maximum and condensation was not
expected for any of Titan's atmospheric gases. Here we show that this cloud is
composed of micron-sized hydrogen cyanide (HCN) ice particles. The presence of
HCN particles at this altitude, together with new temperature determinations
from mid-infrared observations, indicate a very dramatic cooling of Titan's
atmosphere inside the winter polar vortex in early 2012. Such a cooling is
completely contrary to previously measured high-altitude warming in the polar
vortex, and temperatures are a hundred degrees colder than predicted by
circulation models. Besides elucidating the nature of Titan's mysterious polar
cloud, these results thus show that post-equinox cooling at the winter pole is
much more efficient than previously thought.Comment: Published in Nature on 2 October 2014. This is the author version,
before final editing by Natur
Spatial and temporal patterns of snowmelt refreezing in a Himalayan catchment
Recent progress has been made in quantifying snowmelt in the Himalaya. Although the conditions are favorable for refreezing, little is known about the spatial variability of meltwater refreezing, hindering a complete understanding of seasonal snowmelt dynamics. This study aims to improve our understanding about how refreezing varies in space and time. We simulated refreezing with the seNorge (v2.0) snow model for the Langtang catchment, Nepalese Himalaya, covering a 5-year period. Meteorological forcing data were derived from a unique elaborate network of meteorological stations and high-resolution meteorological simulations. The results show that the annual catchment average refreezing amounts to 122 mm w.e. (21% of the melt), and varies strongly in space depending on elevation and aspect. In addition, there is a seasonal altitudinal variability related to air temperature and snow depth, with most refreezing during the early melt season. Substantial intra-annual variability resulted from fluctuations in snowfall. Daily refreezing simulations decreased by 84% (annual catchment average of 19 mm w.e.) compared to hourly simulations, emphasizing the importance of using sub-daily time steps to capture melt-refreeze cycles. Climate sensitivity experiments revealed that refreezing is highly sensitive to changes in air temperature as a 2°C increase leads to a refreezing decrease of 35%
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