568 research outputs found
Detection of deuterium Balmer lines in the Orion Nebula
The detection and first identification of the deuterium Balmer emission
lines, D-alpha and D-beta, in the core of the Orion Nebula is reported.
Observations were conducted at the 3.6m Canada-France-Hawaii Telescope, using
the Echelle spectrograph Gecko. These lines are very narrow and have identical
11 km/s velocity shifts with respect to H-alpha and H-beta. They are probably
excited by UV continuum fluorescence from the Lyman (DI) lines and arise from
the interface between the HII region and the molecular cloud.Comment: 4 pages, latex, 1 figure, 1 table, accepted for publication in
Astronomy & Astrophysics, Letter
Corrigendum to "The upper atmosphere of the exoplanet HD209458b revealed by the sodium D lines: Temperature-pressure profile, ionization layer and thermosphere" [2011, A&A, 527, A110]
An error was detected in the code used for the analysis of the HD209458b
sodium profile (Vidal-Madjar et al. 2011). Here we present an updated T-P
profile and briefly discuss the consequences.Comment: Published in Astronomy & Astrophysics, 533, C
Rayleigh scattering in the transit spectrum of HD 189733b
The transit spectrum of the exoplanet HD 189733b has recently been obtained
between 0.55 and 1.05 microns. Here we present an analysis of this spectrum. We
develop first-order equations to interpret absorption spectra. In the case of
HD 189733b, we show that the observed slope of the absorption as a function of
wavelength is characteristic of extinction proportional to the inverse of the
fourth power of the wavelength (lambda^-4). Assuming an extinction dominated by
Rayleigh scattering, we derive an atmospheric temperature of 1340+/-150 K. If
molecular hydrogen is responsible for the Rayleigh scattering, the atmospheric
pressure at the planetary characteristic radius of 0.1564 stellar radius must
be 410+/-30 mbar. However the preferred scenario is scattering by condensate
particles. Using the Mie approximation, we find that the particles must have a
low value for the imaginary part of the refraction index. We identify MgSiO3 as
a possible abundant condensate whose particle size must be between 0.01 and 0.1
microns. For this condensate, assuming solar abundance, the pressure at 0.1564
stellar radius is found to be between a few microbars and few millibars, and
the temperature is found to be in the range 1340-1540 K, and both depend on the
particle size.Comment: Accepted for publication in A&A Lette
New observations of the extended hydrogen exosphere of the extrasolar planet HD209458b
Atomic hydrogen escaping from the planet HD209458b provides the largest
observational signature ever detected for an extrasolar planet atmosphere.
However, the Space Telescope Imaging Spectrograph (STIS) used in previous
observational studies is no longer available, whereas additional observations
are still needed to better constrain the mechanisms subtending the evaporation
process, and determine the evaporation state of other `hot Jupiters'. Here, we
aim to detect the extended hydrogen exosphere of HD209458b with the Advanced
Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) and to find
evidence for a hydrogen comet-like tail trailing the planet, which size would
depend on the escape rate and the amount of ionizing radiation emitted by the
star. These observations also provide a benchmark for other transiting planets,
in the frame of a comparative study of the evaporation state of close-in giant
planets. Eight HST orbits are used to observe two transits of HD209458b.
Transit light curves are obtained by performing photometry of the unresolved
stellar Lyman-alpha emission line during both transits. Absorption signatures
of exospheric hydrogen during the transit are compared to light curve models
predicting a hydrogen tail. Transit depths of (9.6 +/- 7.0)% and (5.3 +/-
10.0)% are measured on the whole Lyman-alpha line in visits 1 and 2,
respectively. Averaging data from both visits, we find an absorption depth of
(8.0 +/- 5.7)%, in good agreement with previous studies. The extended size of
the exosphere confirms that the planet is likely loosing hydrogen to space.
Yet, the photometric precision achieved does not allow us to better constrain
the hydrogen mass loss rate.Comment: Accepted for publication in Astronomy & Astrophysics. 5 pages, 3
figure
GTC OSIRIS transiting exoplanet atmospheric survey: detection of sodium in XO-2b from differential long-slit spectroscopy
We present two transits of the hot-Jupiter exoplanet XO-2b using the Gran
Telescopio Canarias (GTC). The time series observations were performed using
long-slit spectroscopy of XO-2 and a nearby reference star with the OSIRIS
instrument, enabling differential specrophotometric transit lightcurves capable
of measuring the exoplanet's transmission spectrum. Two optical low-resolution
grisms were used to cover the optical wavelength range from 3800 to 9300{\AA}.
We find that sub-mmag level slit losses between the target and reference star
prevent full optical transmission spectra from being constructed, limiting our
analysis to differential absorption depths over ~1000{\AA} regions. Wider long
slits or multi-object grism spectroscopy with wide masks will likely prove
effective in minimising the observed slit-loss trends. During both transits, we
detect significant absorption in the planetary atmosphere of XO-2b using a
50{\AA} bandpass centred on the Na I doublet, with absorption depths of
Delta(R_pl/R_star)^2=0.049+/-0.017 % using the R500R grism and 0.047+/-0.011 %
using the R500B grism (combined 5.2-sigma significance from both transits). The
sodium feature is unresolved in our low-resolution spectra, with detailed
modelling also likely ruling out significant line-wing absorption over an
~800{\AA} region surrounding the doublet. Combined with narrowband photometric
measurements, XO-2b is the first hot Jupiter with evidence for both sodium and
potassium present in the planet's atmosphere.Comment: 9 pages, 10 figures, 1 table, accepted for publication in MNRA
Observation de la réponse de surfaces agricoles aux pluies par télédétection en hyperfréquence active aéroportée
La tĂ©lĂ©dĂ©tection active en hyperfrĂ©quence Ă partir d'un radar aĂ©roportĂ© a Ă©tĂ© employĂ©e pour suivre le comportement hydrique de parcelles agricoles au cours des pĂ©riodes d'humectation et de ressuyage. On montre l'effet du travail du sol sur le signal rĂ©trodiffusĂ© et la capacitĂ© du radar Ă mesurer l'humiditĂ© de surface du sol. L'utilisation de cette technique permet d'apprĂ©hender la variabilitĂ© spatiale de l'humiditĂ© des sols et la variabilitĂ© de comportement hydrique des surfaces agricoles en fonction du type de couvert vĂ©gĂ©tal et des travaux culturaux antĂ©cĂ©dents. A l'Ă©chelle du bassin versant, on obtient une indication concernant l'Ă©volution de l'Ă©tat hydrique moyen du sol, pouvant ĂȘtre reliĂ©e aux donnĂ©es de pluies et de dĂ©bits mesurĂ©s par ailleurs. On discute de l'intĂ©rĂȘt de cette mĂ©thode en hydrologie, Ă partir des donnĂ©es des futurs satellites Ă©quipĂ©s de radar. (RĂ©sumĂ© d'auteur
The Upper Atmosphere of HD17156b
HD17156b is a newly-found transiting extrasolar giant planet (EGP) that
orbits its G-type host star in a highly eccentric orbit (e~0.67) with an
orbital semi-major axis of 0.16 AU. Its period, 21.2 Earth days, is the longest
among the known transiting planets. The atmosphere of the planet undergoes a
27-fold variation in stellar irradiation during each orbit, making it an
interesting subject for atmospheric modelling. We have used a three-dimensional
model of the upper atmosphere and ionosphere for extrasolar gas giants in order
to simulate the progress of HD17156b along its eccentric orbit. Here we present
the results of these simulations and discuss the stability, circulation, and
composition in its upper atmosphere. Contrary to the well-known transiting
planet HD209458b, we find that the atmosphere of HD17156b is unlikely to escape
hydrodynamically at any point along the orbit, even if the upper atmosphere is
almost entirely composed of atomic hydrogen and H+, and infrared cooling by H3+
ions is negligible. The nature of the upper atmosphere is sensitive to to the
composition of the thermosphere, and in particular to the mixing ratio of H2,
as the availability of H2 regulates radiative cooling. In light of different
simulations we make specific predictions about the thermosphere-ionosphere
system of HD17156b that can potentially be verified by observations.Comment: 31 pages, 42 eps figure
Transit spectrophotometry of the exoplanet HD189733b. I. Searching for water but finding haze with HST NICMOS
We present Hubble Space Telescope near-infrared transit photometry of the
nearby hot-Jupiter HD189733b. The observations were taken with the NICMOS
instrument during five transits, with three transits executed with a narrowband
filter at 1.87 microns and two performed with a narrowband filter at 1.66
microns. Our observing strategy using narrowband filters is insensitive to the
usual HST intra-orbit and orbit-to-orbit measurement of systematic errors,
allowing us to accurately and robustly measure the near-IR wavelength
dependance of the planetary radius. Our measurements fail to reproduce the
Swain et al. absorption signature of atmospheric water below 2 microns at a
5-sigma confidence level. We measure a planet-to-star radius contrast of
0.15498+/-0.00035 at 1.66 microns and a contrast of 0.15517+/-0.00019 at 1.87
microns. Both of our near-IR planetary radii values are in excellent agreement
with the levels expected from Rayleigh scattering by sub-micron haze particles,
observed at optical wavelengths, indicating that upper-atmospheric haze still
dominates the near-IR transmission spectra over the absorption from gaseous
molecular species at least below 2 microns.Comment: 9 pages, 7 figures. Accepted for publication in A&
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