72 research outputs found
Environnements immersifs et dispositifs numériques: Etudes expérimentales et approches distanciées
This paper proposes to discuss the theoretical and methodological issues applied to the study of immersive environments in information and communication sciences. Environments considered are digital devices that generate more or less strong effects of immersion (virtual reality, augmented reality, serious games, etc..). The authors question the renewal of a constructivist and ethno-methodological posture that will be put into test in various experiments. The paper concludes with an illustration of a methodology currently being tested on an industrial project
Near-infrared transmission spectrum of the warm-uranus GJ 3470b with the Wide Field Camera-3 on the Hubble Space Telescope
The atmospheric composition of low-mass exoplanets is the object of intense
observational and theoretical investigations. GJ3470b is a warm uranus recently
detected in transit across a bright late-type star. The transit of this planet
has already been observed in several band passes from the ground and space,
allowing observers to draw an intriguing yet incomplete transmission spectrum
of the planet atmospheric limb. In particular, published data in the visible
suggest the existence of a Rayleigh scattering slope, making GJ3470b a unique
case among the known neptunes, while data obtained beyond 2 um are consistent
with a flat infrared spectrum. The unexplored near-infrared spectral region
between 1 and 2 um, is thus key to undertanding the atmospheric nature of
GJ3470b. Here, we report on the first space-borne spectrum of GJ3470, obtained
during one transit of the planet with WFC3 on board HST, operated in stare
mode. The spectrum covers the 1.1--1.7-um region with a resolution of about
300. We retrieve the transmission spectrum of GJ3470b with a chromatic
planet-to-star radius ratio precision of 0.15% (about one scale height) per
40-nm bins. At this precision, the spectrum appears featureless, in good
agreement with ground-based and Spitzer infrared data at longer wavelengths,
pointing to a flat transmission spectrum from 1 to 5 um. We present new
simulations of transmission spectra for GJ3470b, which allow us to show that
the HST/WFC3 observations rule out cloudless hydrogen-rich atmospheres (>10
sigma) as well as hydrogen-rich atmospheres with tholin haze (>5 sigma). Adding
our near-infrared measurements to the full set of previously published data
from 0.3 to 5 um, we find that a cloudy, hydrogen-rich atmosphere can explain
the full transmission spectrum if, at the terminator, the clouds are located at
low pressures (<1 mbar) or the water mixing ratio is extremely low (<1 ppm).Comment: Astronomy & Astrophysics, in press. 19 figures. 2 table
The HARPS search for southern extra-solar planets: XXVIII. Two giant planets around M0 dwarfs
Fewer giants planets are found around M dwarfs than around more massive
stars, and this dependence of planetary characteristics on the mass of the
central star is an important observational diagnostic of planetary formation
theories. In part to improve on those statistics, we are monitoring the radial
velocities of nearby M dwarfs with the HARPS spectrograph on the ESO 3.6 m
telescope. We present here the detection of giant planets around two nearby M0
dwarfs: planets, with minimum masses of respectively 5 Jupiter masses and 1
Saturn mass, orbit around Gl 676A and HIP 12961. The latter is, by over a
factor of two, the most massive planet found by radial velocity monitoring of
an M dwarf, but its being found around an early M-dwarf is in approximate line
with the upper envelope of the planetary vs stellar mass diagram. HIP 12961
([Fe/H]=-0.07) is slightly more metal-rich than the average solar neighborhood
([Fe/H]=-0.17), and Gl 676A ([Fe/H=0.18) significantly so. The two stars
together therefore reinforce the growing trend for giant planets being more
frequent around more metal-rich M dwarfs, and the 5~Jupiter mass Gl 676Ab being
found around a metal-rich star is consistent with the expectation that the most
massive planets preferentially form in disks with large condensate masses.Comment: Corrected an error in the labelling of one line in Table
The Earth as an extrasolar transiting planet: Earth's atmospheric composition and thickness revealed by Lunar eclipse observations
An important goal within the quest for detecting an Earth-like extrasolar
planet, will be to identify atmospheric gaseous bio-signatures. Observations of
the light transmitted through the Earth's atmosphere, as for an extrasolar
planet, will be the first step for future comparisons. We have completed
observations of the Earth during a Lunar eclipse, a unique situation similar to
that of a transiting planet. We aim at showing what species could be detected
in its atmosphere at optical wavelengths, where a lot of photons are available
in the masked stellar light. We present observations of the 2008 August 16 Moon
eclipse performed with the SOPHIE spectrograph at the Observatoire de
Haute-Provence. Locating the spectrograph fibers in the penumbra of the
eclipse, the Moon irradiance is then a mix of direct, unabsorbed Sun light and
solar light that has passed through the Earth's limb. This mixture essentially
reproduces what is recorded during the transit of an extrasolar planet. We
report here the clear detection of several Earth atmospheric compounds in the
transmission spectra, such as ozone, molecular oxygen, and neutral sodium as
well as molecular nitrogen and oxygen through the Rayleigh signature. Moreover,
we present a method that allows us to derive the thickness of the atmosphere
versus the wavelength for penumbra eclipse observations. We quantitatively
evaluate the altitude at which the atmosphere becomes transparent for important
species like molecular oxygen and ozone, two species thought to be tightly
linked to the presence of life. The molecular detections presented here are an
encouraging first attempt, necessary to better prepare for the future of
extremely-large telescopes and transiting Earth-like planets. Instruments like
SOPHIE will be mandatory when characterizing the atmospheres of transiting
Earth-like planets from the ground and searching for bio-marker signatures.Comment: 15 pages, 14 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysic
Detection of "Flavescence dorée" Grapevine Disease Using Unmanned Aerial Vehicle (UAV) Multispectral Imagery
Flavescence dorée is a grapevine disease affecting European vineyards which has severe economic consequences and containing its spread is therefore considered as a major challenge for viticulture. Flavescence dorée is subject to mandatory pest control including removal of the infected
vines and, in this context, automatic detection of Flavescence dorée symptomatic vines by unmanned aerial vehicle (UAV) remote sensing could constitute a key diagnosis instrument for growers. The objective of this paper is to evaluate the feasibility of discriminating the Flavescence dorée symptoms in red and white cultivars from healthy vine vegetation using UAV multispectral imagery. Exhaustive ground truth data and UAV multispectral imagery (visible and near-infrared domain) have been acquired in September 2015 over four selected vineyards in Southwest France. Spectral signatures of healthy and symptomatic plants were studied with a set of 20 variables computed from the UAV images (spectral bands, vegetation indices and biophysical parameters) using univariate and multivariate classification approaches. Best results were achieved with red cultivars (both using univariate and multivariate approaches). For white cultivars, results were not satisfactory either for the univariate or the multivariate. Nevertheless, external accuracy assessment show that despite problems of Flavescence dorée and healthy pixel misclassification, an operational Flavescence dorée mapping technique using UAV-based imagery can still be proposed
A rocky planet transiting a nearby low-mass star
M-dwarf stars -- hydrogen-burning stars that are smaller than 60 per cent of
the size of the Sun -- are the most common class of star in our Galaxy and
outnumber Sun-like stars by a ratio of 12:1. Recent results have shown that M
dwarfs host Earth-sized planets in great numbers: the average number of M-dwarf
planets that are between 0.5 to 1.5 times the size of Earth is at least 1.4 per
star. The nearest such planets known to transit their star are 39 parsecs away,
too distant for detailed follow-up observations to measure the planetary masses
or to study their atmospheres. Here we report observations of GJ 1132b, a
planet with a size of 1.2 Earth radii that is transiting a small star 12
parsecs away. Our Doppler mass measurement of GJ 1132b yields a density
consistent with an Earth-like bulk composition, similar to the compositions of
the six known exoplanets with masses less than six times that of the Earth and
precisely measured densities. Receiving 19 times more stellar radiation than
the Earth, the planet is too hot to be habitable but is cool enough to support
a substantial atmosphere, one that has probably been considerably depleted of
hydrogen. Because the host star is nearby and only 21 per cent the radius of
the Sun, existing and upcoming telescopes will be able to observe the
composition and dynamics of the planetary atmosphere.Comment: Published in Nature on 12 November 2015, available at
http://dx.doi.org/10.1038/nature15762. This is the authors' version of the
manuscrip
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
Spitzer Observations of GJ 3470 b: A Very Low-density Neptune-size Planet Orbiting a Metal-rich M Dwarf
We present Spitzer/IRAC 4.5 μm transit photometry of GJ 3470 b, a Neptune-size planet orbiting an M1.5 dwarf star with a 3.3 day period recently discovered in the course of the HARPS M-dwarf survey. We refine the stellar parameters by employing purely empirical mass-luminosity and surface brightness relations constrained by our updated value for the mean stellar density, and additional information from new near-infrared spectroscopic observations. We derive a stellar mass of M_* = 0.539^(+0.047)_(-0.043) M_☉ and a radius of R_* = 0.568^(+0.037)_(-0.031)R_☉. We determine the host star of GJ 3470 b to be metal-rich, with a metallicity of [Fe/H] = +0.20 ± 0.10 and an effective temperature of T_(eff) = 3600 ± 100 K. The revised stellar parameters yield a planetary radius R_p = 4.83_(-0.21)^(+0.22)R_⊕ that is 13% larger than the value previously reported in the literature. We find a planetary mass M_p = 13.9^(+1.5)_(-1.4)M_⊕ that translates to a very low planetary density, P_p= 0.72^(+0.13)_(-0.12) g cm^(–3), which is 33% smaller than the original value. With a mean density half of that of GJ 436 b, GJ 3470 b is an example of a very low-density low-mass planet, similar to Kepler-11 d, Kepler-11 e, and Kepler-18 c, but orbiting a much brighter nearby star that is more conducive to follow-up studies
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