545 research outputs found
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Seven-year climatology of dust opacity on Mars
This paper describes the procedure we have used to produce multi-annual dust scenarios for Martian years 24 to 30 from a multi-instrument dataset of total dust opacity observations. This procedure includes gridding the observations on a pre-defined longitude-latitude grid with 1 sol resolution in time, and spatially interpolating the results to obtain complete daily maps of total dust opacity. We used weighted binning as gridding technique, and spatial kriging as method of interpolation. The new dust scenarios are available as NetCDF files, easy to interface to any model including global circulation and mesoscale models for the Martian atmosphere
Study of the scientific potential of a three 40 cm Telescopes Interferometer at Dome C
Recent site testing (see:
http://www-luan.unice.fr/Concordiastro/indexantartic.html) has shown that Dome
C in Antarctica might have a high potential for stellar interferometry if some
solutions related to the surface atmospheric layer are found. A demonstrator
interferometer could be envisioned in order to fully qualify the site and
prepare the future development of a large array.
We analyse the performances of a prototype interferometer for Dome C made
with 3 telescopes of 40 cm diameter. It assumes classical Michelson
recombination. The most recent atmospheric and environmental conditions
measured at Dome C are considered (see K. Agabi "First whole atmosphere
night-time seeing measurements at Dome C, Antarctica"). We also study the
possible science reachable with such a demonstrator. Especially we evaluate
that even such small aperture interferometer could allow the detection and low
resolution spectroscopy of the most favourable pegaside planets.Comment: 6 pages, 5 figures, conferences SPIE, 0rlando, 200
3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability and habitability
The inner edge of the classical habitable zone is often defined by the
critical flux needed to trigger the runaway greenhouse instability. This 1D
notion of a critical flux, however, may not be so relevant for inhomogeneously
irradiated planets, or when the water content is limited (land planets).
Here, based on results from our 3D global climate model, we find that the
circulation pattern can shift from super-rotation to stellar/anti stellar
circulation when the equatorial Rossby deformation radius significantly exceeds
the planetary radius. Using analytical and numerical arguments, we also
demonstrate the presence of systematic biases between mean surface temperatures
or temperature profiles predicted from either 1D or 3D simulations.
Including a complete modeling of the water cycle, we further demonstrate that
for land planets closer than the inner edge of the classical habitable zone,
two stable climate regimes can exist. One is the classical runaway state, and
the other is a collapsed state where water is captured in permanent cold traps.
We identify this "moist" bistability as the result of a competition between the
greenhouse effect of water vapor and its condensation. We also present
synthetic spectra showing the observable signature of these two states.
Taking the example of two prototype planets in this regime, namely Gl581c and
HD85512b, we argue that they could accumulate a significant amount of water ice
at their surface. If such a thick ice cap is present, gravity driven ice flows
and geothermal flux should come into play to produce long-lived liquid water at
the edge and/or bottom of the ice cap. Consequently, the habitability of
planets at smaller orbital distance than the inner edge of the classical
habitable zone cannot be ruled out. Transiting planets in this regime represent
promising targets for upcoming observatories like EChO and JWST.Comment: Accepted for publication in Astronomy and Astrophysics, complete
abstract in the pdf, 18 pages, 18 figure
Evidence of an asymmetrical Keplerian disk in the Br{\gamma} and He I emission lines around the Be star HD 110432
Context. HD 110432 was classified as a "\gamma Cas X-ray analog" since it has
similar peculiar X-ray and optical characteristics, i.e. a hard-thermal X-ray
variable emission and an optical spectrum affected by an extensive disk. Lopes
de Oliveira et al. (2007) suggest that it might be a Be star harboring an
accreting white dwarf or that the X-rays may come from an interaction between
the surface of the star and its disk. Aims. To investigate the disk around this
Be star we used the VLTI/AMBER instrument, which combines high spectral
(R=12000) and high spatial (\theta min =4 mas) resolutions. Methods. We
constrain the geometry and kinematics of its circumstellar disk from the
highest spatial resolution ever achieved on this star. Results. We obtain a
disk extension in the Br{\gamma} line of 10.2 D\ast and 7.8 D\ast in the He I
line at 2.05 \mu m assuming a Gaussian disk model. The disk is clearly
following a Keplerian rotation. We obtained an inclination angle of 55\degree,
and the star is a nearly critical rotator with Vrot /Vc =1.000.2. This
inclination is greater than the value found for \gamma Cas (about 42\degree,
Stee et al. 2012), and is consistent with the inference from optical Fe II
emission profiles by Smith & Balona (2006) that the inclination should be more
than the \gamma Cas value. In the near-IR continuum, the disk of HD 110432 is 3
times larger than \gamma Cas's disk. We have no direct evidence of a companion
around HD 110432, but it seems that we have a clear signature for disk
inhomogeneities as detected for {\zeta} Tau. This asymmetrical disk detection
may be interpreted within the one-armed oscillation viscous disk framework.
Another finding is that the disk size in the near-IR is similar to other Be
stars with different spectral types and thus may be independent of the stellar
parameters, as found for classical Be stars.Comment: 9 page
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Martian meso/micro-scale winds and surface energy budget
Regional, diurnal and seasonal variations of surface
temperature are particularly large on Mars. This is mostly due to the Martian surface remaining close to radiative equilibrium. Contrary to most terrestrial locations, contributions of sensible heat flux (i.e. conduction/convection exchanges between atmosphere and surface) to the surface energy budget [hereinafter SEB] are negligible on Mars owing to lowatmospheric density and heat capacity (e.g. Figure 2 in Savijärvi and Kauhanen, 2008). This radiative control of surface temperature is a key characteristic of the Martian environment and has crucial consequences on the the Martian geology, meteorology, exobiology, etc.
In order to identify the impact of this Martian peculiarity to near-surface regional-to-local atmospheric circulations,
we employ our recently-built Martian limited-area meteorological model (Spiga and Forget, 2009). We use horizontal resolutions adapted to the dynamical phenomena we aim to resolve: from several tens of kilometers to compute regional winds (mesoscale simulations) to several tens of meters to compute atmospheric boundary-layer winds (microscale or turbulent-resolving simulations, also called Large-Eddy Simulations, LES)
Dark spots and cold jets in the polar regions of Mars: new clues from a thermal model of surface CO ice
International audienceObservations of the Martian CO ice cap in late winter and spring have revealed exotic phenomena. Unusual dark spots, fans and blotches form as the south-polar seasonal CO ice cap retreats. The formation mechanisms of these features are not clearly understood. Theoretical models suggest that photons could penetrate deep into the CO ice down to the regolith, leading to basal sublimation and gas and dust ejection. We have developed a detailed thermal model able to simulate the temporal evolution of the regolith-CO ice layer-atmosphere column. It takes into account heat conduction, radiative transfer within the ice and the atmosphere, and latent heat exchange when there is a phase transition. We found that a specific algorithm, fully coupling these 3 components, was needed to properly predict ice sublimation below the surface. Our model allows us to determine under what conditions basal sublimation is possible and thus when and where it can occur on Mars. Our results show that basal sublimation is possible if we consider large pathlengths and very little dust content within the ice. Moreover, the model can explain how dark spots can appear very early after the end of the polar night at high latitudes. We also evaluate the importance of the different parameters in our simulations. Contrary to what was suggested by theoretical models, the role of seasonal thermal waves is found to be limited. Solar radiation alone can initiate basal sublimation, which therefore only depends on the CO ice properties. Three main modes were identified: one where condensation/sublimation only occurs at the surface (in the case of small grains and/or high dust content), one where basal sublimation is possible (large pathlengths and very little dust content) and an intermediate mode where sublimation within the ice may occur. We suggest that these different modes could be keys to understanding many processes that occur at the surface of Mars, like the anticryptic area behavior or the recent reported activity in gullies
VLTI/PIONIER images the Achernar disk swell
Context. The mechanism of disk formation around fast-rotating Be stars is not
well understood. In particular, it is not clear which mechanisms operate, in
addition to fast rotation, to produce the observed variable ejection of matter.
The star Achernar is a privileged laboratory to probe these additional
mechanisms because it is close, presents B-Be phase variations on timescales
ranging from 6 yr to 15 yr, a companion star was discovered around it, and
probably presents a polar wind or jet. Aims. Despite all these previous
studies, the disk around Achernar was never directly imaged. Therefore we seek
to produce an image of the photosphere and close environment of the star.
Methods. We used infrared long-baseline interferometry with the PIONIER/VLTI
instrument to produce reconstructed images of the photosphere and close
environment of the star over four years of observations. To study the disk
formation, we compared the observations and reconstructed images to previously
computed models of both the stellar photosphere alone (normal B phase) and the
star presenting a circumstellar disk (Be phase). Results. The observations
taken in 2011 and 2012, during the quiescent phase of Achernar, do not exhibit
a disk at the detection limit of the instrument. In 2014, on the other hand, a
disk was already formed and our reconstructed image reveals an extended H-band
continuum excess flux. Our results from interferometric imaging are also
supported by several H-alpha line profiles showing that Achernar started an
emission-line phase sometime in the beginning of 2013. The analysis of our
reconstructed images shows that the 2014 near-IR flux extends to 1.7 - 2.3
equatorial radii. Our model-independent size estimation of the H-band continuum
contribution is compatible with the presence of a circumstellar disk, which is
in good agreement with predictions from Be-disk models
Material ejection by the cold jets and temperature evolution of the south seasonal polar cap of Mars from THEMIS/CRISM observations and implications for surface properties
As the seasonal CO_2 ice polar caps of Mars retreat during spring, dark spots appear on the ice in some specific regions. These features are thought to result from basal sublimation of the transparent CO_2 ice followed by ejection of regolith-type material, which then covers the ice. We have used Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) reflectance data, Thermal Emission Imaging System (THEMIS) visible images, and THEMIS-derived temperature retrievals along with a thermal numerical model to constrain the physical and compositional characteristics of the seasonal cap for several areas exhibiting dark spots at both high spatial and temporal resolutions. Data analysis suggests an active period of material ejection (before solar longitude (Ls) 200), accumulation around the ejection points, and spreading of part of the ejected material over the whole area, followed by a period where no significant amount of material is ejected, followed by complete defrosting (≈ Ls 245). Dark material thickness on top of the CO_2 ice is estimated to range from a few hundreds of microns to a few millimeters in the warmest spots, based on numerical modeling combined with the observed temperature evolution. The nature of the venting process and the amount of material that is moved lead to the conclusion that it could have an important impact on the surface physical properties
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