366 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
De-biasing interferometric visibilities in VLTI-AMBER data of low SNR observations
AIMS: We have found that the interferometric visibilities of VLTI-AMBER
observations, extracted via the standard reduction package, are significantly
biased when faint targets are concerned. The visibility biases derive from a
time variable fringing effect (correlated noise) appearing on the detector.
METHODS: We have developed a method to correct this bias that consists in a
subtraction of the extra power due to such correlated noise, so that the real
power spectrum at the spatial frequencies of the fringing artifact can be
restored. RESULTS: This pre-processing procedure is implemented in a software,
called AMDC and available to the community, to be run before the standard
reduction package. Results obtained on simulated and real observations are
presented and discussed.Comment: 7 pages, 9 figure
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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)
The RCB star V854 Cen is surrounded by a hot dusty shell
Aims : The hydrogen-deficient supergiants known as R Coronae Borealis (RCB)
stars might be the result of a double-degenerate merger of two white dwarfs
(WDs), or a final helium shell flash in a planetary nebula central star. In
this context, any information on the geometry of their circumstellar
environment and, in particular, the potential detection of elongated
structures, is of great importance. Methods : We obtained near-IR observations
of V854 Cen with the AMBER recombiner located at the Very Large Telescope
Interferometer (VLTI) array with the compact array (B35m) in 2013 and the
long array (B140m) in 2014. At each time, V854 Cen was at maximum light.
The - and -band continua were investigated by means of spectrally
dependant geometric models. These data were supplemented with mid-IR VISIR/VLT
images. Results : A dusty slightly elongated over density is discovered both in
the - and -band images. With the compact array, the central star is
unresolved (\,mas), but a flattened dusty environment of mas is discovered whose flux increases from about 20% in the
band to reach about 50% at 2.3\micron, which indicates hot
(T1500\,K) dust in the close vicinity of the star. The major axis is
oriented at a position angle (P.A.) of 12629. Adding the long-array
configuration dataset provides tighter constraints on the star diameter
( mas), a slight increase of the overdensity to
mas and a consistent P.A. of 13349. The closure phases, sensitive to
asymmetries, are null and compatible with a centro-symmetric, unperturbed
environment excluding point sources at the level of 3% of the total flux in
2013 and 2014. The VISIR images exhibit a flattened aspect ratio at the 15-20%
level at larger distances (1\arcsec) with a position angle of
9219, marginally consistent with the interferometric observations.
Conclusions : This is the first time that a moderately elongated structure has
been observed around an RCB star. These observations confirm the numerous
suggestions for a bipolar structure proposed for this star in the literature,
which were mainly based on polarimetric and spectroscopic observations.Comment: Accepted by A\&A, new version after language editing, Astronomy and
Astrophysics (2014
VLTI/AMBER spectro-interferometric imaging of VX Sgr's inhomogenous outer atmosphere
Aims. We aim to explore the photosphere of the very cool late-type star VX
Sgr and in particular the existence and characterization of molecular layers
above the continuum forming photosphere. Methods. We obtained interferometric
observations with the VLTI/AMBER interferometer using the fringe tracker FINITO
in the spectral domain 1.45-2.50 micron with a spectral resolution of about 35
and baselines ranging from 15 to 88 meters.We perform independent image
reconstruction for different wavelength bins and fit the interferometric data
with a geometrical toy model.We also compare the data to 1D dynamical models of
Miras atmosphere and to 3D hydrodynamical simulations of red supergiant (RSG)
and asymptotic giant branch (AGB) stars. Results. Reconstructed images and
visibilities show a strong wavelength dependence. The H-band images display two
bright spots whose positions are confirmed by the geometrical toy model. The
inhomogeneities are qualitatively predicted by 3D simulations. At about 2,00
micron and in the region 2,35 - 2,50 micron, the photosphere appears extended
and the radius is larger than in the H band. In this spectral region, the
geometrical toy model locates a third bright spot outside the photosphere that
can be a feature of the molecular layers. The wavelength dependence of the
visibility can be qualitatively explained by 1D dynamical models of Mira
atmospheres. The best-fitting photospheric models show a good match with the
observed visibilities and give a photospheric diameter of theta = 8,82+-0,50
mas. The H2O molecule seems to be the dominant absorber in the molecular
layers. Conclusions. We show that the atmosphere of VX Sgr rather resembles
Mira/AGB star model atmospheres than RSG model atmospheres. In particular, we
see molecular (water) layers that are typical for Mira stars.Comment: 9 Pages, Accepted for publication on Astronomy & Astrophysics, two
references update
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
Imaging the spinning gas and dust in the disc around the supergiant A[e] star HD62623
Context. To progress in the understanding of evolution of massive stars one
needs to constrain the mass-loss and determine the phenomenon responsible for
the ejection of matter an its reorganization in the circumstellar environment
Aims. In order to test various mass-ejection processes, we probed the geometry
and kinematics of the dust and gas surrounding the A[e] supergiant HD 62623.
Methods. We used the combined high spectral and spatial resolution covered by
the VLTI/AMBER instrument. Thanks to a new multiwavelength optical/IR
interferometry imaging technique, we reconstructed the first velocity-resolved
images with a milliarcsecond resolution in the infrared domain. Results. We
managed to disentangle the dust and gas emission in the HD 62623 circumstellar
disc.We measured the dusty disc inner inner rim, i.e. 6 mas, constrained the
inclination angle and the position angle of the major-axis of the disc.We also
measured the inner gaseous disc extension (2 mas) and probed its velocity field
thanks to AMBER high spectral resolution. We find that the expansion velocity
is negligible, and that Keplerian rotation is a favoured velocity field. Such a
velocity field is unexpected if fast rotation of the central star alone is the
main mechanism of matter ejection. Conclusions. As the star itself seems to
rotate below its breakup-up velocity, rotation cannot explain the formation of
the dense equatorial disc. Moreover, as the expansion velocity is negligible,
radiatively driven wind is also not a suitable explanation to explain the disc
formation. Consequently, the most probable hypothesis is that the accumulation
of matter in the equatorial plane is due to the presence of the spectroscopic
low mass companion.Comment: To be published soon in A\&
V838 Monocerotis: the central star and its environment a decade after outburst
Aims. V838 Monocerotis erupted in 2002, brightened in a series of outbursts,
and eventually developed a spectacular light echo. A very red star emerged a
few months after the outburst. The whole event has been interpreted as the
result of a merger. Methods. We obtained near-IR and mid-IR interferometric
observations of V838 Mon with the AMBER and MIDI recombiners located at the
Very Large Telescope Interferometer (VLTI) array. The MIDI two-beam
observations were obtained with the 8m Unit Telescopes between October 2011 and
February 2012. The AMBER three-beam observations were obtained with the compact
array (Bm) in April 2013 and the long array (B140m) in May 2014,
using the 1.8m Auxiliary Telescopes. Results. A significant new result is the
detection of a compact structure around V838 Mon, as seen from MIDI data. The
extension of the structure increases from a FWHM of 25 mas at 8 {\mu}m to 70
mas at 13 {\mu}m. At the adopted distance of D = 6.1 0.6 kpc, the dust is
distributed from about 150 to 400 AU around V838 Mon. The MIDI visibilities
reveal a flattened structure whose aspect ratio increases with wavelength. The
major axis is roughly oriented around a position angle of -10 degrees, which
aligns with previous polarimetric studies reported in the literature. This
flattening can be interpreted as a relic of the 2002 eruption or by the
influence of the currently embedded B3V companion. The AMBER data provide a new
diameter for the pseudo-photosphere, which shows that its diameter has
decreased by about 40% in 10yrs, reaching a radius R = 750 200
R (3.5 1.0 AU). Conclusions. After the 2002 eruption,
interpreted as the merging of two stars, it seems that the resulting source is
relaxing to a normal state. The nearby environment exhibits an equatorial
over-density of dust up to several hundreds of AU.Comment: Astronomy and Astrophysics (2014) Will be set by the publishe
Resolving the dusty circumstellar environment of the A[e] supergiant HD 62623 with the VLTI/MIDI
B[e] stars are hot stars surrounded by circumstellar gas and dust responsible
for the presence of emission lines and IR-excess in their spectra. How dust can
be formed in this highly illuminated and diluted environment remains an open
issue. HD 62623 is one of the very few A-type supergiants showing the B[e]
phenomenon. We obtained nine calibrated visibility measurements using the
VLTI/MIDI instrument in SCI-PHOT mode and PRISM spectral dispersion mode with
projected baselines ranging from 13 to 71 m and with various position angles.
We used geometrical models and physical modeling with a radiative transfer code
to analyze these data. The dusty circumstellar environment of HD 62623 is
partially resolved by the VLTI/MIDI even with the shortest baselines. The
environment is flattened and can be separated into two components: a compact
one whose extension grows from 17 mas at 8 microns to 30 mas at 9.6 microns and
stays almost constant up to 13 microns, and a more extended one that is
over-resolved even with the shortest baselines. Using the radiative transfer
code MC3D, we managed to model HD 62623's circumstellar environment as a dusty
disk with an inner radius of 3.85+-0.6 AU, an inclination angle of 60+-10 deg,
and a mass of 2x10^-7Mo. It is the first time that the dusty disk inner rim of
a supergiant star exhibiting the B[e] phenomenon is significantly constrained.
The inner gaseous envelope likely contributes up to 20% to the total N band
flux and acts like a reprocessing disk. Finally, the hypothesis of a stellar
wind deceleration by the companion's gravitational effects remains the most
probable case since the bi-stability mechanism does not seem to be efficient
for this star.Comment: 13 pages, 11 figures. A&A accepted pape
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The latest (version 4.3) Mars Climate Database
Introduction: The Mars Climate Database (MCD) is a database of meteorological fields derived from General Circulation Model (GCM) numerical simulations of the Martian atmosphere and validated using available observational data. The MCD includes complementary post-processing schemes such as high spatial resolution interpolation of environmental data and means of reconstructing the variability thereof. The GCM is developed at Laboratoire de Météorologie Dynamique du CNRS (Paris, France) [1,2] in collaboration with the Open University (UK), the Oxford University (UK) and the Instituto de Astrofisica de Andalucia (Spain) with support from the European Space Agency (ESA) and the Centre National
d'Etudes Spatiales (CNES)
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