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Simic Z.,Astronomical Observatory | Simic Z.,Isaac Newton Institute of Chile | Dimitrijevic M.S.,Astronomical Observatory | Dimitrijevic M.S.,Isaac Newton Institute of Chile | Sahal-Brechot S.,Observatoire de Paris Meudon
Monthly Notices of the Royal Astronomical Society | Year: 2013

New Stark broadening parameters of interest for the astrophysical, laboratory and technological plasma modelling, investigations and analysis for nine resonant Cr II multiplets have been determined within the semiclassical perturbation approach. In order to demonstrate one possibility for their usage in astrophysical plasma research, obtained results have been applied to the analysis of the Stark broadening influence on stellar spectral line shapes. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Villanueva G.L.,NASA | Villanueva G.L.,Catholic University of America | Mumma M.J.,NASA | Novak R.E.,The College of New Rochelle | And 6 more authors.
Science | Year: 2015

We measured maps of atmospheric water (H2O) and its deuterated form (HDO) across the martian globe, showing strong isotopic anomalies and a significant high deuterium/hydrogen (D/H) enrichment indicative of great water loss. The maps sample the evolution of sublimation from the north polar cap, revealing that the released water has a representative D/H value enriched by a factor of about 7 relative to Earth's ocean [Vienna standard mean ocean water (VSMOW)]. Certain basins and orographic depressions show even higher enrichment, whereas high-altitude regions show much lower values (1 to 3 VSMOW). Our atmospheric maps indicate that water ice in the polar reservoirs is enriched in deuterium to at least 8 VSMOW, which would mean that early Mars (4.5 billion years ago) had a global equivalent water layer at least 137 meters deep. © 2015, American Association for the Advancement of Science. All rights reserved.

Soret L.,University of Liège | Gerard J.-C.,University of Liège | Piccioni G.,National institute for astrophysics | Drossart P.,Observatoire de Paris Meudon
Planetary and Space Science | Year: 2012

Limb spectra of the OH nightglow emission corresponding to the Δv=1 and Δv=2 sequences have been collected with the VIRTIS infrared imaging spectrograph on board Venus Express between April 2006 and October 2008. A detailed statistical analysis shows that the peak intensity and altitude of the two vibrational sequences are significantly correlated, with a mean intensity ratio of the two sequences of 0.38±0.37. The altitude of the maximum of the Δv=2 emission is located ∼1 km lower than Δv=1. A spectral analysis shows that the Δv=1 sequence is composed at 44.6% by the (1-0) band, 9.3% by the (3-2) band and 7.1% by the (4-3) band. The Δv=2 emission is best fitted if solely including the (2-0) band. A non-LTE model of OH vibrational population by the O3+H reaction including radiative and collisional relaxation has been used to compare the expected spectral distribution, the altitude of the emission peak and the emission rate under different assumptions on the quenching processes to those observed with VIRTIS. The adopted carbon dioxide, atomic oxygen and ozone densities are based on recent Venus Express remote sensing measurements. We find that the Sudden Death quenching scheme by CO2 produces inadequate spectral distribution between the various bands and insufficient airglow brightness. Instead, the observed spectral distribution and the total emission intensity are reasonably well reproduced with the Single Quantum jump model, an O density profile peaking at 103.5 km with a maximum value of 1.9×1011 cm-3, a O3 density profile peaking at 5.8×106 cm -3 at 96.5 km and a H density profile close to 108 cm -3 between 90 and 120 km, in agreement with several photochemical models. © 2012 Elsevier Ltd. All rights reserved.

Mouradian Z.,Observatoire de Paris Meudon
Solar Physics | Year: 2013

This article proposes a unified observational model of solar activity based on sunspot number and the solar global activity in the rotation of the structures, both per 11-year cycle. The rotation rates show a variation of a half-century period and the same period is also associated to the sunspot amplitude variation. The global solar rotation interweaves with the observed global organisation of solar activity. An important role for this assembly is played by the Grand Cycle formed by the merging of five sunspot cycles: a forgotten discovery by R. Wolf. On the basis of these elements, the nature of the Dalton Minimum, the Maunder Minimum, the Gleissberg Cycle, and the Grand Minima are presented. © 2012 Springer Science+Business Media Dordrecht.

Short A.,European Space Agency | Prod'Homme T.,Leiden University | Weiler M.,Observatoire de Paris Meudon | Brown S.,University of Cambridge | Brown A.,Leiden University
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

The European Space Agency's Gaia mission1 is scheduled for launch in 2012. It will operate at L2 for 5 years, rotating slowly so that its two optical telescopes will repeatedly observe more than one billion stars. The resulting data set will be iteratively reduced to solve for the relative position, parallax-distance and proper motion of every observed star, yielding a three dimensional dynamical model of our galaxy. The focal plane contains 106 large area silicon CCDs continuously operating in TDI mode at a line rate synchronised with the satellite rotation.2 One of the greatest challenges facing the mission is radiation damage in the CCDs which will cause charge loss and image distortion. This is particularly severe because the large focal plane is difficult to shield and because the launch will coincide with solar maximum. Despite steps taken to minimize the effects of radiation (e.g. regular use of charge injection), the residual distortion will need to be calibrated during the pipeline data processing. Due to the volume of data involved, this requires a trapping model which is physically realistic, yet fast enough and simple enough to implement in the pipeline. The current prototype Charge Distortion Model will be presented. This model was developed specifically for Gaia in TDI mode. However, an imaging mode version has already been applied to other missions, for example, to indicate the potential impact of radiation damage on the proposed Euclid mission. © 2010 SPIE.

Gonzalez O.A.,European Southern Observatory | Rejkuba M.,European Southern Observatory | Zoccali M.,University of Chile | Hill V.,University of Nice Sophia Antipolis | And 9 more authors.
Astronomy and Astrophysics | Year: 2011

Aims. We present the analysis of the [α/Fe] abundance ratios for a large number of stars at several locations in the Milky Way bulge with the aim of constraining its formation scenario. Methods. We obtained FLAMES-GIRAFFE spectra (R = 22 500) at the ESO Very Large Telescope for 650 bulge red giant branch (RGB) stars and performed spectral synthesis to measure Mg, Ca, Ti, and Si abundances. This sample is composed of 474 giant stars observed in 3 fields along the minor axis of the Galactic bulge and at latitudes b = -4°, b = -6°, b = -12°. Another 176 stars belong to a field containing the globular cluster NGC 6553, located at b = -3° and 5° away from the other three fields along the major axis. Stellar parameters and metallicities for these stars were presented in Zoccali et al. (2008, A&A, 486, 177). We have also re-derived stellar parameters and abundances for the sample of thick and thin disk red giants analyzed in Alves-Brito et al. (2010, A&A, 513, A35). Therefore using a homogeneous abundance database for the bulge, thick and thin disk, we have performed a differential analysis minimizing systematic errors, to compare the formation scenarios of these Galactic components. Results. Our results confirm, with large number statistics, the chemical similarity between the Galactic bulge and thick disk, which are both enhanced in alpha elements when compared to the thin disk. In the same context, we analyze [α/Fe] vs. [Fe/H] trends across different bulge regions. The most metal rich stars, showing low [α/Fe] ratios at b = -4° disappear at higher Galactic latitudes in agreement with the observed metallicity gradient in the bulge. Metal-poor stars ([Fe/H] < -0.2) show a remarkable homogeneity at different bulge locations. Conclusions. We have obtained further constrains for the formation scenario of the Galactic bulge. A metal-poor component chemically indistinguishable from the thick disk hints for a fast and early formation for both the bulge and the thick disk. Such a component shows no variation, neither in abundances nor kinematics, among different bulge regions. A metal-rich component showing low [α/Fe] similar to those of the thin disk disappears at larger latitudes. This allows us to trace a component formed through fast early mergers (classical bulge) and a disk/bar component formed on a more extended timescale. © 2011 ESO.

Gerard J.-C.,University of Liège | Soret L.,University of Liège | Piccioni G.,National institute for astrophysics | Drossart P.,Observatoire de Paris Meudon
Icarus | Year: 2014

Images of the nightside limb of Venus have been obtained in the northern hemisphere with the VIRTIS multispectral infrared imager on board Venus Express between April 2006 and October 2008. We analyze the latitudinal distribution of the O2(a1δ) airglow limb profiles at 1.27μm to characterize its distribution and variability. We show that the instantaneous structure of the emission is very different from the statistical global view of an enhanced emission near the equator, decreasing in brightness and slightly increasing in altitude toward the poles. The peak intensity of the limb profiles varies by a factor up to 50 between the brightest spots and the darkest regions. The bright airglow spots correspond to regions of enhanced downward flow of oxygen atoms originating from the dayside. Considerable variations in brightness and morphology are observed in the altitude-latitudinal distribution over a 24-h period. Analysis of the limb profiles indicates that secondary airglow peaks located at altitudes higher than the mean value of 96km are observed on about 30% of the latitudinal cuts, but they are concentrated in narrow latitude areas extending over a few hundred kilometers. Most of them occur in transition regions between two altitude regimes in the 50° to 60°N region, possibly associated with the drop of the cloud top altitude observed equatorward of the "cold collar". We interpret these results as an indication that the strength of vertical transport in this mesosphere-thermosphere transition region is very variable both in location and time. This variability, also observed in nadir airglow images and wind measurements, is a key characteristic of the mesosphere-thermosphere transition region. It may be caused by fluctuations of the global day-to-night circulation generated by gravity waves. We show with a one-dimensional model that local enhancements of eddy transport is a possibility. This variability is currently not accounted for by global circulation models that predict a single stable region of enhanced airglow in the vicinity of the antisolar point. © 2014 Elsevier Inc.

Soret L.,University of Liège | Gerard J.-C.,University of Liège | Piccioni G.,National institute for astrophysics | Drossart P.,Observatoire de Paris Meudon
Icarus | Year: 2014

The dynamical regime of the Venus upper atmosphere is mainly decomposed into three regions. The first one, located below 65km of altitude is governed by the retrograde superrotational zonal (RSZ) circulation. The second region above 130km is dominated by the subsolar to antisolar (SS-AS) circulation. The dynamics of the transition region in between are still not fully understood. However, the O2(a1δ) nightglow emission at 1.27μm, whose emitting layer is located at ~96km, can be used as a tracer of the dynamics in this transition region and the imaging spectrometer VIRTIS-M on board Venus Express, orbiting Venus since April 2006, acquired a large amount of nadir observations at this wavelength. Several previous studies showed that the O2(a1δ) nightglow emission is statistically located near the antisolar point. In this study, individual VIRTIS-M nadir observations have been analyzed to investigate the variability of the phenomenon. Bright patches of 1.27μm airglow have been extracted from every observation. It appears that the location of the bright patch is highly variable, even though the brightest patches occur near the antisolar point.Nadir observations have also been divided into time series, allowing generating animations to follow the intensity and the displacement of bright patches over time. Apparent wind velocities and characteristic decay/rise times and have been deduced from these time series. The speed of the displacements varies from ~0 up to 213ms-1, with a mean value of 54ms-1. Owing to the high variability of the direction of the displacements both in the short and the long terms, no clear trend of a global motion at ~96km can be deduced from these observations. The mean decay time is ~750min while the mean rise time is ~1550min. The decay time can be explained as a combination of radiative decay and atomic oxygen transport. © 2014 Elsevier Inc.

Freytag B.,University of Lyon | Allard F.,University of Lyon | Allard F.,University Pierre and Marie Curie | Ludwig H.-G.,Observatoire de Paris Meudon | And 2 more authors.
Astronomy and Astrophysics | Year: 2010

Context. Observationally, spectra of brown dwarfs indicate the presence of dust in their atmospheres while theoretically it is not clear what prevents the dust from settling and disappearing from the regions of spectrum formation. Consequently, standard models have to rely on ad hoc assumptions about the mechanism that keeps dust grains aloft in the atmosphere. Aims. We apply hydrodynamical simulations to develop an improved physical understanding of the mixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres, in particular of the influence of the underlying convection zone. Methods. We performed two-dimensional radiation hydrodynamics simulations including a description of dust grain formation and transport with the CO5BOLD code. The simulations cover the very top of the convection zone and the photosphere including the dust layers for a sequence of effective temperatures between 900 K and 2800 K, all with log  g = 5 assuming solar chemical composition. Results. Convective overshoot occurs in the form of exponentially declining velocities with small scale heights, so that it affects only the region immediately above the almost adiabatic convective layers. From there on, mixing is provided by gravity waves that are strong enough to maintain thin dust clouds in the hotter models. With decreasing effective temperature, the amplitudes of the waves become smaller but the clouds become thicker and develop internal convective flows that are more efficient in transporting and mixing material than gravity waves. The presence of clouds often leads to a highly structured appearance of the stellar surface on short temporal and small spatial scales (presently inaccessible to observations). Conclusions. We identify convectively excited gravity waves as an essential mixing process in M dwarf and brown dwarf atmospheres. Under conditions of strong cloud formation, dust convection is the dominant self-sustaining mixing component. © 2010 ESO.

Iwata I.,Japan National Astronomical Observatory | Chamaraux P.,Observatoire de Paris Meudon
Astronomy and Astrophysics | Year: 2011

Context. According to models of evolution in the hierarchical structure formation scenarios, voids of galaxies are expected to expand. The Local Void (LV) is the closest large void, and it provides a unique opportunity to test observationally such an expansion. It has been found that the Local Group, which is on the border of the LV, is running away from the void center at ~260 km ∼ s-1. Aims. In this study we investigate the motion of the galaxies at the far-side border of the LV to examine the presence of a possible expansion. Methods. We selected late-type, edge-on spiral galaxies with radial velocities between 3000 km ∼ s-1 and 5000 km ∼ s -1, and carried out HI 21 cm line and H-band imaging observations. The near-infrared Tully-Fisher relation was calibrated with a large sample of galaxies and carefully corrected for Malmquist bias. It was used to compute the distances and the peculiar velocities of the LV sample galaxies. Among the 36 sample LV galaxies with good quality HI line width measurements, only 15 galaxies were selected for measuring their distances and peculiar velocities, in order to avoid the effect of Malmquist bias. Results. The average peculiar velocity of these 15 galaxies is found to be-419+208-251 km ∼ s-1, which is not significantly different from zero. Conclusions. Due to the intrinsically large scatter of Tully-Fisher relation, we cannot conclude whether there is a systematic motion against the center of the LV for the galaxies at the far-side boundary of the void. However, our result is consistent with the hypothesis that those galaxies at the far-side boundary have an average velocity of ~260 km ∼ s-1 equivalent to what is found at the position of the Local Group. © 2011 ESO.

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