Why chromatic imaging matters

During the last two decades, the first generation of beam combiners at the Very Large Telescope Interferometer has proved the importance of optical interferometry for high-angular resolution astrophysical studies in the near- and mid-infrared. With the advent of 4-beam combiners at the VLTI, the u ? v coverage per pointing increases significantly, providing an opportunity to use reconstructed images as powerful scientific tools. Therefore, interferometric imaging is already a key feature of the new generation of VLTI instruments, as well as for other interferometric facilities like CHARA and JWST. It is thus imperative to account for the current image reconstruction capabilities and their expected evolutions in the coming years. Here, we present a general overview of the current situation of optical interferometric image reconstruction with a focus on new wavelength-dependent information, highlighting its main advantages and limitations. As an Appendix we include several cookbooks describing the usage and installation of several state-of-the art image reconstruction packages. To illustrate the current capabilities of the software available to the community, we recovered chromatic images, from simulated MATISSE data, using the MCMC software SQUEEZE. With these images, we aim at showing the importance of selecting good regularization functions and their impact on the reconstruction.     

Colour-differential interferometry for the observation of extrasolar planets

We present the high angular resolution technique of colour-differential interferometry for direct detection of extrasolar giant planets (EGPs). The measurement of differential phase with long-baseline ground-based interferometers in the near-infrared could allow the observation of several hot giant extrasolar planets in tight orbit around the nearby stars, and thus yield their low- or mid-resolution spectroscopy, complete orbital data set and mass. Estimates of potentially achievable signal-to-noise ratios are presented for a number of planets already discovered by indirect methods. The limits from the instrumental and atmospheric instability are discussed, and a subsequent observational strategy is proposed.     

Wind measurements in Mars' middle atmosphere: IRAM Plateau de Bure interferometric CO observations

The IRAM Plateau de Bure Interferometer has been used to map the CO(1-0) rotational line in Mars' middle atmosphere. Absolute winds and thermal profiles were retrieved during the 1999, 2001, 2003 and 2005 planet's oppositions. The observations sampled various seasons (Ls=143, 196, 262, 317 and 322), and different dust situations (clear, global storm, regional storm). The absolute winds were derived by measuring directly the Doppler lineshifts. The main zonal circulation near 50 km is dominated by strong retrograde winds, with typical velocities of 70-170 m/s, strongly varying seasonally, latitudinally, and longitudinally (in particular between morning and evening). Comparison of the retrieved temperature with a general circulation model indicates that the model often underestimates the temperatures in the middle (20-50 km) atmosphere, and overestimates them above 50 km.     

Amazonian northern mid-latitude glaciation on Mars: A proposed climate scenario

Recent geological observations in the northern mid-latitudes of Mars show evidence for past glacial activity during the late Amazonian, similar to the integrated glacial landsystems in the Dry Valleys of Antarctica. The large accumulation of ice (many hundreds of meters) required to create the observed glacial deposits points to significant atmospheric precipitation, snow and ice accumulation, and glacial flow. In order to understand the climate scenario required for these conditions, we used the LMD (Laboratoire de Météorologie Dynamique) Mars GCM (General Circulation Model), which is able to reproduce the present-day water cycle, and to predict past deposition of ice consistent with geological observations in many cases. Prior to this analysis, however, significant mid-latitude glaciation had not been simulated by the model, run under a range of parameters.In this analysis, we studied the response of the GCM to a wider range of orbital configurations and water ice reservoirs, and show that during periods of moderate obliquity (? = 25-35°) and high dust opacity (τ_dust = 1.5-2.5), broad-scale glaciation in the northern mid-latitudes occurs if water ice deposited on the flanks of the Tharsis volcanoes at higher obliquity is available for sublimation. We find that high dust contents of the atmosphere increase its water vapor holding capacity, thereby moving the saturation region to the northern mid-latitudes. Precipitation events are then controlled by topographic forcing of stationary planetary waves and transient weather systems, producing surface ice distribution and amounts that are consistent with the geological record. Ice accumulation rates of ∼10 mm yr⁻¹ lead to the formation of a 500-1000 m thick regional ice sheet that will produce glacial flow patterns consistent with the geological observations.     

The impact of martian mesoscale winds on surface temperature and on the determination of thermal inertia

Radiative control of surface temperature is a key characteristic of the martian environment and its low-density atmosphere. Here we show through meteorological modeling that surface temperature can be far from radiative equilibrium over numerous sloping terrains on Mars, where nighttime mesoscale katabatic winds impact the surface energy budget. Katabatic circulations induce both adiabatic atmospheric heating and enhancement of downward sensible heat flux, which then becomes comparable to radiative flux and acts to warm the ground. Through this mechanism, surface temperature can increase up to 20 K. One consequence is that warm signatures of surface temperature over slopes, observed through infrared spectrometry, cannot be systematically associated with contrasts of intrinsic soil thermal inertia. Apparent thermal inertia maps retrieved thus far possibly contain wind-induced structures. Another consequence is that surface temperature observations close to sloping terrains could allow the validation of model predictions for martian katabatic winds, provided contrasts in intrinsic thermal inertia can be ruled out. The thermal impact of winds is mostly discussed in this paper in the particular cases of Olympus Mons/Lycus Sulci and Terra Meridiani but is generally significant over any sloped terrains in low thermal inertia areas. It is even general enough to apply under daytime conditions, thereby providing a possible explanation for observed afternoon surface cooling, and to ice-covered terrains, thereby providing new insights on how winds could have shaped the present surface of Mars.     

Perspectives of a visible instrument on the VLTI

In this paper we present the most promising science cases for a new generation visible instrument on the VLTI and the conceptual idea for the instrumental configuration. We also present a statistical study of the potential targets that may be accessible for the different classes of objects and for the required spectral resolutions.     

Interferometry basics in practice: Exercises

The following exercises aim to learn the link between the object intensity distribution and the corresponding visibility curves of a long-baseline optical interferometer. They are also intended to show the additional constraints on observability that an interferometer has.This practical session is meant to be carried out with the ASPRO software, from the Jean-Marie Mariotti Center, but can also be done using other observation preparation software, such as viscalc from ESO.There are two main parts with series of exercises and the exercises corrections. The first one aims at understanding the visibility and its properties by practicing with simple examples, and the second one is about UV coverage.     

All you ever wanted to know about optical long baseline stellar interferometry, but were too shy to ask your adviser

I try to present a small view of the properties and issues related to astronomical interferometry observations. I recall a bit of history of the technique, give some basic assessments to the principle of interferometry, and finally, describe physical processes and limitations that affect optical long baseline interferometry and which are, in general, very useful for everyday work. Therefore, this text is not intended to perform strong demonstrations and show accurate results, but rather to transmit the general “feeling” one needs to have to not be destabilised by the first contact to real world interferometry.