C. Ducourant

Gaia Data Release 1 - The photometric data

Context. This paper presents an overview of the photometric data that are part of the first Gaia data release. Aims. The principles of the processing and the main characteristics of the Gaia photometric data are presented. Methods. The calibration strategy is outlined briefly and the main properties of the resulting photometry are presented. Results. Relations with other broadband photometric systems are provided. The overall precision for the Gaia photometry is shown to be at the milli-magnitude level and has a clear potential to improve further in future releases.

Direct imaging of extra-solar planets in star forming regions - Lessons learned from a false positive around IM Lupi

Context. Most exoplanet imagers consist of ground-based adaptive optics coronagraphic cameras which are currently limited in contrast, sensitivity and astrometric precision, but advantageously observe in the near-infrared window (1–5 μm). Because of these practical limitations, our current observational aim at detecting and characterizing planets puts heavy constraints on target selection, observing strategies, data reduction, and follow-up. Most surveys so far have thus targeted young systems (1–100 Myr) to catch the putative remnant thermal radiation of giant planets, which peaks in the near-infrared. They also favor systems in the solar neighborhood (d < 80 pc), which eases angular resolution requirements but also ensures a good knowledge of the distance and proper motion, which are critical to secure the planet status, and enable subsequent characterization. Aims. Because of their youth, it is very tempting to target the nearby star forming regions, which are typically twice as far as the bulk of objects usually combed for planets by direct imaging. Probing these interesting reservoirs sets additional constraints that we review in this paper by presenting the planet search that we initiated in 2008 around the disk-bearing T Tauri star IM Lup, which is part of the Lupus star forming region (140–190 pc). Methods. We show and discuss why age determination, the choice of evolutionary model for both the central star and the planet, precise knowledge of the host star proper motion, relative or absolute (between different instruments) astrometric accuracy (including plate scale calibration), and patience are the key ingredients for exoplanet searches around more distant young stars. Results. Unfortunately, most of the time, precision and perseverance are not paying off: we discovered a candidate companion around IM Lup in 2008, which we report here to be an unbound background object. We nevertheless review in details the lessons learned from our endeavor, and additionally present the best detection limits ever calculated for IM Lup. We also accessorily report on the successful use of innovative data reduction techniques, such as the damped-LOCI and iterative roll subtraction.

Gaia GraL: Gaia DR2 gravitational lens systems: II. The known multiply imaged quasars⋆

Thanks to its spatial resolution the ESA/Gaia space mission offers a unique opportunity to discover new multiply-imaged quasars and to study the already known lensed systems at sub-milliarcsecond astrometric precisions. In this paper, we address the detection of the known multiply-imaged quasars from the Gaia Data Release 2 and determine the astrometric and photometric properties of the individually detected images found in the Gaia DR2 catalogue. We have compiled an exhaustive list of quasar gravitational lenses from the literature to search for counterparts in the Gaia Data Release 2. We then analyze the astrometric and photometric properties of these Gaias detections. To highlight the tremendous potential of Gaia at the sub-milliarcsecond level we finally perform a simple Bayesian modeling of the well-known gravitational lens system HE0435-1223, using Gaia Data Release 2 and HST astrometry. From 478 known multiply imaged quasars, 200 have at least one image found in the Gaia Data Release 2. Among the 41 known quadruply-imaged quasars of the list, 26 have at least one image in the Gaia Data Release 2, 12 of which are fully detected, 6 have three counterparts, 7 have two and 1 has only one. As expected, the modeling of HE0435-1223 shows that the model parameters are significantly better constrained when using Gaia astrometry compared to HST astrometry, in particular the relative positions of the background quasar source and the centroid of the deflector. The Gaia sub-milliarcsecond astrometry also significantly reduces the parameter correlations. Besides providing an up-to-date list of multiply imaged quasars and their detection in the Gaia DR2, this paper shows that more complex modeling scenarios will certainly benefit from Gaia sub-milliarcsecond astrometry.

Gaia GraL: Gaia DR2 gravitational lens systems. I. New quadruply imaged quasar candidates around known quasars

Context. Multiply imaged gravitationally lensed quasars are among the most interesting and useful observable extragalactic phenomena. Because their study constitutes a unique tool in various fields of astronomy, they are highly sought, but difficult to find. Even in this era of all-sky surveys, discovering them remains a great challenge, with barely a few hundred systems currently known. Aims: We aim to discover new multiply imaged quasar candidates in the recently published Gaia Data Release 2 (DR2), which is the astrometric and photometric all-sky survey with the highest spatial resolution that achieves effective resolutions from 0.4″ to 2.2″. Methods: We cross-matched a merged list of quasars and candidates with Gaia DR2 and found 1 839 143 counterparts within 0.5″. We then searched matches with more than two Gaia DR2 counterparts within 6″. We further narrowed the resulting list using astrometry and photometry compatibility criteria between the Gaia DR2 counterparts. A supervised machine-learning method, called extremely randomized trees, was finally adopted to assign a probability of being lensed to each remaining system. Results: We report the discovery of two quadruply imaged quasar candidates that are fully detected in Gaia DR2. These are the most promising new quasar lens candidates from Gaia DR2 and a simple singular isothermal ellipsoid lens model is able to reproduce their image positions to within 1 mas. This Letter demonstrates the discovery potential of Gaia for gravitational lenses.

The CPMDS catalogue of common proper motion double stars in the Bordeaux Carte du Ciel zone

Context. The present knowledge of common proper motion double stars is rather poor and almost entirely restricted to the Luyten LDS catalogue of 6210 pairs, to 439 pairs of Halbwachs catalogue of common proper motion stars in AGK3, and to Greaves new northern common proper motion pairs (975 pairs). Aims. We present a search for new double stars with common proper motion in the Bordeaux, PM2000, proper motion catalogue (+11 ◦ ≤ δ ≤ +18 ◦ ). Methods. The selection of double stars, on the basis of common proper motion, was performed by testing the candidate pairs with a Student t-distribution hypothesis test. A classification was performed by the analysis of the photometric properties of the components. Results. We present a catalogue, which contains 2572 new common proper motion pairs with angular separations ρ ≤ 2 � and PM2000 meridian magnitudes for both components. As a by-product, we present the remeasurement of the parameters of 926 visual pairs from the WDS Washington double star catalogue. Conclusions. The present catalogue, which is a list of candidate binaries, is an important step in the knowledge of double stars because it increases the number of known common proper motion pairs by 30%.

Support Vector Machines and CASGM20 Parameters Applied to Morphological Classification of Reconstructed 2D Images of Extended Objects Within the ESA‐Gaia Mission

In this work we present some parameters that are being studied to perform a purely morphological analysis of reconstructed images of extended objects, particularly galaxies, in the context of the ESA‐Gaia mission. Those parameters, known as Concentration, Asymmetry, Clumpiness, Gini’s coefficient and the Momentum of the brightest 20% of the galaxy, form a set that is becoming commonly used when a limited number of pixels is available to analyse, such as will be the case for Gaia reconstructed images. We comment about small modifications on those parameters that are planned to be performed. We also report tests with a preliminar version of the code that is being written to analyse Gaia images on a sample based on the Frei catalog of galaxies. Finally, we comment on the possibility of using Support Vector Machines to perform the morphological classification based on those measured parameters, and conclude that a very good level of segregation can be obtained for a two‐class discrimination.

KINEMATIC DISTANCES OF PRE-MAIN SEQUENCE STARS IN THE LUPUS STAR-FORMING REGION

Resumen en: The problem of the determination of distances has always played a central role in a stronomy. However, little recent progress has been made in the distan...

Common Proper Motion Visual Double Stars in the Bordeaux Carte du Ciel Zone

We present a preliminary version of CPMDS, a new catalogue of Northern common proper motion visual double stars. All these stars are laying in the Bordeaux Carte du Ciel zone +11°≤δ≤+18°. We applied a statistical hypothesis t‐test in the proper motions of the components. The proper motions used in our list were adopted from the Bordeaux proper motion catalogue PM2000. We detected 2831 new common proper motion double stars covering about 6% of the sky. This new catalogue is a large contribution to the small list of known common proper motion double stars.

The dynamics of an extremely young cluster probed for the first time

We present a proper-motion program that we have developed at ESO to measure, for the first time, proper motions of members of the nearby (125 pc), very young (4 × 10 5 yr) infrared ρ Ophiuchi cluster. Repeated imaging over an extended period will allow us to measure the global proper motion of the cluster and its velocity dispersion and will provide access to the ~ 1 km s −1 velocities for objects as faint as K = 15.7 mag. Access to the velocity field of such an extremely young cluster is of crucial importance to investigate the origin and early evolution of open cluster systems.

Determination of kinematic distances of pre-main-sequence stars in star-forming regions

Many studies of star-forming regions have been carried out since the discovery of compact Hii regions in the late 1960s. The kinematic properties of young stars in the nearest regions with ongoing and recent star formation provide essential tests of their formation mechanisms. The detection of coeval moving groups allows determination of individual distances through the convergent-point method. As a result, the main physical properties of these stars and their early evolutionary stages can be determined if we know how distant they are.