<i>Gaia</i> Data Release 1. Summary of the astrometric, photometric, and survey properties

Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7. Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release. Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue. Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the HIPPARCOS and Tycho-2 catalogues – a realisation of the Tycho-Gaia Astrometric Solution (TGAS) – and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of ∼3000 Cepheid and RR-Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr−1 for the proper motions. A systematic component of ∼0.3 mas should be added to the parallax uncertainties. For the subset of ∼94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr−1. For the secondary astrometric data set, the typical uncertainty of the positions is ∼10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to ∼0.03 mag over the magnitude range 5 to 20.7. Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data

Gaia Data Release 1: Testing parallaxes with local Cepheids and RR Lyrae stars

Context. Parallaxes for 331 classical Cepheids, 31 Type II Cepheids, and 364 RR Lyrae stars in common between Gaia and the Hipparcos and Tycho-2 catalogues are published in Gaia Data Release 1 (DR1) as part of the Tycho-Gaia Astrometric Solution (TGAS). Aims. In order to test these first parallax measurements of the primary standard candles of the cosmological distance ladder, which involve astrometry collected by Gaia during the initial 14 months of science operation, we compared them with literature estimates and derived new period-luminosity (PL), period-Wesenheit (PW) relations for classical and Type II Cepheids and infrared PL, PL-metallicity (PLZ), and optical luminosity-metallicity (M V -[Fe/H]) relations for the RR Lyrae stars, with zero points based on TGAS. Methods. Classical Cepheids were carefully selected in order to discard known or suspected binary systems. The final sample comprises 102 fundamental mode pulsators with periods ranging from 1.68 to 51.66 days (of which 33 with σ Ω /Ω < 0.5). The Type II Cepheids include a total of 26 W Virginis and BL Herculis stars spanning the period range from 1.16 to 30.00 days (of which only 7 with σ Ω /Ω < 0.5). The RR Lyrae stars include 200 sources with pulsation period ranging from 0.27 to 0.80 days (of which 112 with σ Ω /Ω < 0.5). The new relations were computed using multi-band (V,I,J,K s ) photometry and spectroscopic metal abundances available in the literature, and by applying three alternative approaches: (i) linear least-squares fitting of the absolute magnitudes inferred from direct transformation of the TGAS parallaxes; (ii) adopting astrometry-based luminosities; and (iii) using a Bayesian fitting approach. The last two methods work in parallax space where parallaxes are used directly, thus maintaining symmetrical errors and allowing negative parallaxes to be used. The TGAS-based PL,PW,PLZ, and M V - [Fe/H] relations are discussed by comparing the distance to the Large Magellanic Cloud provided by different types of pulsating stars and alternative fitting methods. Results. Good agreement is found from direct comparison of the parallaxes of RR Lyrae stars for which both TGAS and HST measurements are available. Similarly, very good agreement is found between the TGAS values and the parallaxes inferred from the absolute magnitudes of Cepheids and RR Lyrae stars analysed with the Baade-Wesselink method. TGAS values also compare favourably with the parallaxes inferred by theoretical model fitting of the multi-band light curves for two of the three classical Cepheids and one RR Lyrae star, which were analysed with this technique in our samples. The K-band PL relations show the significant improvement of the TGAS parallaxes for Cepheids and RR Lyrae stars with respect to the Hipparcos measurements. This is particularly true for the RR Lyrae stars for which improvement in quality and statistics is impressive. Conclusions. TGAS parallaxes bring a significant added value to the previous Hipparcos estimates. The relations presented in this paper represent the first Gaia-calibrated relations and form a work-in-progress milestone report in the wait for Gaia-only parallaxes of which a first solution will become available with Gaia Data Release 2 (DR2) in 2018. © ESO, 2017

Gaia Data Release 1: Open cluster astrometry: performance, limitations, and future prospects

Context. The first Gaia Data Release contains the Tycho-Gaia Astrometric Solution (TGAS). This is a subset of about 2 million stars for which, besides the position and photometry, the proper motion and parallax are calculated using Hipparcos and Tycho-2 positions in 1991.25 as prior information.Aims. We investigate the scientific potential and limitations of the TGAS component by means of the astrometric data for open clusters.Methods. Mean cluster parallax and proper motion values are derived taking into account the error correlations within the astrometric solutions for individual stars, an estimate of the internal velocity dispersion in the cluster, and, where relevant, the effects of the depth of the cluster along the line of sight. Internal consistency of the TGAS data is assessed.Results. Values given for standard uncertainties are still inaccurate and may lead to unrealistic unit-weight standard deviations of least squares solutions for cluster parameters. Reconstructed mean cluster parallax and proper motion values are generally in very good agreement with earlier HIPPARCOS-based determination, although the Gaia mean parallax for the Pleiades is a significant exception. We have no current explanation for that discrepancy. Most clusters are observed to extend to nearly 15 pc from the cluster centre, and it will be up to future Gaia releases to establish whether those potential cluster-member stars are still dynamically bound to the clusters.Conclusions. The Gaia DR1 provides the means to examine open clusters far beyond their more easily visible cores, and can provide membership assessments based on proper motions and parallaxes. A combined HR diagram shows the same features as observed before using the HIPPARCOS data, with clearly increased luminosities for older A and F dwarfs

The Gaia mission

Gaia is a cornerstone mission in the science programme of the EuropeanSpace Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page. http://www.cosmos.esa.int/gai

Internal doses of French adult population linked to the intake of radionuclides from the decay-chains of uranium and thorium by foodstuffs ingestion

International audienceThis study provides the first dose assessment to the French adult population due to the intake of radionuclides from the decay chains of uranium and thorium by foodstuff ingestion (water consumption excepted). This dose varies widely with the consumption of seafood, from less than 200 μSv.y-1 for people who do not consume shellfish or crustaceans at all, to more than 2,000 μSv.y-1 for the biggest consumers (about 150 kg.y-1 according to specific dietary surveys carried out along the French seaside). For moderate consumers of seafood (around 4.6 kg.y-1), who probably represent a major part of the population, this internal dose would be around 330 μSv.y-1. This variable consumption of seafood overshadows all the other causes of variability of these internal dose estimates. © 2015 EDP Sciences

L’accident de Fukushima : un premier bilan des conséquences pour l’environnement et la population

Dix mois après les rejets radioactifs consécutifs à l’accident de Fukushima, les données disponibles permettent d’ores et déjà de dimensionner ses conséquences sur l’environnement en termes de dépôts radioactifs et de contamination de la chaine alimentaire. De nombreuses similitudes phénoménologiques peuvent être faites avec l’accident de Tchernobyl, mais les surfaces terrestres concernées par les retombées de Fukushima sont de loin beaucoup plus petites. En revanche, une telle contamination du milieu marin côtier n’avait jamais été observée auparavant. S’il est trop tôt pour évaluer précisément l’atteinte des populations, elle a dû être considérablement diminuée par les décisions successives d’éloignement, notamment celle concernant le rayon de 20 km autour de l’installation

Facteurs de transfert du césium au lait et à la viande déterminés à partir de mesures faites en France en 2011 à la suite de l’accident de Fukushima-Daichi

Afin d’évaluer les conséquences des rejets radioactifs provenant des réacteurs accidentés de la centrale nucléaire de Fukushima-Daichi sur l’environnement terrestre français, l’IRSN a renforcé son plan de surveillance en échantillonnant notamment de la végétation de prairie, du lait et de la viande des troupeaux en pâture entre avril et novembre 2011. Ces prélèvements et ces mesures ont, entre autres, permis d’évaluer dans des conditions de dépôts réels la chronologie des transferts du césium-134 de l’herbe au lait et à la viande. Les premières mesures significatives en 134Cs apparaissent dans l’herbe et dans le lait cinq à sept jours après la première mesure significative dans l’air. Les activités maximales atteignent 0,4 Bq.kg–1 d’herbe fraiche et 0,028 Bq.L–1 dans le lait. Dans la viande, les premières mesures significatives apparaissent mi-avril avec une activité maximale mesurée de 0,036 Bq.kg–1, avec de fortes variations d’une espèce animale à une autre. Les coefficients de transfert moyens calculés sont proches de ceux disponibles dans la littérature avec 0,22 j.kg–1 pour la viande de mouton, et de 0,09 et 0,0014 j.L–1 pour le lait de chèvre et de vache, respectivement

Contamination of Japanese foodstuffs of terrestrial origin after the Fukushima nuclear accident and related dose assessments Part 1: foodstuff contamination

During and after the Fukushima accident, the IRSN collected and interpreted the results of radiological measurements performed on foodstuffs of terrestrial origin published by Japan's Ministry of Health between mid-March 2011 and July 2012. Analysis of the findings shows that the accident's date, livestock-rearing practices and the deposits' characteristics had a decisive influence. The fact that radioactive fallout occurred very early in the growing and breeding season largely explains the moderate contamination of most foodstuffs of terrestrial origin, notably in the areas with the largest deposits. In the case of dairy products and meat, feeding imported fodder to livestock in stables, a common practice in Japan, compounded the calendar effect. Measurements published in Japan have also borne out the particular sensitivity of mushrooms, including cultivated species, and game