Gaia is a cornerstone mission in the science programme of the European Space 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.

No abstract

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.

Abstract. The distance dependence of interstellar extinction in the direction of the Aquila Rift is investigated using 473 stars observed in the Vilnius photometric system. The front edge of the dark clouds in the area is found to be at 225 ± 55 pc and the thickness of the cloud system is about 80 pc. The maximum extinction A V in the clouds is close to 3.0 mag. Two stars with larger extinction are found and discussed. Since the new distance of the clouds is larger than the previously accepted distance, the cloud system mass should be increased to 2.7 × 10 5 M which is close to the virial mass estimated from the CO velocity dispersion. Additional arguments are given in favor of the genetic relation between the Serpens and the Scorpio-Ophiuchus dark clouds.

No abstract

Abstract.A short description of possibilities of the Vilnius phe tometric system for classification of faint stars is given. The systerr realized with CCD detectors allows to obtain three-dimensional claL sification of stars down to 18 mag with a 1.5-m telescope or to 2 . mag with a 4-m telescope.

No abstract

Abstract. The interstellar reddening law is derived for 15 heavily reddened stars in the area which includes the North America and Pelican nebulae and the dark cloud between them. The method is based on photometry of these stars in the Vilnius seven-color system and on their MK spectral types. The mean law in this area is very similar to the law for a much wider area in Cygnus derived earlier by other authors. It differs from the normal law by exhibiting somewhat stronger extinction in the violet and the near ultraviolet spectral region, i.e., it shows a smaller "knee" in the blue part of the spectrum.