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.

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.

Context. Water and O 2 are important gas phase ingredients for cooling dense gas when forming stars. On dust grains, H 2 O is an important constituent of the icy mantle in which a complex chemistry is taking place, as revealed by hot core observations. The formation of water can occur on dust grain surfaces, and can impact gas phase composition. Aims. The formation of molecules such as OH, H 2 O, HO 2 and H 2 O 2 , as well as their deuterated forms and O 2 and O 3 is studied to assess how the chemistry varies in different astrophysical environments, and how the gas phase is affected by grain surface chemistry. Methods. We use Monte Carlo simulations to follow the formation of molecules on bare grains as well as the fraction of molecules released into the gas phase. We consider a surface reaction network, based on gas phase reactions, as well as UV photo-dissociation of the chemical species.Results. We show that grain surface chemistry has a strong impact on gas phase chemistry, and that this chemistry is very different for different dust grain temperatures. Low temperatures favor hydrogenation, while higher temperatures favor oxygenation. Also, UV photons dissociate the molecules on the surface, which can subsequently reform. The formation-destruction cycle increases the amount of species released into the gas phase. We also determine the timescales to form ices in diffuse and dense clouds, and show that ices are formed only in shielded environments, as supported by observations.

Context. Although few in number, high-mass stars play a major role in the interstellar energy budget and the shaping of the Galactic environment; however, the formation of high-mass stars is not well understood, because of their large distances, short time scales, and heavy extinction. Aims. The chemical composition of the massive cores forming high-mass stars can put some constraints on the time scale of the massive star formation: sulfur chemistry is of specific interest thanks to its rapid evolution in warm gas and because the abundance of sulfur-bearing species increases significantly with the temperature. Methods. Two mid-infrared quiet and two brighter massive cores were observed in various transitions (E up up to 289 K) of CS, OCS, H 2 S, SO, and SO 2 and of their 34 S isotopologues at mm wavelengths with the IRAM 30m and CSO telescopes. The 1D modeling of the dust continuum is used to derive the density and temperature laws, which were then applied in the RATRAN code to modeling the observed line emission and to deriving the relative abundances of the molecules. Results. All lines are detected, except the highest energy SO 2 transition. Infall (up to 2.9 km s −1 ) may be detected towards the core W43MM1. The inferred mass rate is 5.8-9.4 10 −2 M /yr. We propose an evolutionary sequence of our sources (W43MM1→IRAS18264−1152→IRAS05358+3543→IRAS18162−2048), based on the SED analysis. The analysis of the variations in abundance ratios from source to source reveals that the SO and SO 2 relative abundances increase with time, while CS and OCS decrease. /SO] may be good indicators of evolution, depending on layers probed by the observed molecular transitions. Observations of molecular emission from warmer layers, so that involving higher upper energy levels must be included.

Context. The study of physical and chemical properties of massive protostars is critical for better understanding the evolutionary sequence that leads to the formation of high-mass stars. Aims. IRAS 18151-1208 is a nearby massive region (d = 3 kpc, L ∼ 2 × 10 4 L ) that splits into three cores: MM1, MM2, and MM3 (separated by 1 -2 ). We aim at (1) studying the physical and chemical properties of the individual MM1, MM2, and MM3 cores; (2) deriving their evolutionary stages; (3) using these results to improve our view of the evolutionary sequence of massive cores. Methods. The region was observed in the CS, C 34 S, H 2 CO, HCO + , H 13 CO + , and N 2 H + lines at mm wavelengths with the IRAM 30 m and Mopra telescopes. We use 1D and 2D modeling of the dust continuum to derive the density and temperature distributions, which are then used in the RATRAN code to model the lines and constrain the abundances of the observed species. Results. All the lines were detected in MM1 and MM2. MM3 shows weaker emission, or is even undetected in HCO + and all isotopic species. MM2 is driving a newly discovered CO outflow and hosts a mid-IR-quiet massive protostar. The abundance of CS is significantly greater in MM1 than in MM2, but smaller than in a reference massive protostar such as AFGL 2591. In contrast, the N 2 H + abundance decreases from MM2 to MM1, and is larger than in AFGL 2591. Conclusions. Both MM1 and MM2 host an early-phase massive protostar, but MM2 (and mid-IR-quiet sources in general) is younger and dominated more by the host protostar than MM1 (mid-IR-bright). The MM3 core is probably in a pre-stellar phase. We find that the N 2 H + /C 34 S ratio varies systematically with age in the massive protostars for which the data are available. It can be used to identify young massive protostars.