This paper introduces the The Milky Way Unravelled by Gaia conference proceedings.

This paper gives a brief overview of the Gaia Research for European Astronomy Training (GREAT) network, including a description of the GREAT-ESF Research Network Programme and the GREAT Initial Training Network (GREAT-ITN). Scientific highlights from the GREAT-ITN are noted.

Gaia mission is underway conducting its 5 year lasting survey of the sky. At the moment the commissioning period has been completed and half a year worth of routine phase data has been collected. The status of the mission is outlined with short explanation of some commissioning phase findings and their impact on the mission. With a few examples of early mission data the potential of Gaia is demonstrated and we can conclude that the mission is capable meeting its high expectations.

The standard errors of the end-of-mission Gaia astrometry have been re-assessed after conclusion of the in-orbit commissioning phase of the mission. An analytical relation is provided for the parallax standard error σϖ as function of Gaia G magnitude (and V − I colour) which supersedes the pre-launch relation provided in de Bruijne (2012).

One year before the first release of the first data from Gaia, how robust are our views of the Milky Way stellar populations? Recent results have shown that limits, differences and/or continuities between populations are not where we thought they were just a few years ago. The outer disk (> 10kpc) has properties essentially different from the inner (thin+thick) disk, while the bulge is best explained in terms of disk populations, with a negligible or inexistent classical bulge, suggesting that the Milky Way is a pure disk galaxy. Much less contingent than previously envisaged, the thick disk is probably the main phase of stellar mass creation in the MW, and the parent population of the thin disk. These results lead to fundamental changes in our views on the stellar mass growth of the Galaxy, secular mass redistribution in the disk, and imply a change of paradigm of the chemical evolution. I review these different advances, and discuss some of the key questions.

The Gaia dataset will require a huge leap forward in terms of modelling of the Milky Way. Two problems are highlighted here. First, models of the Galactic Bar remain primitive as compared to the Galactic Disk and Stellar Halo. Although Schwarzschild and N-body methods are useful, the future belongs to Made-to-Measure (M2M) models which have significant advantages in terms of storage and flexibility. Second, the Milky Way potential will need much better representation than hitherto. Most models still use very simple building blocks (Miyamoto-Nagai disks or Hernquist bulges) and these will not be fit for purpose in the Gaia Era. Expansions in terms of basis functions offer the possibility of incorporating cosmological information as priors, as well as mych greater adaptability.

Unprecedented astrometric accuracy of Gaia gives a unique opportunity to perform a suite of tests of fundamental physics. The tests range from a test of the Local Lorenz Invariance and the gravitational defection of light to measuring the mass of black hole candidates in certain compact binary systems with visible components. Gaia data can also be used to estimate the energy flux of the gravitational waves in certain frequency ranges.

Before Gaia data are published, they will undergo scrutiny in several different validation processes. Each leg of the data reduction will have its own specialised validation with focus on the difficult points for that task. In addition, when the combined catalogue is ready for release, it will undergo an independent, more global, validation. All kinds of properties of the catalogue are checked, and it is of particular interest to test if the error estimates are realistic, and if the parameters are unbiased. We also look briefly at the experiences from other projects like 2MASS and Hipparcos, and address recent worries that Hipparcos was wrong and that Gaia may therefore also be wrong.

Gaia, the space astrometry satellite of the European Space Agency, was successfully launched on 2013 December 19th. A vast amount of data has already been received by the on-ground data processing systems running at the European Space Astronomy Centre near Madrid. In this paper we describe the mentioned systems, focusing on the Initial Data Treatment system which is coordinated by the Gaia group at the University of Barcelona. We present some of the results obtained during the first months of nominal operations.

The ESA Gaia mission uses two telescopes to create the most ambitious survey of the Galaxy. The angle between them must be known with exquisite precision and accuracy. An interferometer: the Basic Angle Monitoring system measures its variations. High quality data have been retrieved and analysed for more than a year. A summary of the in-orbit performance and some early results are presented.

We present the initial performance of the Gaia Radial Velocity Spectrometer, providing an overview of its performance, which is essentially nominal in terms of spectral resolution, throughput and operation, except for the presence of unexpectedly high levels of scattered background. This is mainly Solar in origin, and reduces the limiting magnitude for radial velocity measurements by ∼1 magnitude to V  ∼ 16. Radial velocity calibration accuracies are compliant with requirements.

We present the variability processing and analysis that is foreseen for the Gaia mission within Coordination Unit 7 (CU7) of the Gaia Data Processing and Analysis Consortium (DPAC). A top level description of the tasks is given.

I describe dynamical modelling of the Milky Way using action-angle coordinates. I explain what action-angle coordinates are, and what progress has been made in the past few years to ensuring they can be used in reasonably realistic Galactic potentials. I then describe recent modelling efforts, and progress they have made in constraining the potential of the Milky Way and the local dark matter density.

We have developed our original made-to-measure (M2M) algorithm, primal, with the aim of modelling the Galactic disc from upcoming Gaia data. From a Milky Way like N-body disc galaxy simulation, we have created mock Gaia data using M0III stars as tracers, taking into account extinction and the expected Gaia errors. In primal, observables calculated from the N-body model are compared with the target stars, at the position of the target stars. Using primal, the masses of the N-body model particles are changed to reproduce the target mock data, and the gravitational potential is automatically adjusted by the changing mass of the model particles. We have also adopted a new resampling scheme for the model particles to keep the mass resolution of the N-body model relatively constant. We have applied primalto this mock Gaia data and we show that primalcan recover the structure and kinematics of a Milky Way like barred spiral disc, along with the apparent bar structure and pattern speed of the bar despite the galactic extinction and the observational errors.

We use a 3D test particle simulation evolved under a potential that includes a Galactic bar. The test particles are given the properties of the disc Red Clump stars in order to convolve with the Gaia errors and to generate a mock catalogue with the stars up to Gaia magnitude, G, 20. In this work, we work in the space of Gaia observables, that is, trigonometric parallaxes. Thus, we show first how the bar structure is mapped in the parallax cartesian space derived from (parallax, galactic longitude). Secondly, we show the effect of the magnitude cut and the introduction o the Gaia errors. Finally, we also consider other possible error sources, for instance from IR photometric distances.

The era of high-precision astrometry has dawned upon us. The potential of Gaia μas-level precision in positional measuraments is about to be unleashed in the field of extrasolar planetary systems. The Gaia data hold the promise for much improved global characterization of planetary systems around stars of all types, ages, and chemical composition, particularly when synergistically combined with other indirect and direct planet detection and characterization programs.

The search for extrasolar planets has developed rapidly and, today, more than 1700 planets have been found orbiting stars. Thanks to Gaia, we will collect high-accuracy astrometric orbits of thousands of new low-mass celestial objects, such as extra-solar planets and brown dwarfs. These measurements in combination with spectroscopy and with present day and future extrasolar planet search programs (like HARPS, ESPRESSO) will have a crucial contribution to several aspects of planetary astrophysics (formation theories, dynamical evolution, etc.). Moreover, Gaia will have a strong contribution on the stellar chemical and kinematic characterisation studies. In this paper we present a short overview of the importance of Gaia in the context of exoplanet research. As preparatory work for Gaia, we will then present a study where we derived stellar parameters for a sample of field giant stars.

We explore a sample of 148 solar-like stars to search for a possible correlation between the slopes of the abundance trends versus condensation temperature (known as the Tc slope) both with stellar parameters and Galactic orbital parameters in order to understand the nature of the peculiar chemical signatures of these stars and the possible connection with planet formation. We find that the Tc slope correlates at a significant level with the stellar age and the stellar surface gravity. We also find tentative evidence that the Tc slope correlates with the mean galactocentric distance of the stars (Rmean), suggesting that stars that originated in the inner Galaxy have fewer refractory elements relative to the volatile ones. We found that the chemical peculiarities (small refractory-to-volatile ratio) of planet-hosting stars is probably a reflection of their older age and their inner Galaxy origin. We conclude that the stellar age and probably Galactic birth place are key to establish the abundances of some specific elements.

We present the Gaia-Groundbased Observation Service for Asteroids (Gaia-GOSA) which release is planned for April 2015. This web service aims to lay the foundation of a worldwide collaborative network of observers with the goal of collecting lightcurves around isolated photometric observations by the European Space Agency (ESA) Gaia mission. Such data will allow us to extract physical parameters of the observed asteroids, such as rotational period. Moreover, we will obtain a minimum threshold of the asteroid's lightcurve amplitude. The resulting catalogue might be used to enhance some of the problematic inversion scenarios or also for validation purposes.

The Gaia-ESO Survey (GES) is a large public spectroscopic survey that aims at observing with FLAMES@VLT the main stellar components of our Galaxy. The study of the population of open clusters is one of the main objectives of GES. We present some results from the first 18 months of observations, among them, a preliminary view of the radial metallicity gradient as traced by open cluster data and a comparison of the chemical patterns of clusters located in different parts of the Galactic disk.