Author(s): Jos de Bruijne
Whereas astrometry originated several millennia ago (for an overview, see Perryman 2012), the last centuries – and the last decades in particular – have shown exponential progress in the number of objects and the accuracy with which their positions, proper motions, and parallaxes are determined. These improvements have arisen as a result of improved technologies and instrumentation and of the possibility to eliminate the effects of the Earth’s atmosphere by going to space. Astrometry from space was pioneered by the European Space Agency’s Hipparcos mission, which operated from 1989 till 1993. The Hipparcos Catalogue was published in 1997 (ESA 1997; van Leeuwen 2007a). An overview of the science revolution that Hipparcos has brought about is presented by Perryman (2009).
The successor of Hipparcos, originally named GAIA, was proposed in the early 1990s by Perryman and Lindegren as an interferometric concept (Lindegren and Perryman 1995). Later, the mission design was changed to a direct-imaging approach but the name was kept for continuity reasons yet spelt as of then in lower case, i.e., Gaia. For more details about the history of Gaia, see Høg (2011, 2014).
As explained in Gaia Collaboration et al. (2016), the science case for the Gaia mission was compiled around the year 2000 (Perryman et al. 2001). The scientific goals of the design reference mission were relying heavily on astrometry, combined with its photometric and spectroscopic surveys. Nearly 20 years later, the astrometric part of the science case remains unique, and so do the photometric and spectroscopic data, despite various, large ground-based surveys having materialised in the last decade(s). The space environment and design of Gaia enable a combination of accuracy, sensitivity, dynamic range, and sky coverage, which is practically impossible to obtain with ground-based facilities targeting photometric or spectroscopic surveys of a similar scientific scope. The spectra collected by the radial-velocity spectrometer have sufficient signal to noise for bright stars to make the Gaia spectroscopic survey the biggest of its kind. The astrometric part of Gaia is unique simply because global, micro-arcsecond astrometry is possible only from space. Therefore, the science case outlined nearly two decades ago remains largely valid and the Gaia data releases are still needed to address the scientific questions (for a recent overview of the expected yield from Gaia, see Walton et al. 2014). A non-exhaustive list of scientific topics is provided in this section with an outline of the most important Gaia contributions.
The fundamental scientific-performance requirements for Gaia stem, to a large extent, from the main scientific target of the mission: the Milky Way galaxy. Gaia is built to address the question of the formation and evolution of the Galaxy through the analysis of the distribution and kinematics of the luminous and dark mass in the Galaxy. By also providing measurements to deduce the physical properties of the constituent stars, it is possible to study the structure and dynamics of the Galaxy. Although the Gaia sample will only cover about 1% of the stars in the Milky Way, it will consist of more than 1000 million stars covering a large volume (out to many kpc, depending on spectral type), allowing thorough statistical analysis work to be conducted. The dynamical range of the Gaia measurements facilitates reaching stars and clusters in the Galactic disk out to the Galactic centre as well as far out in the halo, while providing extremely high accuracies in the solar neighbourhood. In addition to using stars as probes of Galactic structure and the local, Galactic potential in which they move, stars can also be used to map the interstellar matter. By combining extinction deduced from stars, it is possible to construct the three-dimensional distribution of dust in our Galaxy. In this way, Gaia will address not only the stellar contents, but also the interstellar matter in the Milky Way.
The current understanding of galaxy formation is based on a combination of theories and observations, both of (high-redshift) extragalactic objects and of individual stars in our Milky Way. The Milky Way galaxy provides the single possibility to study details of the processes, but the observational challenges are different in comparison with measuring other galaxies. From our perspective, the Galaxy covers the full sky, with some components far away in the halo requiring sensitivity, while stars in the crowded Galactic centre region require spatial resolving power. Both these topics can be addressed with the Gaia data. Gaia distances will allow the derivation of absolute luminosities for stars which, combined with metallicities, allow the derivation of accurate individual ages, in particular for old subgiants, which are evolving from the main-sequence turn-off to the bottom of the red giant branch. By combining the structure and dynamics of the Galaxy with the information of the physical properties of the individual stars and, in particular, ages, it is possible to deduce the star formation histories of the stellar populations in the Milky Way.
Distances are one of the most fundamental quantities needed to understand and interpret various astronomical observations of stars. Yet direct distance measurement using trigonometric parallax of any object outside the immediate solar neighbourhood or not emitting in radio wavelengths is challenging from the ground. The Gaia revolution will be in the parallaxes, with hundreds of millions being accurate enough to derive high-quality colour-magnitude diagrams and to make significant progress in stellar astrophysics. The strength of Gaia is also in the number of objects that are surveyed as many phases of stellar evolution are fast. With 1000 million parallaxes, Gaia will cover most phases of evolution across the stellar-mass range, including pre-main-sequence stars and (chemically) peculiar objects. In addition to parallaxes, the homogeneous, high-accuracy photometry will allow fine tuning of stellar models to match not only individual objects, but also star clusters and populations as a whole. The combination of Gaia astrometry and photometry will also contribute significantly to star formation studies.
On average, each star is measured astrometrically 70 times during the five-year nominal operations phase. At each epoch, photometric measurements are also made: ten in the Gaia broadband filter and one each with the red and blue photometer. For the variable sky, this provides a systematic survey with the sampling and cadence of the scanning law of Gaia. This full-sky survey will provide a census of variable stars with tens of millions of new variables, including rare objects. Sudden photometric changes in transient objects can be captured and the community can be alerted for follow-up observations. Pulsating stars, especially RR Lyrae and Cepheids, can easily be discovered from the Gaia data stream, allowing, in combination with the parallaxes, calibration of the period-luminosity relations to better accuracies, thereby improving the quality of the cosmic-distance ladder and scale.
Gaia is a powerful mission to improve our understanding of multiple stars. The instantaneous spatial resolution, in the scanning direction, is comparable to that of the Hubble Space Telescope and Gaia is surveying the whole sky. In addition to resolving many binaries, all instruments in Gaia can complement our understanding of multiple systems. The astrometric wobbles of unresolved binaries, seen superimposed on parallactic and proper motions, can be used to identify multiple systems. Periodic changes in photometry can be used to find (eclipsing) binaries and an improved census of double-lined systems based on spectroscopy will follow from the Gaia data. It is again the large number of objects that Gaia can provide that will help address the fundamental questions of mass distributions and orbital eccentricities among binaries.
From the whole spectrum of scientific topics that Gaia can address, the exoplanet research area has been the most dynamic in the past two decades. The field has expanded from hot, giant planets to smaller planets, to planets further away from their host star, and to multiple planetary systems. These advancements have been achieved both with space- and ground-based facilities. Nevertheless, the Gaia astrometric capabilities remain unique, probing a poorly explored area in the parameter space of exoplanetary systems and providing astrophysical parameters not obtainable by other means. A strong point of Gaia in the exoplanet research field is the provision of an unbiased, volume-limited sample of Jupiter-mass planets in multiyear orbits around their host stars. These are logical prime targets for future searches of terrestrial-mass exoplanets in the habitable zone in an orbit protected by a giant planet further out. In addition, the astrometric data of Gaia allow actual masses (rather than lower limits) to be measured. Finally, the data of Gaia will provide the detailed distributions of giant exoplanet properties (including the transition regime between giant planets and brown dwarf) as a function of stellar-host properties with unprecedented resolution.
Although Gaia is designed to detect and observe stars, it will provide a full census of all sources that appear point-like on the sky. The movement of solar system objects with respect to the stars smears their images and makes them less point-like. As long as this smearing is modest, Gaia will still detect the object. The most relevant solar system object group for Gaia are asteroids. Unlike planets, which are too big in size (and, in addition, sometimes too bright) to be detected by Gaia, asteroids remain typically point-like and have brightness in the dynamical range of Gaia. Gaia astrometry and photometry will provide a census of orbital parameters and taxonomy in a single, homogeneous photometric system. The full-sky coverage of Gaia will also provide this census far away from the ecliptic plane as well as for locations inside the orbit of the Earth. An alert can be made of newly discovered asteroids to trigger ground-based observations to avoid losing the object again. For near-Earth asteroids, Gaia is not going to be very complete as the high apparent motion of such objects often prevents Gaia detection, but in those cases where Gaia observations are made, the orbit determination can be very precise. Gaia will provide fundamental mass measurements of those asteroids that experience encounters with other solar system bodies during the Gaia operational lifetime.
In the Local Group, the spatial resolution of Gaia is sufficient to resolve and observe the brightest individual stars. Tens of Local Group galaxies will be covered, including the Andromeda galaxy and the Magellanic Clouds. While for the faintest dwarf galaxies only a few dozen of the brightest stars are observed, this number increases to thousands and millions of stars in Andromeda and the Large Magellanic Cloud, respectively. In dwarf spheroidals such as Fornax, Sculptor, Carina, and Sextans, thousands of stars will be covered. A major scientific goal of Gaia in the Local Group concerns the mutual, dynamical interaction of the Magellanic Clouds and the interaction between the Clouds and the Galaxy. In addition to providing absolute proper motions for transverse-velocity determination, needed for orbits, it is possible to explore internal stellar motions within dwarf galaxies. These kinds of data may reveal the impact of dark matter, among other physical processes in the host galaxy, to the motions of its stars.
Gaia will provide a homogeneous, magni-tude-limited sample of unresolved galaxies. For resolved galaxies, the sampling function is complicated as the onboard detection depends on the contrast between any point-like, central element (bulge) and any extended structure, convolved with the scanning direction. For unresolved galaxies, the most valuable measurements are the photometric observations. Millions of galaxies across the whole sky will be measured systematically. As the same Gaia system is used for stellar work, one can anticipate that, in the longer term, the astrophysical interpretation of the photometry of extragalactic objects will be based on statistically sound fundamentals obtained from Galactic studies. Quasars form a special category of extragalactic sources for Gaia as not only their intrinsic properties can be studied, but they can also be used in comparisons of optical and radio reference frames. Such a comparison will, among others, answer questions of the coincidence of quasar positions across different wavelengths.
Relativistic corrections are part of the routine data processing for Gaia. Given the huge number of measurements, it is possible to exploit the redundancy in these corrections to conduct relativity tests or to use (residuals of) the Gaia data in more general fundamental-physics experiments. Specifically for light bending, it is possible to determine the parameter in the parametrised post-Newtonian formulation very precisely. Another possible experiment is to explore light bending of star images close to the limb of Jupiter to measure the quadrupole moment of the gravitational field of the giant planet. A common element in all fundamental physics tests using Gaia data is the combination of large sets of measurements. This is meaningful only when all systematic effects are under control, down to micro-arcsecond levels. Therefore, Gaia results for relativistic tests can be expected only towards the end of the mission, when all calibration aspects have been handled successfully.